The language
************

This chapter of the manual is a full guide to the librep Lisp programming language, including documentation for most of the built-in functions.

This manual still fails to document the following functions: default-boundp, default-value, recursive-edit, regexp-cache-control, sdbm-close, sdbm-delete, sdbm-error, sdbm-fetch, sdbm-firstkey, sdbm-nextkey, sdbm-open, sdbm-rdonly, sdbm-store, sdbmp, set-default, setq-default,

Introduction
============

As you have probably gathered by now, librep provides a dialect of the Lisp programming language---a dialect originally inspired by Emacs Lisp, but later adapted to include many features from various Scheme implementations and Common Lisp. The language dialect aims to be convenient for both extending applications and writing large stand-alone programs.

All programs written using only the information in this manual should be compatible with future revisions of librep.

This following sections explain some of the most important Lisp concepts and the conventions I've used in this manual.

nil and t
---------

In the rep Lisp dialect there is a single data value representing boolean ``false''---the empty list, written as (). All other values are considered ``not-false'', i.e. ``true''.

By convention the constants nil and t are used to represent the canonical boolean values. The constant variable nil evaluates to the empty list (i.e. ``false''), while t evaluates to itself (i.e. not-``false'', therefore ``true'').

Reiterating, all of the conditional operations regard anything which is not () as being true (i.e. non-false). The actual symbol t should be used where a true boolean value is explicitly stated, to increase the clarity of the code.

So, (), and its alias nil, represent both the empty list and boolean falsehood. Most Lisp programmers write () where its value as a list should be emphasized, and nil where its value as boolean false is intended. Although neither of these values need be quoted (see Quoting), most programmers will quote the empty list to emphasize that it is a constant value. However nil should not be quoted, doing so would produce the symbol nil, not boolean falsehood. For example:

     (append '() '()) => ()          ;Emphasize use of empty lists

     (not nil) => t                  ;Emphasize use as boolean false

     (get 'nil 'color)               ;Use the symbol nil

When a function is said to ``return false'', it means that it returns the false boolean value, i.e. the empty list. When a function is said to ``return true'', this means that any non-false value is returned.

The Lisp Reader
---------------

Lisp programs and functions are stored internally as Lisp data objects, the Lisp Reader is the mechanism that translates from textual descriptions of Lisp objects to the internal data structures representing them.

The Lisp Reader is the collection of internal functions accessed by the read Lisp function. It reads a character at a time from an input stream until a whole Lisp object has been parsed.

See Data Types.

Notation
--------

Wherever an example of evaluating a Lisp form is shown it will be formatted like this,

     (+ 1 2)
         => 3

The glyph => is used to show the computed value of a form.  (In this case the list (+ 1 2) (i.e. the list containing three elements, the symbol + and, the numbers 1 and 2), represents a function application. The first element in the list is the name of the function to be called, all other elements are the arguments to apply to it. Since the + function adds a series of numbers, the above function call is actually performing the computation 1 + 2.)

When two forms are shown as being exactly equivalent to one another the glyph == is used, for example,

     (car some-variable) == (nth 0 some-variable)

Evaluating some forms result in an error being signalled, this is denoted by the error--> glyph.

     (open-file "/tmp/foo" 'read)
         error--> File error: No such file or directory, /tmp/foo

Descriptions
------------

In this document the simplest type of descriptions are those defining variables (see Variables), they look something like:

 -- Variable: grains-of-sand
     This imaginary variable contains the number of grains of sand in a one-mile long stretch of an averagely sandy beach.

Hooks (see Hooks) are also described in this format, the only difference is that Variable: is replaced by Hook:.

Functions (see Functions) and macros (see Macros) have more complex descriptions; as well as the name of the object being described, they also have a list of parameters which the object will accept. Each parameter in the list is named and may be referred to in the body of the description.

Three keyword parameters may also be used: #!optional, #!key and #!rest. They have the same meaning as when used in the lambda-list of a function definition (see Lambda Expressions). That is, #!optional means that all further parameters are optional, and #!rest means that the following parameter actually receives a list of any unused argument values.

An example function definition follows.

 -- Function: useless-function first #!optional second #!rest tail
     This function returns a list consisting of the values SECOND (when undefined the number 42 is used), all the items in the list TAIL and FIRST.

          (useless-function 'foo 'bar 'xyz 20)
              => (bar xyz 20 foo)

          (useless-function '50)
              => (42 50)

Macros and interactive commands are defined in the same way with Macro: or Command: replacing Function:.

Special forms (see Special Forms) are described similarly to functions except that the argument list is formatted differently, since special forms are, by definition, more flexible in how they treat their arguments. Optional values are enclosed in square brackets ([OPTIONAL-ARG]) and three dots (REPEATED-ARG...) indicate where zero or more arguments are allowed.

Data Types
==========

The way that data is represented in Lisp is fundamentally different to languages such as C or Fortran. In Lisp each piece of data (or value) has two basic attributes: the data and the type of the data. This means that type checking is performed at run-time on the actual data itself, not at compile-time on the ``variable'' holding the data.

Also, there are no ``pointers'' in Lisp. As in the Java programming language, all values are references to data structures, with each actual data structure (or Lisp Object) being able to have as many values referring to it concurrently as necessary. Because of this lack of pointers, there can be no memory-leakage in Lisp---when an object has no more extant references, it is automatically deallocated (see Garbage Collection).

Most Lisp objects are a member of one of the primitive types; these are types built into the Lisp system and can represent things like strings, numbers, cons cells, vectors, etc... Other primitive types may be defined at run-time.

More complex objects may be constructed from these primitive types, for example a vector of three elements could be regarded as a type triple if necessary. In general, each separate type provides a predicate function which returns true when applied to an object of its own type.

Finally, one of the most important differences between Lisp and other languages is that there is no distinction between programs and data. But this will be explained later.

Types Summary
-------------

Each separate data type is documented in its own section, this is a just a summary of the more common types.

Numbers
     Numbers: fixnums, bignums, rationals and floats. See Numbers.

Cons cell
     An object referring to two other Lisp objects. See Cons Cells.

List
     A sequence of objects, in Lisp lists are not primitive types, instead they are made by chaining together Cons cells. See Lists.

Vector
     A one-dimensional array of objects. See Vectors.

String
     A vector of characters. See Strings.

Array
     An ordered sequence of objects which can be accessed in constant time, either a vector or a string. See Sequences.

Sequence
     An ordered sequence of objects, either a list or an array. See Sequences.

Symbol
     A symbol is a named object; they are used to provide named variables and functions. See Symbols.

File
     A link to a notional file in the filing system. This file may be in the local filing system, or on a FTP server, or wherever. See Files.

Process
     An object through which processes may be created and controlled. See Processes.

Stream
     Serial data sinks and sources. These may include files, functions, and processes. See Streams.

Void
     The empty type, only used in symbols to represent an undefined value. Note that this is not the same as (), which is the empty list, or false truth value.

Read Syntax
-----------

As previously noted the Lisp reader translates textual descriptions of Lisp objects into the object they describe (source files are simply descriptions of objects). However, not all data types can be created in this way: in fact the only types which can are numbers, strings, symbols, cons cells (or lists) and vectors, all others have to be created by calling functions.

Single line comments are introduced by a semi-colon character (;). Whenever the Lisp reader encounters a semi-colon where it's looking for the read syntax of a new Lisp object it will discard the rest of the line of input. Block comments are also supported, introduced by the string #| and terminated by |#. See Comment Styles.

The read syntax of an object is the string which when given to the reader as input will produce the object. The read syntax of each type of object is documented in that type's main section of this manual but here is a small summary of how to write each type.

Numbers
     A number is number written as an integer---decimal, octal (when the number is preceded by #o) or hexadecimal (when the number is preceded by #x)---or a decimal rational or floating point value. An optional minus sign may be the first character in a number. Some examples are,

          42
              => 42

          #o177
              => 127

          #x-ff
              => -255

          3/2
              => 3/2

          1.23
              => 1.23

Strings
     The read syntax of a string is simply the string with a double-quote character (") at each end, for more details see Strings.

          "This is a string"

Cons cells
     A cons cell is written in what is known as dotted pair notation, an opening left-parenthesis, followed by the read syntax of the first object, followed by a dot, then the second object, and finally a closing right-parenthesis. For example:

          ("car" . "cdr")

Lists
     The syntax of a list is similar to a cons cell, but the dot is removed and zero or more objects may be written:

          (0 1 2 3)

          ("foo" ("bar" "baz") 100)

     The second example is a list of three elements, a string, an inner list and a number.

Vectors
     The read syntax of a vector is similar to that of a list, but with square brackets instead of parentheses,

          [0 1 2 3]

Symbols
     The read syntax of a symbol is its name, for example the read syntax of the symbol called my-symbol is,

          my-symbol

Printed Representation
----------------------

As well as translating textual descriptions to Lisp objects, the process may be reversed, converting a value back to a textual description. The resulting text is known as the printed representation of the object, and will usually be very similar to the read syntax of the object (see Read Syntax).

Objects which do not have a read syntax do have a printed representation, it will normally be of the form,

     #<RELEVANT-TEXT>

where the RELEVANT-TEXT is object-dependent and usually describes the object and its contents. The reader will signal an error if it encounters a description of an object in the format #<...>.

Equality Predicates
-------------------

 -- Function: eq arg1 arg2
     Returns true when ARG1 and ARG2 refer to the same object. Two objects are the same when they occupy the same place in memory and hence modifying one object would alter the other. The following Lisp fragments may illustrate this,

          (eq "foo" "foo")        ;the objects are distinct
              => ()

          (eq t t)                ;the same object -- the symbol t
              => t

     Note that the result of eq is undefined when called on two integer objects with the same value, see eql.

 -- Function: equal arg1 arg2
     The function equal compares the structure of the two objects ARG1 and ARG2. If they are considered to be equivalent then returns true, otherwise returns false.

          (equal "foo" "foo")
              => t

          (equal 42 42)
              => t

          (equal 42 0)
              => ()

          (equal '(x . y) '(x . y))
              => t

 -- Function: eql arg1 arg2
     This function is a cross between eq and equal: if ARG1 and ARG2 are both numbers then the value of these numbers are compared. Otherwise it behaves in exactly the same manner as eq does.

          (eql 3 3)
              => t

          (eql 1 2)
              => ()

          (eql "foo" "foo")
              => ()

          (eql 'x 'x)
              => t

Comparison Predicates
---------------------

These functions compare their two arguments in a scalar fashion, the arguments may be of any type but the results are only meaningful for numbers, strings (ASCII values of each byte compared until a non-matching pair is found then those two values are compared as numbers) and cons cells (cars compared before cdrs).

Unlike the eql function, inexact and exact numbers will be compared by first coercing the exact number to be inexact.

 -- Function: = arg1 arg2 arg3 ... argn
     Returns true if all arguments represent the same value.

 -- Function: /= arg1 arg2 arg3 ... argn
     Returns true if no two arguments represent the same value.

 -- Function: > arg1 arg2 arg3 ... argn
     Returns true when ARG1 is `greater than' ARG2, and ARG2 is greater than ARG3, and so on, upto ARGN.

 -- Function: >= arg1 arg2 arg3 ... argn
     Similar to >, but for the ``greater than or equal to'' relation.

 -- Function: < arg1 arg2 arg3 ... argn
     Similar to >, but for the ``less than'' relation.

 -- Function: <= arg1 arg2 arg3 ... argn
     Similar to >, but for the ``less than or equal to'' relation.

There are two related functions for finding the maximum or minimum of a sequence of values.

 -- Function: max #!rest args
     Return the maximum value from the list of ARGS. When comparing numbers, any inexact arguments cause the result to be inexact.

 -- Function: min #!rest args
     Return the minimum value from the list of ARGS. When comparing numbers, any inexact arguments cause the result to be inexact.

Type Predicates
---------------

Each type has a corresponding predicate which defines the objects which are members of that type. Each predicate function has a single parameter, if that parameter is of the correct type it returns true.

integerp, numberp, null, consp, listp, vectorp, subrp, functionp, sequencep, stringp, symbolp, processp, filep.

The documentation for these functions is with the documentation for the relevant type.

Garbage Collection
------------------

In Lisp, data objects are used very freely; a side effect of this is that it is not possible to (easily) know when an object is stale, that is, no references to it exist and it can therefore be reused.

The garbage collector is used to overcome this problem; whenever enough memory has been allocated to make it worthwhile, evaluation stops and the garbage collector works its way through memory deciding which objects may still be referenced, and which are stale. The stale objects are then recorded as being available for reuse and evaluation continues. (But see Guardians)

 -- Function: garbage-collect
     Runs the garbage collector, usually this function doesn't need to be called manually.

 -- Variable: garbage-threshold
     The number of bytes of data that must have been allocated since the last garbage collection before evaluation pauses and the garbage collector is invoked. Its default value is about 100K.

 -- Variable: idle-garbage-threshold
     When the input loop is idle (due to a lack of input), this is the number of bytes of data that must have been allocated since the garbage collection, for another collection to be triggered.

     This is usually set to a lot less than garbage-threshold since the small delay caused by garbage collection is unnoticeable if the system is already idle.

 -- Variable: after-gc-hook
     A hook (see Normal Hooks) called immediately after each invocation of the garbage collector.

Numbers
=======

Librep distinguishes between numbers that are represented exactly and numbers that may not be. This is similar to the Scheme dialect of Lisp. Quoting from the Scheme standard:

     ... numbers are either exact or inexact. A number is exact if it was written as an exact constant or was derived from exact numbers using only exact operations. A number is inexact if it was written as an inexact constant, if it was derived using inexact ingredients, or if it was derived using inexact operations. Thus inexactness is a contagious property of a number.

Exact numbers include both integers and rational numbers, there is no theoretical limit to the range of the values that may be represented (However, depending on implementation restrictions, very large integers may be coerced to an inexact representation.). Inexact numbers are currently implemented using double precision floating point values.

The read syntax of any number is: [PFX...][SGN]DATA..., where the optional SGN is one of the characters - or +, DATA is the representation of the number, and PFX is zero or more of the following prefix strings:

#b
#B
     Integers are described in binary,

#o
#O
     Integers are in octal,

#d
#D
     Integers are in decimal (the default),

#x
#X
     Integers are in hexadecimal,

#e
#E
     Coerce the number to an exact representation after parsing it,

#i
#I
     Coerce to an inexact representation.

The representation of an integer is simply the digits representing that integer, in the radix chosen by any given prefix (defaults to decimal). Examples of valid integer read syntaxes for the number 42 could be 42, #x2a, #o52, #o+52, ...

The representation of a rational number is two sequences of digits, separated by a / character. For example, 3/2 represents the rational number three divided by two.

Inexact numbers are parsed from one of two representations: decimal point form, which is simply a decimal number containing a decimal point, and exponential form, which is a decimal number followed by the letter e and a decimal exponent multiplying the first part of the number by that power of ten. For example, 10.0, 10. and 1e1 all read as the inexact number ten. Note that the radix prefixes currently have no effect when parsing inexact numbers, decimal is used exclusively.

An integer's printed representation is simply the number printed in decimal with a preceding minus sign if it is negative. Rational numbers are printed as two integers separated by a / character. Inexact numbers are printed in their decimal form.

 -- Function: numberp object
     Returns true if OBJECT is a number.

Arithmetic Functions
--------------------

There are a number of functions which perform arithmetic operations on numbers, they take a varying number of values as their arguments returning a new number as their result. When given only exact arguments, an exact result will be returned.

 -- Function: + number1 #!rest numbers
     This functions adds its arguments then returns their sum.

 -- Function: - number1 #!rest numbers
     If this function is just given one argument (NUMBER1) that number is negated and returned. Otherwise each of NUMBERS is subtracted from a running total starting with the value of NUMBER1.

          (- 20)
              => -20

          (- 20 10 5)
              => 5

 -- Function: * number1 #!rest numbers
     This function multiplies its arguments then returns the result.

 -- Function: / number1 #!rest numbers
     This function performs division, a running-total (initialised from NUMBER1 is successively divided by each of NUMBERS then the result is returned.

          (/ 100 2)
              => 50

          (/ 200 2 5)
              => 20

          (/ 3 2)
              => 3/2

          (/ 3.0 2)
              => 1.5

 -- Function: 1+ number
     This function returns the result of adding one to NUMBER.

          (1+ 42)
              => 43

 -- Function: 1- number
     Returns NUMBER minus one.

Integer Functions
-----------------

The functions described in this section all operate on, and return, integer values.

 -- Function: quotient dividend divisor
     Return the integer part of dividing DIVIDEND by DIVISOR.

 -- Function: remainder dividend divisor
     Returns the integer remainder from dividing the DIVIDEND by DIVISOR. The remainder is either zero or has the same sign as DIVIDEND.

 -- Function: modulo dividend divisor
 -- Function: mod dividend divisor
     Return the value of DIVIDEND modulo DIVISOR. Unlike the remainder function the modulo function always has the sign of the DIVISOR, not of the DIVIDEND

 -- Function: gcd args...
     Returns the greatest common divisor of the integers ARGS... If no arguments are given, returns zero.

 -- Function: lcm args...
     Return the lowest common multiple of the integers ARGS... If no arguments are given, returns one.

Rational Functions
------------------

These functions operate on rational numbers.

 -- Function: numerator x
     Returns the exact numerator of X.

 -- Function: denominator x
     Returns the exact denominator of X.

 -- Function: exact->inexact x
     Returns an inexact version of rational number X.

Real Number Functions
---------------------

 -- Function: abs x
     Returns the magnitude of X.

 -- Function: floor x
     Round X downwards to the nearest integer less than or equal to X.

 -- Function: ceiling x
     Round X upwards to the nearest integer less than or equal to X.

 -- Function: truncate x
     Round X to the nearest integer between X and zero.

 -- Function: round x
     Round X to the nearest integer. Halfway cases are rounded to the nearest even integer.

 -- Function: inexact->exact x
     Returns an exact representation of X. This may involve a loss of accuracy.

Mathematical Functions
----------------------

 -- Function: exp x
     Return `e' (the base of natural logarithms) raised to the power X.

 -- Function: log x
     Return the natural logarithm of X. An arithmetic error is signalled if X is less than zero.

 -- Function: sin x
     Return the sine of angle X; x is in terms of radians.

 -- Function: cos x
     Return the cosine of angle X.

 -- Function: tan x
     Return the tangent of angle X.

 -- Function: asin x
     Return the arc sine of X (the value whose sine is X), in radians.

 -- Function: acos x
     Return the arc cosine of X.

 -- Function: atan x
     Return the arc tangent of X.

 -- Function: sqrt x
     Return the non-negative square root of X. Currently, if X is negative, an arithmetic error is signalled.

 -- Function: expt x y
     Returns X raised to the power Y.

     If X is negative and Y is a non-integer, then an arithmetic error is signalled (mathematically should return a complex number).

Bitwise Functions
-----------------

These functions operate on the bit string which an integer represents, assuming a two's complement representation.

 -- Function: lsh number count
     This function shifts the integer NUMBER COUNT bits to the left, if COUNT is negative NUMBER is shifted to the right instead.

          (lsh 1 8)
              => 256

          (lsh 256 -8)
              => 1

 -- Function: logand number1 #!rest numbers
     This function uses a bit-wise logical `and' operation to combine all its arguments (there must be at least one argument).

          (logand 15 8)
              => 8

          (logand 15 7 20)
              => 4

 -- Function: logior number1 #!rest numbers
     Uses a bit-wise logical `inclusive-or' to combine all its arguments (there must always be at least one argument).

          (logior 1 2 4)
              => 7

 -- Function: logxor number1 #!rest numbers
     Uses a bitwise logical `exclusive-or' to combine all its arguments (there must be at least one).

          (logxor 7 3)
              => 4

 -- Function: lognot number
     This function inverts all the bits in NUMBER.

          (lognot 0)
              => -1

          (lognot 2)
              => -3

          (lognot -1)
              => 0

Numeric Predicates
------------------

For the documentation of the functions =, /=, >, <, >=, <=, max and min, see Comparison Predicates.

 -- Function: exactp object
     Returns true when OBJECT is an exact number.

 -- Function: inexactp object
     Returns true when OBJECT is an inexact number.

 -- Function: integerp object
     Returns true when OBJECT is an integer.

 -- Function: rationalp object
     Returns true when OBJECT is a rational number (including integers).

 -- Function: realp object
     Returns true when OBJECT is a real number.

 -- Function: oddp x
     Return true if X is an odd number.

 -- Function: evenp x
     Return true if X is an even number.

 -- Function: positivep x
     Return true if X is a number greater than zero.

 -- Function: negativep x
     Return true if X is a number less than zero.

 -- Function: zerop x
     Returns true if X is equal to zero.

Pseudo-Random Numbers
---------------------

The random function allows pseudo-random numbers to be generated.

 -- Function: random #!optional limit
     Return a pseudo-random number between zero and LIMIT-1 inclusive. If LIMIT is undefined, it is taken as being the largest positive integer representable in a fixnum.

     Calling random with LIMIT equal to the symbol t seeds the generator with the current time of day.

Characters
----------

In librep characters are stored in integers. Their read syntax is a question mark followed by the character itself, which may be an escape sequence introduced by a backslash. For details of the available escape sequences see Strings.

     ?a
         => 97

     ?\n
         => 10

     ?\177
         => 127

 -- Function: alpha-char-p character
     This function returns true when CHARACTER is one of the alphabetic characters.

          (alpha-char-p ?a)
              => t

 -- Function: upper-case-p character
     When CHARACTER is one of the upper-case characters this function returns true.

 -- Function: lower-case-p character
     Returns true when CHARACTER is lower-case.

 -- Function: digit-char-p character
     This function returns true when CHARACTER is one of the decimal digit characters.

 -- Function: alphanumericp character
     This function returns true when CHARACTER is either an alphabetic character or a decimal digit character.

 -- Function: space-char-p character
     Returns true when CHARACTER is a white-space character (space, tab, newline or form feed).

 -- Function: char-upcase character
     This function returns the upper-case equivalent of CHARACTER. If CHARACTER is already upper-case or has no upper-case equivalent it is returned unchanged.

          (char-upcase ?a)
              => 65                       ;`A'

          (char-upcase ?A)
              => 65                       ;`A'

          (char-upcase ?!)
              => 33                       ;`!'

 -- Function: char-downcase character
     Returns the lower-case equivalent of the character CHARACTER.

Sequences
=========

Sequences are ordered groups of objects, there are several primitive types which can be considered sequences, each with their pros and cons.

A sequence is either an array or a list, where an array is either a vector or a string.

 -- Function: sequencep object
     This function returns true if OBJECT is a sequence.

Cons Cells
----------

A cons cell is an ordered pair of two objects, the car and the cdr.

The read syntax of a cons cell is an opening parenthesis followed by the read syntax of the car, a dot, the read syntax of the cdr and a closing parenthesis. For example a cons cell with a car of 10 and a cdr of the string foo would be written as,

     (10 . "foo")

 -- Function: cons car cdr
     This function creates a new cons cell. It will have a car of CAR and a cdr of CDR.

          (cons 10 "foo")
              => (10 . "foo")

 -- Function: consp object
     This function returns true if OBJECT is a cons cell.

          (consp '(1 . 2))
              => t

          (consp '())
              => ()

          (consp (cons 1 2))
              => t

The strange syntax '(1 . 2) is known as quoting (see Quoting), it tells the evaluator that the object following the quote-mark is a constant, and therefore should not be evaluated. This will be explained in more detail later.

In Lisp an atom is any object which is not a cons cell (and is, therefore, atomic).

 -- Function: atom object
     Returns true if OBJECT is an atom (not a cons cell).

Given a cons cell there are a number of operations which can be performed on it.

 -- Function: car cons-cell
     This function returns the object which is the car (first element) of the cons cell CONS-CELL.

          (car (cons 1 2))
              => 1

          (car '(1 . 2))
              => 1

 -- Function: cdr cons-cell
     This function returns the cdr (second element) of the cons cell CONS-CELL.

          (cdr (cons 1 2))
              => 2

          (cdr '(1 . 2))
              => 2

 -- Function: rplaca cons-cell new-car
     This function sets the value of the car (first element) in the cons cell CONS-CELL to NEW-CAR. The value returned is CONS-CELL.

          (setq x (cons 1 2))
              => (1 . 2)
          (rplaca x 3)
              => (3 . 2)
          x
              => (3 . 2)

 -- Function: rplacd cons-cell new-cdr
     This function is similar to rplacd except that the cdr slot (second element) of CONS-CELL is modified.

Lists
-----

A list is a sequence of zero or more objects, the main difference between lists and vectors is that lists are more dynamic: they can change size, be split, reversed, concatenated, etc... very easily.

In Lisp lists are not a primitive type; instead singly-linked lists are formed by chaining cons cells together (see Cons Cells). The empty list is represented by the special value ().

 -- Function: listp arg
     This functions returns true when its argument, ARG, is a list (i.e. either a cons cell or ()).

 -- Function: null arg
     Returns a true value if ARG is the empty list.

List Structure
..............

Each element in a list is given its own cons cell and stored in the car of that cell. The list is then constructed by having the cdr of a cell point to the cons cell containing the next element (and hence the entire rest of the list). The cdr of the cell containing the last element in the list is (). A list of zero elements is represented by ().

The read syntax of a list is an opening parenthesis, followed by the read syntax of zero or more space-separated objects, followed by a closing parenthesis. Alternatively, lists can be constructed `manually' using dotted-pair notation.

All of the following examples result in the same list of five elements: the numbers from zero to four.

     (0 1 2 3 4)

     (0 . (1 . (2 . (3 . (4 . ())))))

     (0 1 2 . (3 4))

An easy way to visualise lists and how they are constructed is to see each cons cell in the list as a separate box with pointers to its car and cdr,

     +-----+-----+
     |  o  |  o----> cdr
     +--|--+-----+
        |
         --> car

Complex box-diagrams can now be drawn to represent lists. For example the following diagram represents the list (1 2 3 4).

     +-----+-----+   +-----+-----+   +-----+-----+   +-----+-----+
     |  o  |  o----> |  o  |  o----> |  o  |  o----> |  o  |  o----> ()
     +--|--+-----+   +--|--+-----+   +--|--+-----+   +--|--+-----+
        |               |               |               |
         --> 1           --> 2           --> 3           --> 4

A more complex example, the list ((1 2) (foo bar)) can be drawn as,

     +-----+-----+                          +-----+-----+
     |  o  |  o---------------------------> |  o  |  o----> ()
     +--|--+-----+                          +--|--+-----+
        |                                      |
     +-----+-----+   +-----+-----+          +-----+-----+   +-----+-----+
     |  o  |  o----> |  o  |  o----> ()     |  o  |  o----> |  o  |  o----> ()
     +--|--+-----+   +--|--+-----+          +--|--+-----+   +--|--+-----+
        |               |                      |               |
         --> 1           --> 2                  --> foo         --> bar

Building Lists
..............

It has already been described how you can create lists using the Lisp reader; this method does have a drawback though: the list created is effectively static. If you modify the contents of the list and that list was created when a function was defined the list will remain modified for all future invocations of that function. This is not usually a good idea, consider the following function definition,

     (defun bogus-function (x)
       "Return a list whose first element is nil and whose second element is X."
       (let
           ((result '(nil nil)))     ;Static list which is filled in each time
         (rplaca (cdr result) x)     ; the function is called
         result))

This function does in fact do what its documentation claims, but a problem arises when it is called more than once,

     (setq x (bogus-function 'foo))
         => (nil foo)
     (setq y (bogus-function 'bar))
         => (nil bar)               ;The first result has been destroyed
     x
         => (nil bar)               ;See!

This example is totally contrived---no one would ever write a function like the one in the example but it does demonstrate the need for a dynamic method of creating lists.

 -- Function: list #!rest elements
     This function creates a list out of its arguments, if zero arguments are given the empty list, (), is returned.

          (list 1 2 3)
              => (1 2 3)

          (list (major-version-number) (minor-version-number))
              => (3 2)

          (list)
              => ()

 -- Function: list* arg1 arg2 ... argn-1 argn
     Creates a new list (ARG1 ARG2 ... ARGN-1 . ARGN).

          (list* 1 2 '(3 4))
              => (1 2 3 4)

 -- Function: make-list length #!optional initial-value
     This function creates a list LENGTH elements long. If the INITIAL-VALUE argument is given it defines the value of all elements in the list, if it is not defined they are all ().

          (make-list 2)
              => (() ())

          (make-list 3 t)
              => (t t t)

          (make-list 0)
              => ()

 -- Function: append #!rest lists
     This function creates a new list with the elements of each of its arguments (which must be lists). Unlike the function nconc this function preserves the structure of all its arguments.

          (append '(1 2 3) '(4 5))
              => (1 2 3 4 5)

          (append)
              => ()

     What actually happens is that all arguments but the last are copied, then the last argument is linked on to the end of the list (uncopied).

          (setq foo '(1 2))
              => (1 2)
          (setq bar '(3 4))
              => (3 4)
          (setq baz (append foo bar))
              => (1 2 3 4)
          (eq (nthcdr 2 baz) bar)
              => t

     The following diagram shows the final state of the three variables more clearly,

          foo--> +-----+-----+   +-----+-----+
                 |  o  |  o----> |  o  |     |
                 +--|--+-----+   +--|--+-----+
                    |               |
                    o--> 1          o--> 2   bar
                    |               |          ->
          baz--> +--|--+-----+   +--|--+-----+   +-----+-----+   +-----+-----+
                 |  o  |  o----> |  o  |  o----> |  o  |  o----> |  o  |     |
                 +-----+-----+   +-----+-----+   +--|--+-----+   +--|--+-----+
                                                    |               |
                                                     --> 3           --> 4

     Note how foo and the first half of baz use the same objects for their elements---copying a list only copies its cons cells, its elements are reused. Also note how the variable bar actually references the mid-point of baz since the last list in an append call is not copied.

 -- Function: remove elt list
     Return a copy of LIST, with all elements the same as ELT discarded (using the equal function to compare).

 -- Function: remq elt list
     Similar to the remove function, except that comparisons are made using eq.

 -- Function: reverse list
     This function returns a new list; it is made from the elements of the list LIST in reverse order. Note that this function does not alter its argument.

          (reverse '(1 2 3 4))
              => (4 3 2 1)

As a postscript to this section, the function used as an example at the beginning could now be written as,

     (defun not-so-bogus-function (x)
       (list nil x))

Also note that the cons function can be used to create lists by hand and to add new elements onto the front of a list. For example:

     (setq x (list 1 2 3))
         => (1 2 3)
     (setq x (cons 0 x))
         => (0 1 2 3)

Accessing List Elements
.......................

The most flexible method of accessing an element in a list is via a combination of the car and cdr functions. There are other functions which provide an easier way to get at the elements in a flat list. These will usually be faster than a string of car and cdr operations.

 -- Function: nth count list
     This function returns the element COUNT elements down the list, therefore to access the first element use a COUNT of zero (or even better the car function). If there are too few elements in the list and no element number COUNT can be found () is returned.

          (nth 3 '(0 1 2 3 4 5))
              => 3

          (nth 0 '(foo bar)
              => foo

 -- Function: nthcdr count list
     This function takes the cdr of the list LIST COUNT times, returning the last cdr taken.

          (nthcdr 3 '(0 1 2 3 4 5))
              => (3 4 5)

          (nthcdr 0 '(foo bar))
              => (foo bar)

 -- Function: last list
     This function returns the last element in the list LIST. If the list has zero elements () is returned.

          (last '(1 2 3))
              => 3

          (last '())
              => ()

 -- Function: member object list
     This function scans through the list LIST until it finds an element which is equal to OBJECT. The tail of the list (the cons cell whose car is the matched object) is then returned. If no elements match OBJECT then the empty list () is returned.

          (member 'c '(a b c d e))
              => (c d e)

          (member 20 '(1 2))
              => ()

 -- Function: memq object list
     This function is similar to member except that comparisons are performed by the eq function not equal.

Modifying Lists
...............

The nthcdr function can be used in conjunction with the rplaca function to modify an arbitrary element in a list. For example,

     (rplaca (nthcdr 2 '(0 1 2 3 4 5)) 'foo)
         => foo

sets the third element of the list (0 1 2 3 4 5) to the symbol called foo.

There are also functions which modify the structure of a whole list. These are called destructive operations because they modify the actual structure of a list---no copy is made. This can lead to unpleasant side effects if care is not taken.

 -- Function: nconc #!rest lists
     This function is the destructive equivalent of the function append, it modifies its arguments so that it can return a list which is the concatenation of the elements in its arguments lists.

     Like all the destructive functions this means that the lists given as arguments are modified (specifically, the cdr of their last cons cell is made to point to the next list). This can be seen with the following example (similar to the example in the append documentation).

          (setq foo '(1 2))
              => (1 2)
          (setq bar '(3 4))
              => (3 4)
          (setq baz (nconc foo bar))
              => (1 2 3 4)
          foo
              => (1 2 3 4)                ;`foo' has been altered!
          (eq (nthcdr 2 baz) bar)
              => t

     The following diagram shows the final state of the three variables more clearly,

          foo-->                           bar-->
          baz--> +-----+-----+   +-----+-----+   +-----+-----+   +-----+-----+
                 |  o  |  o----> |  o  |  o----> |  o  |  o----> |  o  |     |
                 +--|--+-----+   +--|--+-----+   +--|--+-----+   +--|--+-----+
                    |               |               |               |
                     --> 1           --> 2             --> 3           --> 4

 -- Function: nreverse list
     This function rearranges the cons cells constituting the list LIST so that the elements are in the reverse order to what they were.

          (setq foo '(1 2 3))
              => (1 2 3)
          (nreverse foo)
              => (3 2 1)
          foo
              => (1)                      ;`foo' wasn't updated when the list
                                          ; was altered.

 -- Function: delete object list
     This function destructively removes all elements of the list LIST which are equal to OBJECT then returns the modified list.

          (delete 1 '(0 1 0 1 0))
              => (0 0 0)

     When this function is used to remove an element from a list which is stored in a variable that variable must be set to the return value of the delete function. Otherwise, if the first element of the list has to be deleted (because it is equal to OBJECT) the value of the variable will not change.

          (setq foo '(1 2 3))
              => (1 2 3)
          (delete 1 foo)
              => (2 3)
          foo
              => (1 2 3)
          (setq foo (delete 1 foo))
              => (2 3)

 -- Function: delq object list
     This function is similar to the delete function, the only difference is that the eq function is used to compare OBJECT with each of the elements in LIST, instead of the equal function which is used by delete.

 -- Function: sort list #!optional predicate
     Destructively sorts (i.e. by modifying cdrs) the list of values LIST, to satisfy the function PREDICATE, returning the sorted list. If PREDICATE is undefined, the < function is used, sorting the list into ascending order.

     PREDICATE is called with two values, it should return true if the first is considered less than the second.

          (sort '(5 3 7 4))
              => (3 4 5 7)

     The sort is stable, in that elements in the list which are equal will preserve their original positions in relation to each other.

Association Lists
.................

An association list (or alist) is a list mapping keys to to. Each element of the alist is a cons cell, the car of which is the key, the cdr the value that it associates to. For example an alist could look like,

     ((fred . 20)
      (bill . 30))

this alist has two keys, fred and bill which both associate to an integer (20 and 30 respectively).

It is possible to make the associated values lists, this looks like,

     ((fred 20 male)
      (bill 30 male)
      (sue  25 female))

in this alist the symbol fred is associated with the list (20 male).

There are a number of functions which let you interrogate an alist with a given key for its association.

 -- Function: assoc key alist
     This function scans the association list ALIST for the first element whose car is equal to KEY, this element is then returned. If no match of KEY is found false is returned.

          (assoc 'two '((one . 1) (two . 2) (three . 3)))
              => (two . 2)

 -- Function: assq key alist
     Similar to the function assoc except that the function eq is used to compare elements instead of equal.

     It is not usually wise to use assq when the keys of the alist may not be symbols---eq won't think two objects are equivalent unless they are the same object!

          (assq "foo" '(("bar" . 1) ("foo" . 2)))
              => ()
          (assoc "foo" '(("bar" . 1) ("foo" . 2)))
              => ("foo" . 2)

 -- Function: rassoc association alist
     This function searches through ALIST until it finds an element whose cdr is equal to ASSOCIATION, that element is then returned. false will be returned if no elements match.

          (rassoc 2 '((one . 1) (two . 2) (three . 3)))
              => (two . 2)

 -- Function: rassq association alist
     This function is equivalent to rassoc except that it uses eq to make comparisons.

Infinite Lists
..............

Sometimes it is useful to be able to create `infinite' lists---that is, lists which appear to have no last element---this can easily be done in Lisp by linking the cdr of the last cons cell in the list structure back to the beginning of the list.

      -----------------------------------
     |                                   |
      --> +-----+-----+   +-----+-----+  |
          |  o  |  o----> |  o  |  o-----
          +--|--+-----+   +--|--+-----+
             |               |
              --> 1           --> 2

The diagram above represents the infinite list (1 2 1 2 1 2 ...).

Infinite lists have a major drawback though, many of the standard list manipulation functions can not be used on them. These functions work by moving through the list until they reach the end. If the list has no end the function may never terminate and the only option is to send the interpreter an interrupt signal.

The only functions which may be used on circular lists are: the cons cell primitives (cons, car, cdr, rplaca, rplacd), nth and nthcdr.

Also note that infinite lists can't be printed. But note the print-length and print-level variables, see Output Functions.

Vectors
-------

A vector is a fixed-size sequence of Lisp objects, each element may be accessed in constant time---unlike lists where the time taken to access an element is proportional to the position of the element.

The read syntax of a vector is an opening square bracket, followed by zero or more space-separated objects, followed by a closing square bracket. For example,

     [zero one two three]

In general it is best to use vectors when the number of elements to be stored is known and lists when the sequence may grow or shrink.

 -- Function: vectorp object
     This function returns true if its argument, OBJECT, is a vector.

 -- Function: vector #!rest elements
     This function creates a new vector containing the arguments given to the function.

          (vector 1 2 3)
              => [1 2 3]

          (vector)
              => []

 -- Function: make-vector size #!optional initial-value
     Returns a new vector, SIZE elements big. If INITIAL-VALUE is defined each element of the new vector is set to INITIAL-VALUE, otherwise they are all ().

          (make-vector 4)
              => [() () () ()]

          (make-vector 2 t)
              => [t t]

Strings
-------

A string is a vector of characters (see Characters), they are generally used for storing and manipulating pieces of text. librep puts no restrictions on the values which may be stored in a string---specifically, the null character (^@) may be stored with no problems.

The read syntax of a string is a double quote character, followed by the contents of the string, the object is terminated by a second double quote character. For example, "abc" is the read syntax of the string abc.

Any backslash characters in the string's read syntax introduce an escape sequence; one or more of the following characters are treated specially to produce the next actual character in the string.

The following escape sequences are supported (all are shown without their leading backslash \ character).

n
     A newline character.

r
     A carriage return character.

f
     A form feed character.

t
     A TAB character.

a
     A `bell' character (this is Ctrl-g).

\
     A backslash character.

^C
     The `control' code of the character C. This is calculated by toggling the seventh bit of the upper-case version of C.

     For example,

          \^C             ;A Ctrl-c character (ASCII value 3)
          \^@            ;The NUL character (ASCII value 0)

012
     The character whose ASCII value is the octal value 012. After the backslash character the Lisp reader reads up to three octal digits and combines them into one character.

x12
     The character whose ASCII value is the hexadecimal value 12, i.e. an x character followed by one or two hex digits.

 -- Function: stringp object
     This function returns true if its argument is a string.

 -- Function: make-string length #!optional initial-character
     Creates a new string containing LENGTH characters, each character is initialised to INITIAL-CHARACTER (or to spaces if INITIAL-CHARACTER is not defined).

          (make-string 3)
              => "   "

          (make-string 2 ?$)
              => "$$"

 -- Function: concat #!rest args
     This function concatenates all of its arguments, ARGS, into a single string which is returned. If no arguments are given then the null string results.

     Each of the ARGS may be a string, a character or a list or vector of characters. Characters are stored in strings modulo 256.

          (concat "foo" "bar")
              => "foobar"

          (concat "a" ?b)
              => "ab"

          (concat "foo" [?b ?a ?r])
              => "foobar"

          (concat)
              => ""

 -- Function: substring string start #!optional end
     This function creates a new string which is a partial copy of the string STRING. The first character copied is START characters from the beginning of the string. If the END argument is defined it is the index of the character to stop copying at, if it is not defined all characters until the end of the string are copied.

          (substring "xxyfoozwx" 3 6)
              => "foo"

          (substring "xyzfoobar" 3)
              => "foobar"

 -- Function: string= string1 string2
     This function compares the two strings STRING1 and STRING2---if they are made from the same characters in the same order then true is returned.

          (string= "one" "one")
              => t

          (string= "one" "two")
              => ()

     Note that an alternate way to compare strings (or anything!) is to use the equal function.

 -- Function: string-equal string1 string2
     Returns true if STRING1 and STRING2 are the same, ignoring differences in character case.

 -- Function: string< string1 string2
     This function returns true if STRING1 is `less' than string2. This is determined by comparing the two strings a character at a time, the first pair of characters which do not match each other are then compared with a normal `less-than' function.

     In librep the standard < function understands strings so string< is just a macro calling that function.

          (string< "abc" "abd")
              => t

          (string< "abc" "abb")
              => ()

 -- Function: string-lessp string1 string2
     Similar to string< but ignores character case in comparisons.

See String Functions for a few more string manipulating functions, and Regular Expressions for a method of pattern matching in strings.

Array Functions
---------------

 -- Function: arrayp object
     This function returns true if OBJECT is an array.

 -- Function: aref array position
     Returns the element of the array (vector or string) ARRAY POSITION elements from the first element (i.e. the first element is numbered zero). If no element exists at POSITION in ARRAY, false is returned.

          (aref [0 1 2 3] 2)
              => 2

          (aref "abcdef" 3)
              => 100                      ;`d'

 -- Function: aset array position value
     This function sets the element of the array ARRAY with an index of POSITION (counting from zero) to VALUE. An error is signalled if element POSITION does not exist. The result of the function is VALUE.

          (setq x [0 1 2 3])
              => [0 1 2 3]
          (aset x 2 'foo)
              => foo
          x
              => [0 1 foo 3]

Sequence Functions
------------------

 -- Function: sequencep arg
     Returns true if ARG is a sequence, i.e. a list or an array.

 -- Function: length sequence
     This function returns the length (an integer) of the sequence SEQUENCE.

          (length "abc")
              => 3

          (length '(1 2 3 4))
              => 4

          (length [x y])
              => 2

 -- Function: copy-sequence sequence
     Returns a new copy of the sequence SEQUENCE. Where possible (in lists and vectors) only the `structure' of the sequence is newly allocated: the same objects are used for the elements in both sequences.

          (copy-sequence "xy")
              => "xy"

          (setq x '("one" "two"))
              => ("one" "two")
          (setq y (copy-sequence x))
              => ("one" "two")
          (eq x y)
              => ()
          (eq (car x) (car y))
              => t

 -- Function: elt sequence position
     This function returns the element of SEQUENCE POSITION elements from the beginning of the sequence.

     This function is a combination of the nth and aref functions.

          (elt [0 1 2 3] 1)
              => 1

          (elt '(foo bar) 0)
              => foo

Symbols
=======

Symbols are objects with a name (almost always a unique name). They are one of the most important data types in Lisp since they are used to provided named variables (see Variables) and functions (see Functions).

 -- Function: symbolp arg
     This function returns true when its argument is a symbol.

Symbol Syntax
-------------

The read syntax of a symbol is usually its name; however, if the name contains any meta-characters (whitespace or any from ()[]'";|\) they will have to be entered specially. There are two ways to tell the reader that a meta-character is actually part of the symbol's name:

 1. Precede the meta-character by a backslash character (\), for example:

          xy\(z\)                 ;the symbol whose name is xy(z)

 2. Enclose part of the name in vertical bars (two | characters). All characters after the starting vertical line are copied as-is until the closing vertical line is encountered. For example:

          xy|(z)|                 ;the symbol xy(z)

Here are some example read syntaxes.

     setq                    ; setq
     |setq|                  ; setq
     \s\e\t\q                ; setq
     1                       ; the number 1
     \1                      ; the symbol 1
     |!$%zf78&|              ; !$%zf78&
     foo|(bar)|              ; foo(bar)
     foo\(bar\)              ; foo(bar)

Symbol Attributes
-----------------

All symbols have two basic attributes: print name and property list. Most important is the print name of the symbol. This is a string naming the symbol, after it has been defined (when the symbol is first created) it may not be changed.

 -- Function: symbol-name symbol
     This function returns the print name of the symbol SYMBOL.

          (symbol-name 'unwind-protect)
              => "unwind-protect"

The symbol's property list (or plist) is similar to an alist (see Association Lists), though stored differently, and provides a method of storing arbitrary extra values in each symbol. See Property Lists.

Although not strictly an attribute of the symbol, symbols also provide a means of associating values with names (i.e. variables). Within a defined context, a symbol may have a binding, this binding associates the symbol with a memory location within which a value may be stored. When writing Lisp programs, the value of a symbol's current binding is accessed by writing the print name of the symbol. Similarly the binding may be modified by using the setq special form. See Variables.

Obarrays
--------

An obarray is the structure used to ensure that no two symbols have the same name and to provide quick access to a symbol given its name. An obarray is a vector, each element of the vector is a chain of symbols whose names share the same hash-code (a bucket). These symbols are chained together through links which are invisible to Lisp programs: if you examine an obarray you will see that each bucket looks as though it has at most one symbol stored in it.

The normal way to reference a symbol is simply to type its name in the program, when the Lisp reader encounters a name of a symbol it looks in the default obarray for a symbol of that name. If the named symbol doesn't exist it is created and hashed into the obarray---this process is known as interning the symbol, for more details see Interning.

 -- Variable: obarray
     This variable contains the obarray that the read function uses when interning symbols.

 -- Function: make-obarray size
     This function creates a new obarray with SIZE hash buckets (this should probably be a prime number for the fewest hash collisions).

     This is the only way of creating an obarray. make-vector is not suitable.

 -- Function: find-symbol symbol-name #!optional obarray
     This function scans the specified obarray (OBARRAY or the value of the variable obarray if OBARRAY is undefined) for a symbol whose name is the string SYMBOL-NAME. The value returned is the symbol if it can be found or false otherwise.

          (find-symbol "setq")
              => setq

 -- Function: apropos regexp #!optional predicate obarray
     Returns a list of symbols from the obarray OBARRAY (or the default) whose print name matches the regular expression REGEXP (see Regular Expressions). If PREDICATE is true, each symbol which matches REGEXP is applied to the function PREDICATE, if the value is true it is considered a match.

     The PREDICATE argument is useful for restricting matches to a certain type of symbol, for example only commands.

          (apropos "^yank" 'commandp)
              => (yank-rectangle yank yank-to-mouse)

Creating Symbols
----------------

It is possible to allocate symbols dynamically, this is normally only necessary when the symbol is to be interned in a non-default obarray or the symbol is a temporary object which should not be interned (for example: labels in a compiler).

 -- Function: make-symbol print-name
     This function creates and returns a new, uninterned, symbol whose print name is the string PRINT-NAME. Its value cell is void (undefined) and it will have an empty property list.

          (make-symbol "foo")
              => foo

 -- Function: gensym
     This function returns a new, uninterned, symbol that has a unique print name.

          (gensym)
              => G0001

          (gensym)
              => G0002

Interning
---------

Interning a symbol means to store it in an obarray so that it can be found in the future: all variables and named-functions are found through interned symbols.

When a symbol is interned a hash function is applied to its print name to determine which bucket in the obarray it should be stored in. Then it is simply pushed onto the front of that bucket's chain of symbols.

Normally all interning is done automatically by the Lisp reader. When it encounters the name of a symbol which it can't find in the default obarray (the value of the variable obarray) it creates a new symbol of that name and interns it. This means that no two symbols can have the same print name, and that the read syntax of a particular symbol always produces the same object (unless the value of obarray is altered).

     (eq 'some-symbol 'some-symbol)
         => t

 -- Function: intern symbol-name #!optional obarray
     This function uses find-symbol to search the OBARRAY (or the standard obarray) for a symbol called SYMBOL-NAME. If a symbol of that name is found it is returned, otherwise a new symbol of that name is created, interned into the obarray, and returned.

          (intern "setq")
              => setq

          (intern "my-symbol" my-obarray)
              => my-symbol

 -- Function: intern-symbol symbol #!optional obarray
     Interns the symbol SYMBOL into the obarray OBARRAY (or the standard one) then returns the symbol. If SYMBOL is currently interned in an obarray an error is signalled.

          (intern-symbol (make-symbol "foo"))
              => foo

          (intern-symbol 'foo)
              error--> Error: Symbol is already interned, foo

 -- Function: unintern symbol #!optional obarray
     This function removes the symbol SYMBOL from the obarray OBARRAY then returns the symbol.

     Beware! this function should be used with extreme caution---once you unintern a symbol there may be no way to recover it.

          (unintern 'setq)                ;This is extremely stupid
              => setq

Property Lists
--------------

Each symbol has a property list (or plist), this is a structure which associates an arbitrary Lisp object with a key (usually a symbol). The keys in a plist may not have any duplications (so that each property is only defined once).

The concept of a property list is very similar to an association list (see Association Lists) but there are two main differences:

 1. Structure; each element of an alist represents one key/association pair. In a plist each pair of elements represents an association: the first is the key, the second the property. For example, where an alist may be,

          ((one . 1) (two . 2) (three . 3))

     a property list would be,

          (one 1 two 2 three 3)

 2. Plists have their own set of functions to modify the list. This is done destructively, altering the property list (since the plist is stored in only one location, the symbol, this is quite safe).

 -- Function: get symbol property
     This function searches the property list of the symbol SYMBOL for a property equal to PROPERTY. If such a property is found it is returned, otherwise false is returned.

          (get 'if 'lisp-indent)
              => 2

          (get 'set 'lisp-indent)
              => ()

 -- Function: put symbol property new-value
     put sets the value of the property PROPERTY to NEW-VALUE in the property list of the symbol SYMBOL. If there is an existing value for this property (using equal to compare keys) it is overwritten. The value returned is NEW-VALUE.

          (put 'foo 'prop 200)
              => 200

 -- Function: symbol-plist symbol
     Returns the property list of the symbol SYMBOL.

          (symbol-plist 'if)
              => (lisp-indent 2)

 -- Function: setplist symbol plist
     This function sets the property list of the symbol SYMBOL to PLIST.

          (setplist 'foo '(zombie yes))
              => (zombie yes)

Keyword Symbols
---------------

Keywords are a special class of symbols. They evaluate to themselves, and have the read syntax #:SYMBOL, where SYMBOL is anything satisfying the usual symbol syntax. These objects are normally used to mark keyword parameters in function applications (see Lambda Expressions).

 -- Function: make-keyword symbol
     Return the keyword symbol that could be used to mark an argument value for the keyword parameter SYMBOL.

          (make-keyword 'x)
              => #:x

 -- Function: keywordp arg
     Returns true if ARG is a keyword symbol.

Evaluation
==========

So far only the primitive data types have been discussed, and how the Lisp reader converts textual descriptions of these types into Lisp objects. Obviously there has to be a way of actually computing something---it would be difficult to write a useful program otherwise.

What sets Lisp apart from other languages is that in Lisp there is no difference between programs and data: a Lisp program is just a sequence of Lisp objects which will be evaluated as a program when required.

The subsystem which does this evaluation is called the Lisp evaluator and each expression to be evaluated is called a form. The evaluator (the function eval) examines the structure of the form that is applied to it and computes the value of that form within the current Lisp environment.

A form can be any type of data object; the only types which the evaluator treats specially are symbols (which describe variables) and lists (subroutine applications), anything else is returned as-is (and is called a self-evaluating form).

 -- Function: eval form
     This function computes and returns the value of FORM within the current module and dynamic environment, and a null lexical environment.

However, eval is rarely explicitly invoked, except in the read-eval-print loop. Lisp provides many other methods of evaluation that are usually much more suitable within a program.

 -- Variable: max-lisp-depth
     This variable limits the number of nested calls to eval. If more than this many nested calls to eval exist, an error is signalled. The intention is to detect infinite recursion before hitting the stack size limit (causing a segmentation fault).

Symbol Forms
------------

When the evaluator is applied to a symbol the computed value of the form is the value associated with the symbol in the current environment. Basically this means that to get the value of a variable you simply write its name. For example,

     rep-version
         => "1.0"

this extract from a Lisp session shows the read syntax of a form to get the value of the variable rep-version and the result when this form is evaluated.

Since forms are evaluated within the current environment the value of a variable is its most-recent extant binding (with slight differences for lexical and special variables). See Variables.

If an evaluated symbol has no current binding, an error is signalled.

List Forms
----------

Forms which are lists are used to invoke a subroutine. The first element of the list defines the subroutine to be called; all further elements are arguments to be applied to that subroutine invocation.

There are several different types of subroutines available: functions, macros, special forms and autoloads. When the evaluator finds a form which is a list it tries to classify the form into one of these four types.

First of all it evaluates the first element of the list; the computed value of this element decides how the rest of the elements in the list are treated. For example, if the first element is a symbol whose value is a function, then that function is called with the other values in the list.

Function Call Forms
...................

When the first element of a list form evaluates to a function object (either a primitive subroutine or a closure), all other elements in the list are evaluated sequentially from left-to-right, then these values are applied to the function definition. The result returned by the function is then taken as the value of the whole list form.

For example, consider the form (/ 100 (1+ 4)). This is a function call to the function stored in the variable /. First the / form is evaluated, it is a variable containing a data value representing the primitive subroutine for integer division. Then the 100 form is evaluated: it is a number, so self-evaluates to the value 100. Next the form (1+ 4) is evaluated. This is also a function call and computes to a value of 5 which becomes the second argument to the / function. Now the / function is applied to its evaluated arguments of 100 and 5, and returns the value 20. This then becomes the value of the form (/ 100 (1+ 4)).

     (/ 100 (1+ 4))
     == (/ 100 5)
     => 20

Or another example,

     (+ (- 10 (1- 7)) (* (1+ 2) 4)
     == (+ (- 10 6) (* (1+ 2) 4)
     == (+ 4 (* (1+ 2) 4)
     == (+ 4 (* 3 4))
     == (+ 4 12)
     => 16

The system is also capable of eliminating tail calls where possible, allowing tail-recursive function definitions to run with bounded space requirements.

A tail-call is a function call that occurs immediately before exiting the containing function. Since the containing function need not receive the result of the function call, it is possible to, in effect, exit from the containing function before invoking the called function.

Note however, that this is only possible where none of the dynamic features of the language (i.e. bindings to special variables, unwind-protect, condition-case, catch, etc...) are currently active in the containing function.

Consider, for example, the following function:

     (defun print-list (l)
       (unless (null l)
         (format standard-output "%s\n" (car l))
         (print-list (cdr l))))

the call to print-list occurs in the tail-position of the function. This means that the call may be made after removing the previous call to print-list from the interpreter's stack of active functions.

[ XXX currently the interpreter is incapable of eliminating tail calls to subrs, i.e. Lisp functions implemented in C ]

Macro Call Forms
................

Macros are source code expansions, the general idea is that a macro is a function which using the unevaluated arguments applied to it, computes another form (the expansion of the macro and its arguments) which is then evaluated to provide the value of the form.

Macros are generally used to implement control-flow operations, where not all arguments may be evaluated, or evaluated in an unusual order. For more details see Macros.

Special Forms
.............

Special forms are built-in subroutines which the evaluator knows must be handled specially. The main difference between a special form and a function is that the arguments applied to a special form are not automatically evaluated---if necessary the special form will evaluate arguments itself. This will be noted in the documentation of the special form.

Special forms are generally used to provide control structures, for example, the primitive conditional constructs are special forms (if all of their arguments, including the forms to be conditionally evaluated, were evaluated automatically this would defeat the object of being conditional!).

The special forms supported by librep are: cond, defvar, progn, quote, setq.

 -- Function: special-form-p arg
     Returns true if ARG is a special form.

          (special-form-p quote)
              => t

Autoload Forms
..............

Not all parts of librep are needed at once, autoload forms provide a means of marking that a function (or macro) is contained by a specific Lisp library. The first time that the function is accessed the autoload form will be evaluated; this loads the file containing the function, then re-evaluates the original form. By then the autoload form will have been overwritten in the symbol's function slot by the true function (when it was loaded) so the form will execute properly.

For more details see Autoloading.

Self-Evaluating Forms
---------------------

The computed value of any form which is not a symbol or a list will simply be the form itself and the form is said to be a self-evaluating form.

Usually the only forms to be evaluated in this way will be numbers, strings and vectors (since they are the only other data types which have read syntaxes) but the effect is the same for other types of data.

This means that forms you know are self-evaluating do not have to be quoted to be used as constants (like lists and symbols do).

     "foo"
         => "foo"

Quoting
-------

As the above sections explain some types of Lisp object have special meaning to the Lisp evaluator (namely the symbol and list types) this means that if you want to refer to a symbol or a list in a program you can't because the evaluator will treat the form as either a variable reference or a function call respectively.

To get around this Lisp uses an idea called quoting. The special form quote simply returns its argument without evaluating it. For example,

     (quote my-symbol)
         => my-symbol

the quote form prevents the my-symbol being treated as a variable---it is effectively `hidden' from the evaluator.

Writing quote all the time would be a bit time-consuming so there is a shortcut: the Lisp reader treats any form X preceded by a single quote character (') as the form (quote X). So the example above would normally be written as,

     'my-symbol
         => my-symbol

The general way to prevent evaluation of a form is to simply precede it by a single quote-mark.

 -- Special Form: quote form
     This special form returns its single argument without evaluating it. This is used to quote constant objects to prevent them from being evaluated.

For another form of quoting, see Backquoting.

Variables
=========

In Lisp, symbols are used to represent variables. Each symbol contains a value slot that is used to contain the value of the symbol when it used as a variable.

The normal way to obtain the current value of a variable is simply to evaluate the symbol of the same name (i.e. write the name of the variable in your program). The symbol-value function can be used to evaluate variables whose names not known statically.

 -- Function: symbol-value variable
     This function returns the value of the symbol VARIABLE in the current environment.

Local Variables
---------------

A local variable is a variable which has a temporary value. For example, when a function is called the variables which are the names of its arguments are temporarily bound to the values of the arguments passed to the function. When the function call exits its arguments are unbound and the previous definitions of the variables come back into view.

A binding is a particular instance of a local variable. Even if a variable has more than one binding currently in place, only the most recent is available---there is no way the previous binding can be accessed until the previous binding is removed.

One way of visualising variable binding is to think of each variable as a stack. When the variable is bound to, a new value is pushed onto the stack, when it is unbound the top of the stack is popped. Similarly when the stack is empty the value of the variable is void (see Void Variables). Assigning a value to the variable (see Setting Variables) overwrites the top value on the stack with a new value. When the value of the variable is required it is simply read from the top of the stack.

Apart from function applications there are two special forms which perform variable binding (i.e. creating local variables), let and let*.

 -- Macro: let bindings body-forms...
     let creates new variable bindings as specified by the BINDINGS argument, then evaluates the BODY-FORMS in order. The bindings are then removed, returning all variables to their state before the let statement was entered. The value of the statement is the value of the implicit progn.

     The BINDINGS argument is a list of the bindings to perform. Each binding is either a symbol, in which case that variable is bound to (), or a list whose car is a symbol. The cdr of this list is a list of forms which, when evaluated as a progn, gives the value to bind to that variable.

          (setq foo 42)
              => 42
          (let
              ((foo (+ 1 2))
               bar)
            ;; Body forms
            (setq foo (1+ foo))   ;This sets the new binding
            (cons foo bar))
              => (4 . ())
          foo
              => 42        ;The original values is back

     No bindings are made until all new values have been computed. For example:

          (setq foo 42)
              => 42
          (let
              ((foo 100)
               (bar foo))
            (cons foo bar))
              => (100 . 42)

     Although foo is given a new binding this is not actually done until all the new values have been computed, hence bar is bound to the old value of foo.

 -- Macro: let* bindings body-forms...
     This special form is exactly the same as let except for one important difference: the new bindings are installed as they are computed.

     You can see the difference by comparing the following example with the last example in the let documentation (above),

          (setq foo 42)
              => 42
          (let*                   ;Using let* this time
              ((foo 100)
               (bar foo))
            (cons foo bar))
              => (100 . 100)

     By the time the new value of bar is computed the new binding of foo is already active.

 -- Macro: letrec bindings body-forms...
     letrec is similar to let and let*, with the differerence being that the values of bindings are evaluated with all other bindings in scope. This means that recursive functions may be defined with letrec. For example, a local factorial function (from SICP):

          (letrec ((fact
                    (lambda (n)
                      (if (= n 1)
                          1
                        (* n (fact (1- n)))))))
            (fact 10))

     Note also that letrec allows groups of mutually recursive functions to be defined, as in the following example (also from SICP):

          (defun f (x)
            (letrec ((evenp
                      (lambda (n)
                        (if (= n 0)
                            t
                          (oddp (1- n)))))
                     (oddp
                      (lambda (n)
                        (if (= n 0)
                            nil
                          (evenp (1- n))))))
              ...

Setting Variables
-----------------

Setting a variable means to overwrite its current value (that is, the value of its most recent active binding) with a new one. In the variable-as-stack analogy, this is analogous to overwriting the top of the stack. The old value is irretrievably lost (unlike when a new value is bound to a variable, see Local Variables).

The setq special form is the usual method of altering the value of a variable.

 -- Special Form: setq variable form ...
     Each VARIABLE is set to the result of evaluating its corresponding FORM. The last value assigned becomes the value of the setq form.

          (setq x 20 y (+ 2 3))
              => 5

     In the above example the variable x is set to 20 and y is set to the value of the form (+ 2 3) (5).

 -- Function: set variable new-value
     The value of the variable VARIABLE (a symbol) is set to NEW-VALUE and the NEW-VALUE is returned.

     This function is used when the VARIABLE is unknown until run-time, and therefore has to be computed from a form.

          (set 'foo 20)
          ==
          (setq foo 20)           ;setq means `set-quoted'
              => 20

     Note: currently the set function may be used to set any type of variable (i.e. lexical or special). However this likely to change in the future, such that only special variables will be allowed to be modified using the set function. It is strongly advised to avoid using this function on lexical bindings! (Moreover the compiler may generate incorrect code in certain circumstances.)

Scope and Extent
----------------

In the librep dialect of Lisp by default variables have lexical scope. This means that bindings are associated with textual regions of programs, and may be accessed by any forms within this associated textual region. Moreover, the bindings are persistent, even when the flow of control is currently outside the associated region.

Consider the following example:

     (let
         ((counter 0))
       (defun count ()
         (setq counter (1+ counter))
         counter))

the value of the counter variable persists, and is incremented each time the count function is called. The counter variable is accessible from nowhere but the forms written inside the let statement declaring it.

     (count)
       => 1
     (count)
       => 2

An alternative method of scoping variables is also available. Any variables declared using the defvar special form are said to be special variables, they have indefinite scope and dynamic extent, often simplified to dynamic scope. What this means is that references to these variables may occur anywhere in a program (i.e. bindings established in one function are visible within functions called from the original function) and that references may occur at any point in time between the binding being created and it being unbound.

Dynamic scoping is easy to abuse, making programs hard to understand and debug. A quick example of the use of dynamic scope,

     (defvar *foo-var* nil)

     (defun foo (x)
       (let
           ;; a dynamically-scoped binding
           ((*foo-var* (* x 20)))
         (bar x)
         ...

     (defun bar (y)
       ;; Since this function is called from
       ;; the function foo it can refer
       ;; to *foo-var*
       (setq y (+ y *foo-var*))
       ...

As shown in the previous example, a common convention is to mark special variables by enclosing their names within asterisks.

Void Variables
--------------

A variable which has no value is said to be void, attempting to reference the value of such a symbol will result in an error. It is possible for the most recent binding of a variable to be void even though the inactive bindings may have values.

 -- Function: boundp variable
     Returns true if the symbol VARIABLE has a value.

 -- Function: makunbound variable
     This function makes the current binding of the symbol VARIABLE be void, then returns VARIABLE.

          (setq foo 42)
              => 42
          foo
              => 42
          (boundp 'foo)
              => t
          (makunbound 'foo)
              => foo
          (boundp 'foo)
              => ()
          foo
              error--> Value as variable is void: foo

Defining Variables
------------------

The special forms define, defvar and defconst allow you to define the global variables that will be used by a program.

 -- Macro: define variable form
     Defines a lexically scoped global variable called VARIABLE. It will have the result of evaluating FORM assigned to it.

     Note that the define special form may also be used to declare block-structured functions, Definitions.

 -- Special Form: defvar variable [form [doc-string]]
     This special form defines a special (i.e. dynamically scoped) variable, the symbol VARIABLE. If the value of VARIABLE is void the FORM is evaluated and its value is stored as the value of VARIABLE (note that only the default value is modified, never a buffer-local value). If no FORM is given the assigned value defaults to false.

     If the DOC-STRING argument is defined it is a string documenting VARIABLE. This string is then stored as the symbol's variable-documentation property and can be accessed by the describe-variable function.

          (defvar *my-variable* '(x y)
            "This variable is an example showing the usage of the defvar
          special form.")
              => *my-variable*

 -- Macro: defconst constant form [doc-string]
     defconst defines a global constant, the symbol CONSTANT. Its value is set to the result of evaluating FORM. Note that unlike defvar the value of the symbol is always set, even if it already has a value.

     The DOC-STRING argument, if defined, is the documentation string for the constant.

          (defconst the-answer 42
            "An example constant.")
              => the-answer

Fluid Variables
---------------

Special variables have a number of drawbacks, especially when used in conjunction with the module system (see Modules and Special Variables). As a consequence of these drawbacks, rep provides a second method of implementing dynamically scoped variables, known as fluid variables, or just fluids.

A fluid is a first class Lisp object that may be passed around like any other Lisp object. Its sole function is to provide a location from which dynamic bindings may be created. Fluids are anonymous objects, they are usually named by being stored in lexically scoped variables.

 -- Function: make-fluid #!optional value
     Create and return a new fluid, it will have an initial binding of VALUE (or false if VALUE is undefined).

 -- Function: fluid fluid
     Return the value of the most recently created binding of the fluid variable object FLUID.

 -- Function: fluid-set fluid value
     Set the value of the most recently created binding of the fluid variable object FLUID to VALUE.

 -- Function: with-fluids fluids values thunk
     Call the zero parameter function THUNK (and return the value that it returns) with new bindings created for each of the fluid variables specified in the list FLUIDS.

     For each member of FLUIDS the corresponding member of the VALUES list provides the initial value of the new binding.

     If the lists FLUIDS and VALUES are not of the same length, an error is signalled.

 -- Macro: let-fluids bindings body ...
     A convenient wrapper around with-fluids, similar to the let syntax.

     The list BINDINGS associates the names of lexical variables containing fluid objects, with the values to bind to those fluid objects. Once the bindings have been installed, the BODY ... forms are evaluated, and the bindings removed. The value of the last of the BODY ... forms is returned.

Here is an example code fragment using fluid variables and let-fluids:

     (define a (make-fluid))
     (define b (make-fluid))

     (let-fluids ((a 1)
                  (b 2))
       (+ (fluid a) (fluid b))) => 3

Functions
=========

A function is a Lisp object which, when applied to a sequence of argument values, produces another value---the function's result. It may also induce side-effects (e.g. changing the environment of the calling function). All Lisp functions return results --- there is nothing like a procedure in Pascal.

Note that special forms (see Special Forms) and macros (see Macros) are not functions since they do not guarantee to evaluate all of their arguments.

Functions are the main building-block in Lisp programs, each program is usually a system of interrelated functions.

There are two types of function: primitive functions are functions written in the C language, these are sometimes called built-in functions, the object containing the C code itself is called a subr. All other functions are defined in Lisp.

 -- Function: functionp object
     Returns true if OBJECT is a function (i.e. it can be used as the function argument of funcall.

          (functionp set)
              => t

          (functionp setq)
              => ()

          (functionp (lambda (x) (+ x 2)))
             => t

 -- Function: subrp arg
     Returns true is ARG is a primitive subroutine object.

 -- Function: subr-name subr
     Returns a string naming the primitive subroutine SUBR.

Lambda Expressions
------------------

Lambda expressions are used to create functions from other Lisp objects. A lambda expression is a list whose first element is the symbol lambda. All functions written in Lisp (as opposed to the primitive functions in C) are defined using lambda expressions.

The general format of a lambda expression is:

     (lambda LAMBDA-LIST [DOC] [INTERACTIVE-DECLARATION] BODY-FORMS... )

Where LAMBDA-LIST is a list defining the formal parameters of the function, DOC is an optional documentation string, INTERACTIVE-DECLARATION is only required by interactive commands (Only used when librep is embedded within another application.) and BODY-FORMS is the sequence of forms making up the function body, evaluated using an implicit progn.

The LAMBDA-LIST is a list, it defines how the values applied to the function are bound to local variables which represent the parameters of the function. At its simplest it is simply a list of symbols, each symbol will have the corresponding argument value bound to it. For example, the lambda list (x y) defines two parameters, x and y. When called with two arguments the first will be bound to the variable x, the second to y. When used in a full lambda expression this looks like:

     (lambda (x y) (+ x y))

this evaluates to an anonymous function with two parameters, x and y, which when called evaluates to their sum.

Note that a lambda expression itself is not a function, it must be associated with a lexical environment, this conjunction is usually called a closure; it is the closure that may be called as a function.

However, to confuse matters, a lambda expression evaluates to the closure of itself and the current environment. Consider the following example:

     (lambda (x) (1+ x))
         => #<closure>

     (functionp (lambda (x) (1+ x)))
         => t

     (functionp '(lambda (x) (1+ x)))
         => ()

There are several lambda-list keywords which modify the meaning of symbols in the lambda-list. The syntax of the lambda list is:

     ([REQUIRED-PARAMETERS...]
      [#!optional OPTIONAL-PARAMETERS...]
      [#!key KEYWORD-PARAMETERS...]
      [#!rest REST-PARAMETER | . REST-PARAMETER])

Each lambda list keyword is a symbol whose name begins #!, they are interpreted as follows:

#!optional
     All variables following this keyword are considered optional (all variables before the first keyword are required: an error will be signalled if a required argument is undefined in a function call).

     OPTIONAL-PARAMETERS may either be of the form SYMBOL or of the form (SYMBOL DEFAULT). If no argument is supplied for this parameter the DEFAULT form is evaluated to give the bound value (The DEFAULT form is evaluated in the environment of the closure being called, but without any of the bindings created by the lambda expression.). If no DEFAULT form is given, then the variable is bound to a false value.

     Note that optional parameters must be specified if a later parameter is also specified.

          ((lambda (#!optional a b) (list a b)))
              => (() ())
          ((lambda (#!optional a b) (list a b)) 1)
              => (1 ())
          ((lambda (#!optional a b) (list a b)) nil 1)
              => (() 1)
          ((lambda (#!optional (a 1)) (list a)))
              => (1)
          ((lambda (#!optional (a 1)) (list a)) 2)
              => (2)

#!key
     This object marks that the parameters up to the next lambda list keyword are keyword parameters. The values bound to these parameters when the function is called are determined not by position (as with normal parameters), but by being marked by a preceding keyword symbol. Keyword symbols have the syntax #:SYMBOL.

     As with optional parameters, default values may be supplied through the use of the (SYMBOL DEFAULT) syntax. If no default value is given and no keyword argument of the specified kind is available, the variable is bound to a false value.

     For example, the lambda list (a #!key b c) accepts one required argument, and two optional keyword arguments. The variable a would be bound to the first supplied argument; the variable b would be bound to the argument preceded by the keyword #:b, or () if no such argument exists. (After extracting required and optional arguments, each remaining pair of values is checked for associating a value with each keyword.)

          ((lambda (a #!key b c) (list a b c)) 1 2 3)
              => (1 () ())
          ((lambda (a #!key b c) (list a b c)) 1 #:b 2 3)
              => (1 2 ())
          ((lambda (a #!key b c) (list a b c)) 1 #:b 2 #:c 3)
              => (1 2 3)
          ((lambda (a #!key b c) (list a b c)) 1 #:c 3 #:b 2)
              => (1 2 3)

#!rest
     The #!rest keyword allows a variable number of arguments to be applied to a function, all the argument values which have not been bound to argument variables (or used to mark keyword arguments) are made into a list and bound to the variable following the #!rest keyword. For example, in

          (lambda (x #!rest y) ...)

     the first argument, x, is required. Any other arguments applied to this function are made into a list and this list is bound to the variable y.

     Variable argument functions may also be defined through the Scheme method of using an improper lambda-list. The previous example is exactly equivalent to:

          (lambda (x . y) ...)

When a function represented by a lambda-list is called the first action is to bind the argument values to the formal parameters. The LAMBDA-LIST and the list of argument values applied to the function are worked through in parallel. Any required arguments which are left undefined when the end of the argument values has been reached causes an error.

After the arguments have been processed the BODY-FORMS are evaluated by an implicit progn, the value of which becomes the value of the function call. Finally, all parameters are unbound and control passes back to the caller.

Defining Functions
------------------

Globally accessible functions are usually defined by the defun special form.

 -- Macro: defun name lambda-list body-forms...
     defun initialises the function definition of the symbol NAME to the lambda expression resulting from the concatenation of the symbol lambda, LAMBDA-LIST and the BODY-FORMS.

     The BODY-FORMS may contain a documentation string for the function as its first form and an interactive calling specification as its first (if there is no doc-string) or second form if the function may be called interactively by the user (see Lambda Expressions).

An example function definition taken from the librep source code is:

     (defun load-all (file)
       "Try to load files called FILE (or FILE.jl, etc) from all
     directories in the Lisp load path."
       (mapc (lambda (dir)
               (let
                   ((full-name (expand-file-name file dir)))
                 (when (or (file-exists-p full-name)
                           (file-exists-p (concat full-name ".jl"))
                           (file-exists-p (concat full-name ".jlc")))
                   (load full-name nil t))))
             load-path))

Anonymous Functions
-------------------

When supplying functions as arguments to other functions it is often useful to give an actual function definition (i.e. an enclosed lambda expression) instead of the name of a function.

In Lisp, unlike most other programming languages, functions have no inherent name. As seen in the last section named-functions are created by storing a function object in a variable, if you want, a function can have many different names: simply store the function in many different variables!

So, when you want to pass a function as an argument there is the option of just writing down its definition. This is especially useful with functions like mapc and delete-if. For example, the following form removes all elements from the LIST which are even and greater than 20.

     (setq LIST (delete-if (lambda (x)
                             (and (zerop (% x 2)) (> x 20)))
                           LIST))

The above lambda expression combines two predicates applied to its argument.

In certain cases it may be necessary to create a non-constant function, for example by using backquoting (see Backquoting). In these cases the make-closure function may be used to create a function object from a lambda expression.

 -- Function: make-closure arg
     Return the closure of ARG and the current lexical environment.

 -- Function: closurep arg
     Returns true if ARG is a closure.

 -- Function: closure-function closure
     Returns the function object associated with the lexical closure CLOSURE.

Predicate Functions
-------------------

In Lisp, a function which returns a boolean `true' or boolean `false' value is called a predicate. As is the convention in Lisp a value of () means false, anything else means true. The symbols nil and t are often used to represent constant false and true values (see nil and t).

Another Lisp convention is that the names of predicate functions should name the quality that the predicate is testing followed by either a p or -p string. The p variant is used when the first string does not contain any hyphens.

For example, the predicate to test for the quality const-variable (a variable which has a constant value, see Defining Variables) is called const-variable-p. On the other hand the predicate to test for the quality cons (a Cons cell) is called consp.

Local Functions
---------------

The defun special form allows globally-accessible functions to be defined. It is often desirable to declare functions local to the current lexical environment. The let and let* special form that were introduced earlier allow this since named functions are simply functional values stored in variables.

For example,

     (let
         ((temporary-function (lambda (x)
                                (+ x 42))))
       ...
       (temporary-function 20)
       ...

Calling Functions
-----------------

Most of the time function applications are made by the evaluator when it finds a functional value after evaluating the first element of a list form. However two functions are available for manually calling functions.

 -- Function: funcall function #!rest args
     Applies the argument values ARGS to the function FUNCTION, then returns its result.

 -- Function: apply function #!rest args
     Similar to funcall except that the last of its arguments is a list of arguments which are appended to the other members of ARGS to form the list of argument values to apply to the function FUNCTION.

          (apply + 1 '(2 3))
              => 6

          (apply + (make-list 1000000 1))
              => 1000000

Mapping Functions
-----------------

A mapping function applies a function to each of a collection of objects. librep currently has two mapping functions, mapcar and mapc.

 -- Function: mapcar function list
     Each element of LIST is individually applied to the function FUNCTION. The values returned are made into a new list which is returned.

     The FUNCTION must accept a single argument value.

          (mapcar 1+ '(1 2 3 4 5))
              => (2 3 4 5 6)

 -- Function: mapc function list
     Similar to mapcar except that the values returned when each element is applied to the function FUNCTION are discarded. The value returned is undefined.

     This function is generally used where the side effects of calling the function are the important thing, not the results. It is often the most efficient way of traversing all items in a list, for example:

          (mapc (lambda (x)
                  (print x standard-error)) list)

The two following functions are also mapping functions of a sort. They are variants of the delete function (see Modifying Lists) and use predicate functions to classify the elements of the list which are to be deleted.

 -- Function: delete-if predicate list
     This function is a variant of the delete function. Instead of comparing each element of LIST with a specified object, each element of LIST is applied to the predicate function PREDICATE. If it returns true then the element is destructively removed from LIST.

          (delete-if stringp '(1 "foo" 2 "bar" 3 "baz"))
              => (1 2 3)

 -- Function: delete-if-not predicate list
     This function does the inverse of delete-if. It applies PREDICATE to each element of LIST, if it returns false then the element is destructively removed from the list.

          (delete-if-not stringp '(1 "foo" 2 "bar" 3 "baz"))
              => ("foo" "bar" "baz")

The filter function is similar to delete-if-not, except that the original list isn't modified, a new list is created.

 -- Function: filter predicate list
     Return a new list, consisting of the elements in LIST which the function PREDICATE returns true when applied to. This function is equivalent to:

          (mapcar nconc (mapcar (lambda (x)
                                  (and (PREDICATE x) (list x)))
                                LIST))

Macros
======

Macros are used to extend the Lisp language. They consist of a function which instead of returning a computed value, transform their unevaluated arguments into a new form that, when evaluated, produces the actual value of the original form.

For example, the when macro (see Conditional Structures) implements a new conditional operation by transforming its arguments into a cond statement. That is,

     (when CONDITION FORM ...)
         ==> (cond (CONDITION FORM ...))

Since macros do not evaluate their arguments, instead just transforming them, they may be expanded at compile-time. The resulting form is then compiled as usual.

 -- Function: macrop arg
     Returns true if ARG is a macro object.

Defining Macros
---------------

Macros are defined in the same style as functions, the only difference is the name of the special form used to define them.

A macro object is a list whose car is the symbol macro, its cdr is the function which creates the expansion of the macro when applied to the macro calls unevaluated arguments.

 -- Macro: defmacro name lambda-list body-forms...
     Defines the macro stored in the function cell of the symbol NAME. LAMBDA-LIST is the lambda-list specifying the arguments to the macro (see Lambda Expressions) and BODY-FORMS are the forms evaluated when the macro is expanded. The first of BODY-FORMS may be a documentation string describing the macro's use.

Here is a simple macro definition, it is the definition of the when macro shown in the previous section.

     (defmacro when (condition #!rest body)
       "Evaluates CONDITION, if it's true evaluates the BODY
     forms."
       (list 'cond (list* condition body)))

When a form of the type (when C B ...) is evaluated the macro definition of when expands to the form (cond (C (progn B ...))) which is then evaluated to perform the when-construct.

When you define a macro ensure that the forms which produce the expansion have no side effects; otherwise undefined effects will occur when programs using the macro are compiled.

Backquoting
-----------

As seen in the previous sections, macros are a very powerful mechanism of defining new control structures. However due to the need to create the expansion, i.e. the form that will be actually evaluated, they can often be complex to write and understand.

We have already seen that constants may be produced through the use of the quote-mark (see Quoting), here another form of quoting is described, where only some of the quoted object is actually constant. This is known as backquoting, since it is introduced by the backquote character `, a shortcut for the backquote macro.

 -- Macro: backquote arg
     Constructs a new version of ARG (a list). All parts of LIST are preserved except for expressions introduced by comma (,) characters, which are evaluated and spliced into the list. For example:

          `(1 2 ,(+ 1 2))
              => (1 2 3)

     Also, the ,@ prefix will splice the following list into the output list, at the same level:

          `(1 2 ,@(list 3))
              => (1 2 3)

Backquoting allows macros expansions to be created from static templates. For example the when macro shown in the previous sections can be rewritten as:

     (defmacro when (condition #!rest body)
       `(cond (,condition ,@body)))

which is easier to read, since it is a lot closer to the actual expansion.

Macro Expansion
---------------

When a macro call is detected (see List Forms) the function which is the cdr of the macro's definition (see Defining Macros) is applied to the macro call's arguments. Unlike in a function call, the arguments are not evaluated, the actual forms are the arguments to the macro's expansion function. This is to allow these forms to be rearranged by the macro's expansion function, creating the form that will finally be evaluated.

There is a function which performs macro expansion, its main use is to let the Lisp compiler expand macro calls at compile time.

 -- Function: macroexpand form #!optional environment
     If FORM is a macro call macroexpand will expand that call by calling the macro's expansion function (the cdr of the macro definition). If this expansion is another macro call the process is repeated until an expansion is obtained which is not a macro call, this form is then returned.

     The optional ENVIRONMENT argument is a function to call to do the actual expansion.

          (defmacro when (condition #!rest body)
            "Evaluates CONDITION, if it's true evaluates the BODY
          forms."
            (list 'if condition (cons 'progn body)))
              => when

          (macroexpand '(when x (setq foo bar)))
              => (cond (x (progn (setq foo bar))))

     While a macro is being expanded, the special variable macro-environment is bound to value of the ENVIRONMENT parameter in the containing call to macroexpand. This allows macros to expand inner macros correctly.

 -- Function: macroexpand-1 form #!optional environment
     Similar to macroexpand, but only a single macro expansion is ever performed, i.e. if FORM is a macro call the result of expanding that call will be returned, otherwise FORM is returned.

          (macroexpand-1 '(when x (setq foo bar)))
              => (if x (progn (setq foo bar)))

Compiling Macros
----------------

Although it may seem odd that macros return a form to produce a result and not simply the result itself, this is actually their most important feature. It allows the expansion and the evaluation of the expansion to occur at different times.

The Lisp compiler makes use of this; when it comes across a macro call in a form it is compiling it uses the macroexpand function to produce the expansion of that form. This expansion is then compiled straight into the object code. Obviously this is good for performance (why evaluate the expansion every time it is needed when once will do?).

Some rules do need to be observed to make this work properly:

  * The macro expansion function (i.e. the definition of the macro) should not have any side effects or evaluate its arguments (the value of a symbol at compile-time probably won't be the same as its value at run-time).

  * Macros which are defined by another file must be loaded so they can be recognised. Use the require function, the compiler will evaluate any require forms it sees loading any macro definitions used.

Note however, that the librep compiler does allow macros to be used before they are defined (two passes are made through the source file).

Block-Structured Definitions
============================

Previous sections of this document have described several special forms and macros for defining top-level functions and variables. librep also provides a higher-level method of creating these definitions, the define statement. define originates in the Scheme dialect of Lisp, it allows block-structured programs to be defined intuitively.

The most basic use of define is very similar to defun, e.g. the two following forms have exactly the same effect:

     (defun foo (x) (1+ x))

     (define (foo x) (1+ x))

But note the different position of the parentheses. This is because define may also be used to define (lexical) variables. Hence the following is also equivalent:

     (define foo (lambda (x) (1+ x)))

However this is the most uninteresting aspect of define. More interesting is that it allows internal definitions.

Within a define form, any inner calls to define (that occur in a contiguous block at the start of the body of a let, let*, letrec, lambda, or define form) are also used to create definitions, but definitions that are local to the containing scope. For example:

     (define (foo x)
       (define (bar)
         (* x 42))
       (1+ (bar)))

This defines a top-level function called foo. However it also contains an inner function named bar, that is only accessible within foo. Since bar is defined inside foo, and librep uses lexical scope by default, the variable x defined by foo may also be accessed by bar.

 -- Macro: define name form
 -- Macro: define (name . args) body-forms...
     Define a global lexical variable called NAME, whose value will be set to FORM.

     If the first argument to the macro is a list, then a function is defined whose name is NAME and whose formal parameters are specified by ARGS. The body of the function is defined by the BODY-FORMS. The body forms have any macros expanded, and are scanned for internal definitions (at the start of the body of let, let*, lambda special forms)

 -- Macro: define-macro name form
 -- Macro: define-macro (name . args) body-forms...
     Similar to define, except that it creates a macro definition (see Macros).

 -- Macro: with-internal-definitions body-forms
     Recursively expand macros in BODY-FORMS, while scanning out any internal definitions into letrec statements.

Modules
=======

When creating large programs from many separate components, it is important to be able to encapsulate these components, such that the interfaces they present to other components are well defined, and the implementations of these interfaces may be modified without affecting any other components. To this end rep provides a module system for managing the scope of global definitions. This module system was inspired by the Scheme48, Xerox Scheme and Standard ML module systems.

Modules are known as structures and may be anonymous or named. Each structure specifies and implements an interface, essentially a list of names listing the definitions within that module that may be accessed by other modules. Each structure is a separate global namespace, with a number of variable bindings. Each closure contains a reference to the structure it was instantiated in, for accessing the bindings of any free variables referenced by that closure.

As well as specifying its name and interface, each module also lists the other modules whose bindings it may reference. Structures may either open or access other structures; when opening a structure all its exported bindings are immediately referenceable from the importing module. Exported bindings from accessed structures are referenced using the `structure-ref' form.

Module Interfaces
-----------------

Each module implements an interface---the set of bindings (i.e. functions, macros or variables) that it exports to other modules. Interfaces may either be defined and then referenced by name, written literally, or combined from a number of sources.

The syntax of interface definitions is as follows:

     INTERFACE -> (export ID ...)
               |  NAME
               |  (compound-interface INTERFACE ...)
               |  (structure-interface MODULE-NAME)

where each ID is the name of a binding to export, and each NAME is the name of an interface previously defined using define-interface.

 -- Macro: define-interface name interface
     Associate the symbol NAME with the module interface INTERFACE (using one of the forms listed above.

Here is an example defining an interface called foo:

     (define-interface foo (compound-interface bar (export baz quux)))

It includes the interface called bar and adds two extra exported symbols: baz and quux.

Module Definition
-----------------

Two special forms are used to define modules, one for anonymous modules, one for named modules. When storing modules in files, each file often contains a single instance of one of these forms.

 -- Macro: structure interface config body...
 -- Macro: define-structure name interface config body...
     These special forms each create a new module with interface INTERFACE (using the syntax described in the previous section), and configuration CONFIG.

     After configuring the module as specified, the sequence of forms BODY... is evaluated; it should include the definitions required by the interface that the module has promised to implement.

     The CONFIG form is either a list of configuration clauses, or a single configuration clause. Each such clause must be of the following syntax:

          CLAUSE -> (open NAME ...)
                 |  (access NAME ...)

     Each NAME specifies the name of a module, in the case of open clauses, the named module(s) will be loaded such that their exported bindings may be referenced from within the current module with no qualification (i.e. as if they had been defined within the module itself).

     Alternatively, if an access clause was used, the named module(s) will be loaded, but their exported bindings will only be accessible from within the current module using the structure-ref form. E.g. if a module foo has been accessed and it exports a binding named bar, then the following form could be used to access its value:

          (structure-ref foo bar)

     Since this form is used so often, the reader allows the abbreviation foo#bar to be used instead, it is expanded to the form above when read. Note that no whitespace is allowed between the three tokens.

Note that to access the standard features of the rep language described in this manual, modules need to import the rep module. Alternatively, they may import the scheme module to load a minimal R4RS Scheme environment.

Here is an example module definition, defining a module named test that exports two functions foo and bar.

     (define-structure test (export foo bar)
         (open rep)
       (define (foo x) (* x 42))
       (define (bar x y) (+ (foo x) (1+ y))))

It is also possible to export multiple views of a single underlying set of bindings, by using the define-structures form to create a number of modules.

 -- Macro: define-structures ((name interface) ...) config body...
     Create a module for each (NAME INTERFACE) pair. The module is called NAME and exports the interface defined by INTERFACE.

     The CONFIG and BODY... forms are as in define-structure.

     Here is a trivial example:

          (define-structures ((foo (export foo both))
                              (bar (export bar both)))
              (open rep)
            (define both 1)
            (define foo 2)
            (define bar 3))

     the underlying environment has three bindings. Each created module exports two of these.

Module Loading
--------------

As described above, the common way of loading modules is to use the open and access clauses of the configuration language.

If the modules named by these clauses are not currently loaded into the interpreter, then the system will attempt to load them from the filing system, using the standard load-path variable to define the directories to search.

To allow modules names to be hierarchical, any dot characters in a module's name are replaced by the operating system's directory separator string (i.e. on unix, all . characters are simply replaced by / characters).

When searching for files to load, the standard filename suffixes are used to differentiate Lisp files from other types of files (see Load Function). This file should contain a define-structure form (as described in the previous section) as the last top-level form in the file.

For backwards compatibility, the require function can also be used to import modules. If a module of the same name as the requested feature has already been loaded, then it is imported into the current module. Otherwise if a file is loaded that contains a module definition as its last top-level form, this module is imported into the current module. See Features.

Modules and Special Variables
-----------------------------

As described earlier, the defvar special form may be used to create variables that are scoped dynamically, known as special variables, see Defining Variables. Due to their dynamic scope, special variables do not fit well with the lexically scoped module system described here.

As a result of this mismatch, special variables are stored in a separate namespace. This means that modules defining special variables must take the necessary steps to avoid the names of these variables clashing with those declared in other modules (The usual convention is to prefix the variable name with a unique string derived from the module name.).

In fact, it is often advisable to avoid using special variables as much as possible, especially when writing modules of Lisp code. An alternative method of creating dynamically scoped variables is to use fluid variable objects. These use first class Lisp objects to represent anonymous dynamically scoped variables. Since they are just Lisp objects, they may be stored in lexically scoped variables---this gives the benefits of both lexical (i.e. encapsulation) and dynamic scoping. See Fluid Variables.

Control Structures
==================

Control structures are special forms or macros that control which forms get evaluated, when they get evaluated and the number of times to evaluate them. This includes conditional structures, loops, etc...

The simplest control structures are the sequencing structures; they are used to evaluate a list of forms in left to right order.

Sequencing Structures
---------------------

Each of the special forms in this section simply evaluates its arguments in left-to-right order. The only difference is the result returned.

The most widely used sequencing special form is progn: it evaluates all its argument forms and returns the computed value of the last one. Many other control structures are said to perform an implicit progn, this means that internally they call progn with a list of forms.

progn in Lisp is nearly analogous to a begin...end block in Pascal; it is used in much the same places---to allow you to evaluate a sequence of form where only one form was allowed (for example the ``true'' clause of an if structure).

 -- Special Form: progn forms...
     All of the FORMS are evaluated sequentially (from left-to-right), the result of the last evaluated FORM is the return value of the special form. If no arguments are given to progn it returns false.

          (progn 'one (+ 1 1) "three")
              => "three"

          (progn)
              => ()

 -- Macro: prog1 first forms...
     This special form evaluates its FIRST form then performs an implicit progn on the rest of its arguments. The result of this structure is the computed value of the FIRST form.

          (prog1 'one (+ 1 1) "three")
              => one

 -- Macro: prog2 first second forms...
     This is similar to prog1 except that the evaluation of its SECOND form is returned.

     The FIRST form is evaluated, then its SECOND, then it performs an implicit progn on the remaining arguments.

          (prog2 'one (+ 1 1) "three")
              => 2

Conditional Structures
----------------------

Lisp provides a number of conditional constructs, the most complex of which (cond) takes a list of conditions, the first of which evaluates to true has its associated list of forms evaluated. Theoretically this is the only conditional special form necessary---all others can be implemented as macros.

 -- Macro: if condition true-form else-forms...
     The if form is the nearest thing in Lisp to the if-then-else construct found in most programming languages.

     First the CONDITION form is evaluated, if it returns true the TRUE-FORM is evaluated and its result returned. Otherwise the result of an implicit progn on the ELSE-FORMS is returned. If there are no ELSE-FORMS false is returned.

     Note that one of the TRUE-FORM or the ELSE-FORMS is completely ignored---it is not evaluated.

          (if (special-form-p if)
              "`if' is a special form"
            "`if' is not a special form")
              => "`if' is not a special form"

 -- Macro: when condition true-forms...
     CONDITION is evaluated, if it is true the result of an implicit progn on the TRUE-FORMS is returned, otherwise false is returned.

          (when t
            (message "Pointless")
            'foo)
              => foo

 -- Macro: unless condition else-forms...
     This special form evaluates CONDITION, if its computed value is true, () is returned. Otherwise the ELSE-FORMS are evaluated sequentially, the value of the last is returned.

 -- Special Form: cond clause...
     The cond special form is used to choose between an arbitrary number of conditions. Each CLAUSE is a list; the car of which is a CONDITION, the cdr is a list of forms to evaluate (in an implicit progn) if the CONDITION evaluates to true. This means that each CLAUSE looks something like:

          (CONDITION BODY-FORMS...)

     and a whole cond form looks like:

          (cond
           (CONDITION-1 BODY-FORMS-1...)
           (CONDITION-2 BODY-FORMS-2...)
           ...)

     The CONDITION in each CLAUSE is evaluated in sequence (CONDITION-1, then CONDITION-2, ...), the first one which evaluates to a true value has an implicit progn performed on its BODY-FORMS. The value of this progn is also the value of the cond statement.

     If the true CONDITION has no BODY-FORMS the value returned is the value of the CONDITION. If none of the clauses has a true CONDITION the value of the cond statement is false.

     Often you want a default clause which has its BODY-FORMS evaluated when none of the other clauses are true. The way to do this is to add a clause with a CONDITION of t and BODY-FORMS of whatever you want the default action to be.

          (cond
           ((stringp buffer-list))        ;Clause with no BODY-FORMS
           ((consp buffer-list)
            (setq x buffer-list)          ;Two BODY-FORMS
            t)
           (t                             ;Default clause
            (error "`buffer-list' is corrupted!")))
              => t

 -- Macro: case key clauses...
     This special form is similar to cond, but switches on the result of evaluating a single form KEY, checking for equality with a number of other values, defined by the CLAUSES. If any of these other values is the same as the result of evaluating KEY, then a sequence of forms associated with the value is evaluated.

     Each element of the CLAUSES list has the format:

          ((VALUE-1 VALUE-2 ... VALUE-N) FORMS...)

     Each of the values in the car of the clause is tested for equality with KEY, using the eql function. If any test positively, then the associated FORMS are evaluated and the resulting value becomes the result of the special form.

     Instead of supplying a list of possible values, it is also possible to just specify the symbol t. If such a clause is encountered, and no other clauses have matched the value of KEY, then this clause is assumed to match by default.

     If any of the values in the CLAUSES appear multiply, then the behaviour of the construct is undefined.

     Here is an example use of case:

          (case foo
            ((bar baz)
              (print "It was either bar or baz"))
            ((quux)
              (print "It was quux"))
            (t
              (print "I've no idea what it was...")))

There are also a number of special forms which combine conditions together by the normal logical rules.

 -- Macro: or forms...
     The first of the FORMS is evaluated, if it is true its value is the value of the or form and no more of forms are evaluated. Otherwise this step is repeated for the next member of FORMS.

     If all of the FORMS have been evaluated and none have a true value the or form evaluates to false.

          (or nil 1 nil (beep))           ;(beep) won't be evaluated
              => 1

 -- Macro: and forms...
     The first of the FORMS is evaluated. If it is false no more of the FORMS are evaluated and false is the value of the and statement. Otherwise the next member of FORMS is evaluated and its value tested. If none of the FORMS are false the computed value of the last member of FORMS is returned from the and form.

          (and 1 2 nil (beep))            ;(beep) won't be evaluated
              => ()

          (and 1 2 3)                     ;All forms are evaluated
              => 3

 -- Function: not object
     This function inverts the truth value of its argument. If OBJECT is true, false is returned, otherwise true is returned.

          (not nil)
              => t

          (not t)
              => ()

          (not (not 42))
              => t

Looping Structures
------------------

The librep Lisp dialect only has one method of creating looping control structures---recursion. Any looping construct found in an imperative language can be represented as a recursive function. For example the common while statement:

     (while CONDITION BODY...)
     ==
     (letrec ((loop (lambda ()
                      (when CONDITION
                        BODY
                        (loop)))))
       (loop))

Each successive iteration of the loop is simply another call to the function. Also note that the recursive call to the (loop) function occurs in the tail-position of the function. When combined with the system's ability to eliminate tail-calls (see Function Call Forms) the above example loop has bounded space requirements. This is important when loops make a large number of iterations.

Although tail-recursion is the only primitive method of looping, the language offers a number of looping forms for convenience.

 -- Macro: do vars (test expr...) body...
     do is an iteration construct; VARS specifies a set of variable bindings to be created, how they are initialized and how they are updated on each iteration. TEST specifies the termination condition of the loop, any EXPR... forms are evaluated immediately prior to exiting the `do' construct. The BODY... forms specify the side effecting body of the loop.

     VARS is a list of variable clauses, each of which has the structure (VARIABLE INIT STEP) where VARIABLE is the name of a variable, INIT defines the initial value of its binding, and STEP defines how the next value of the binding is computed. An alternative form is (VARIABLE INIT), in this case the value of the binding does not change across loop iterations.

     Each iteration begins by evaluating TEST, if the result is false, then the BODY... expressions are evaluated, and the variables bound to new locations initialized to the results of evaluating the associated STEP forms.

     If the result of evaluating TEST is true then the EXPR... forms are evaluated, and the do construct returns the value of the last EXPR form evaluated.

          (do ((vec (make-vector 5))
               (i 0 (1+ i)))
              ((= i 5) vec)
            (aset vec i i))

              => [0 1 2 3 4]

The ``named-let'' variant of the let form also provides a convenient looping construct.

 -- Macro: let variable bindings body...
     This is the same as the (let BINDINGS BODY...) form described in Local Variables, but within the BODY... forms, the symbol VARIABLE is bound to a function whose parameters are the bound variables defined by BINDINGS and whose body is the sequence of forms BODY...

     This means that the body of the let may be repeated by invoking the function VARIABLE with suitable parameters.

          (let loop ((rest '(1 2 3))
                     (total 0))
            (if (null rest)
                total
              (loop (cdr rest) (+ total (car rest)))))

              => 6

Finally, the imperative while form shown at the start of the section is also provided:

 -- Macro: while condition body...
     The CONDITION form is evaluated. If it is true an implicit progn is performed on the BODY forms and the whole procedure is repeated.

     This continues until the CONDITION form evaluates to false. The value of every while structure that terminates is false.

Non-Local Exits
---------------

A non-local exit is a transfer of control from the current point of evaluation to a different point (somewhat similar to the much-maligned goto statement in imperative languages).

Non-local exits can either be used explicitly (catch and throw) or implicitly (errors).

Catch and Throw
...............

The catch and throw structures are used to perform explicit transfers of control. First a catch form is used to setup a tag; this acts like a label for a goto statement. To transfer control a throw form is then used to transfer to the named tag. The tag is destroyed and the catch form exits with the value provided by the throw.

In a program this looks like,

     (catch 'TAG
       ;; Forms which may `throw' back to TAG
       ...
       (throw 'TAG VALUE)
       ;; Control has now passed to the `catch',
       ;; no more forms in this progn will be evaluated.
       ...)
         => VALUE

where TAG is the tag to be used (this is normally a symbol) and VALUE is the result of the catch form.

When a throw actually happens all catches in scope are searched for one with a tag which is eq to the tag in the throw. If more than one exists the innermost is selected. Now that the catch has been located the environment is `wound-back' to the catch's position (i.e. local variables are unbound, cleanup forms executed, unused catches removed, etc...) and all Lisp constructs between the current point of control and the catch are immediately exited.

For example,

     (let
         ((test 'outer))
       (cons (catch 'foo
               (let
                   ((test 'inner))
                 (throw 'foo test)
                 (setq test 'unreachable)))  ;Never reached
             test))
         => (inner . outer)

when the throw executes the second binding of test is unwound and the first binding comes back into effect. For more details on variable binding see Local Variables.

Note that catch tags are dynamically scoped, the thrower does not have to be within the same lexical scope (this means that you can throw through functions).

 -- Macro: catch tag body-forms...
     This special form defines a catch tag which will be accessible while the BODY-FORMS are evaluated.

     TAG is evaluated and recorded as the tag for this catch. Next the BODY-FORMS are evaluated as an implicit progn. The value of the catch form is either the value of the progn, or, if a throw happened, the value specified in the THROW form.

     Before exiting, the tag installed by this form is removed.

 -- Function: throw tag #!optional catch-value
     This function transfers the point of control to the catch form with a tag which is eq to TAG. The value returned by this catch form is either CATCH-VALUE or false if CATCH-VALUE is undefined.

     If there is no catch with a tag of TAG an error is signalled and the interpreter returns to the top-level of evaluation.

There are a number of pre-defined throw tags:

quit
     Terminate the librep interpreter, returning the value of the throw (if a number).

exit
     Exit the innermost event loop, unless currently in the outermost event loop, when control just passes back to the event loop.

user-interrupt
     As if a SIGINT or C-c signal has been received. Control passes back to the top-level event loop.

term-interrupt
     Triggered when a SIGTERM or SIGHUP signal is received. Tries to clean up any existing state, then terminates the interpreter.

Note that it is the event loop that catches these tags. If no event loop is active (i.e. just in read-eval-print on the console mode), any uncaught throws will result in termination.

Function Exits
..............

librep has no explicit return statement, as found in most other languages. Where a value has to returned from a function before the function would normally exit, a catch/throw pair may be used.

For example:

     (defun foo (x y)
       (catch 'return
          (when (= x 2)
            (throw 'return nil))
          ...

Cleanup Forms
.............

It is sometimes necessary ensure that a certain form is always evaluated, even when a non-local exit would normally bypass that form. The unwind-protect special form is used in this case.

 -- Macro: unwind-protect body-form cleanup-forms...
     The BODY-FORM is evaluated, if it exits normally the CLEANUP-FORMS are evaluated sequentially then the value which the BODY-FORM returned becomes the value of the unwind-protect form. If the BODY-FORM exits abnormally though (i.e. a non-local exit happened) the CLEANUP-FORMS are evaluated anyway and the non-local exit continues.

One use of this is to ensure that an opened file is always closed, for example,

     (catch 'foo
       (unwind-protect
           (let
               ((temporary-file (open-file (make-temp-name) 'write)))
             ;; Use temporary-file
             (write temporary-file "A test\n")
             ;; Now force a non-local exit
             (throw 'foo))
         ;; This is the CLEANUP-FORM it will always
         ;; be evaluated, despite the throw.
         (close temporary-file)))
         => ()

Errors
......

Errors are a type of non-local exit; when a form can not be evaluated for some reason an error is normally signalled. If an error-handler has been installed for that type of error, control is passed to the handler for that error, and evaluation continues. If there is no suitable handler, control is passed back to the innermost input loop and a suitable error message is printed.

 -- Function: signal error-symbol data
     Signals that an error has happened. ERROR-SYMBOL is a symbol classifying the type of error, it should have a property error-message (a string) which is the error message to be printed.

     DATA is a list of objects which are relevant to the error --- they will be made available to any error-handler or printed with the error message otherwise.

          (signal 'void-value '(some-symbol))
              error--> Value as variable is void: some-symbol

 -- Variable: debug-on-error
     This variable is consulted by the function signal. If its value is either t or a list containing the ERROR-SYMBOL to signal as one of its elements, the Lisp debugger is entered. When the debugger exits the error is signalled as normal.

 -- Variable: backtrace-on-error
     Similar to debug-on-error, but if an error is matched, the current backtrace is printed to the standard error stream, and control continues.

When you expect an error to occur and need to be able to regain control afterwards the condition-case special form may be used.

 -- Macro: condition-case symbol body-form error-handlers...
     condition-case evaluates the BODY-FORM with the ERROR-HANDLERS in place. If an error occurs and one of the handles matches the error, then it is evaluated with the value of SYMBOL set to the error information.

     Each of the ERROR-HANDLERS is a list whose car is a symbol defining the type of error which this handler catches. The cdr of the list is a list of forms to be evaluated in a progn if the handler is invoked.

     While the forms of the error handler are being evaluated the variable SYMBOL is bound to the value (ERROR-SYMBOL . DATA) (these were the arguments to the signal form which caused the error). If SYMBOL is the symbol nil (or the empty list ()), then the error information is not available to the handler.

     The special value, the symbol error, in the car of one of the ERROR-HANDLERS will catch all types of errors.

          (condition-case data
              (signal 'file-error '("File not found" "/tmp/foo"))
            (file-error
             data)
            (error
             (setq x z)))         ;Default handler
              => (file-error "File not found" "/tmp/foo")

Continuations
=============

Whenever a function is called, there is a control path waiting to receive the result of the function, e.g. often the form following the function invocation. This waiting control path is called the continuation of the function, since control will continue down this path when the called function exits.

These continuations are usually not paid much thought, but in some cases it may be useful to be able to directly manipulate the continuation of a function. For this purpose rep provides the call-with-current-continuation function (often shortened to call/cc) that is standard in the Scheme dialect of Lisp.

 -- Function: call/cc function
     FUNCTION is a function with a single parameter; it will be immediately invoked with this parameter bound to an object representing the current continuation (i.e. the control path that would be taken after FUNCTION exits).

     The continuation object passed to FUNCTION is itself a function accepting a single argument, when called it transfers control to the continuation of FUNCTION, as if FUNCTION had returned the argument applied to the continuation object.

 -- Function: call-with-current-continuation function
     This is an alias for call/cc.

In its simplest form, call/cc can mimic the catch and throw procedures (see Catch and Throw), for example:

     (defun foo (bar)
       (call/cc (lambda (esc)
                  (when (null bar)
                    ;; throws out of the call/cc
                    (esc nil))
                  ;; do something with bar
                  ...

this is roughly equivalent to:

     (defun foo (bar)
       (catch 'tag
         (when (null bar)
           (throw 'tag nil))
         ;; do something with bar
         ...

This is only half the story---the most powerful feature of call/cc is that since continuations have dynamic extent (that is, no object is freed until no references to it exist) it is possible to return control to scopes that have already exited.

For example, consider the following fragment of a lisp interaction:

     (prog1 (call/cc (lambda (esc)
                       (setq cont esc)))
       (message "foo!"))
         -| foo!
         => #<closure>

     cont
         => #<closure>

     (cont 10)
         -| foo!
         => 10

The continuation of the prog1 form is saved into the variable cont. When subsequently called with a single argument, it has exactly the same effect as the first time that the second form in the prog1 construct was evaluated.

Implementation Notes
--------------------

call/cc works by making a copy of the process' entire call stack. For this reason, it is likely to be less efficient than using the control structures described in the previous parts of this section. Of course, it is much more powerful than the other constructs, so this often outweighs the slight inefficiency.

Also note that currently no attempt is made to save or restore the dynamic state of the Lisp system, apart from variable bindings (both lexical and special). This means that any unwind-protect, condition-case or catch forms that are active when invoking a continuation are all ignored.

Another restriction is that invoking a continuation may not cause control to pass across a dynamic root (see Threads).

Threads
=======

librep supports a simple model of multi-threaded programming. Multiple threads of execution may be created, with control preemptively being switched between them.

Unless otherwise noted, all definitions described in this section are provided by the rep.threads module.

Thread Contexts
---------------

Every thread created by rep is a member of a thread context, this context is defined by the current position in the lisp call stack. At any point in time, only threads which are members of the current context may be executing.

 -- Function: call-with-dynamic-root thunk
     Call the function of zero-parameters THUNK in a new thread context. The new context will contain a single thread, that executing THUNK.

     The call to call-with-dynamic-root will only return once all threads in the newly created context have been deleted, or a non-local exit causes control to leave forcibly.

Creating Threads
----------------

The make-thread function may be used to create threads that execute within the current thread context (dynamic root). Each thread is represented by a lisp object.

 -- Function: threadp arg
     Return true if lisp object ARG represents a thread of execution in the lisp environment.

 -- Function: make-thread thunk #!optional name
     Create and return a new thread of execution; it will initially invoke the zero-parameter function THUNK. If the call to THUNK returns the thread is automatically deleted.

     If NAME is defined, it is a string naming the current thread.

 -- Function: make-suspended-thread #!optional name
     Similar to make-thread, except that the newly created thread will be immediately suspended from running.

 -- Function: current-thread
     Returns the currently executing thread. If no threads have been created yet in the current dynamic root (i.e. there is a single ``implicit'' thread) then false is returned.

 -- Function: all-threads
     Returns a newly-created list containing all threads in the current dynamic root. If no threads have been created yet, returns a null list.

Deleting Threads
----------------

A thread may be deleted by either returning from the function specified when it was created, or by explicit deletion. Also, the implicit thread created by the call-with-dynamic-root function may be deleted by exiting from the function called in the new context.

 -- Function: thread-delete #!optional thread
     Mark THREAD (or the current thread), as being deleted. It will not be switched to in the future. If the current thread is deleted, control will be passed to the next runnable thread. Deleting the last runnable thread results forces the containing dynamic root to be closed.

 -- Function: thread-deleted-p thread
     Returns true if THREAD has been deleted.

Manipulating Threads
--------------------

 -- Function: thread-yield
     This function may be used to pass control away from the current thread if other threads are waiting to run. There is usually no need to call this function since running threads will be preempted after a period of time.

 -- Function: thread-suspend #!optional thread milliseconds
     Mark THREAD (or the current thread) as being suspended. It will not be selected until either it has had this status removed, or MILLISECONDS milliseconds time has passed.

     Suspending the current thread will pass control to the next runnable thread in the same dynamic root. If there are no runnable threads, then the interpreter will sleep until the next thread becomes runnable.

 -- Function: thread-join thread #!optional timeout default-value
     Suspends the current thread until either THREAD has exited, or TIMEOUT milliseconds have passed.

     If THREAD exits normally, then the value of the last form it evaluated is returned; otherwise DEFAULT-VALUE is returned.

     It is an error to call thread-join on a THREAD that is not a member of the current dynamic root.

 -- Function: thread-wake thread
     Remove the suspended state from thread THREAD. It will then be scheduled for execution sometime subsequently, if its dynamic root is active.

 -- Function: thread-suspended-p thread
     Returns true if THREAD is currently suspended.

Thread preemption may be forbidden at times, to allow atomic operations to take place. Each dynamic root has its own ``forbid counter''. Only when this counter is zero may the current thread be preempted.

 -- Function: thread-forbid
     Increment the forbid count.

 -- Function: thread-permit
     Decrement the forbid count.

 -- Macro: without-interrupts #!rest forms
     Evaluate the list of forms FORMS with thread preemption temporarily disabled.

Mutual Exclusion Devices
------------------------

Mutexes are lisp objects used to coordinate access to data shared across multiple threads (where interleaved access would be bad). These functions are exported by the rep.threads.mutex module (see Modules).

 -- Function: make-mutex
     Create and return a mutex object. No thread will own the new mutex.

 -- Function: mutexp arg
     Return true if ARG is a mutex object.

 -- Function: obtain-mutex mutex
     Obtain the mutex MUTEX for the current thread. Will suspend the current thread until the mutex is exclusively available.

 -- Function: maybe-obtain-mutex mutex
     Attempt to obtain mutex MUTEX for the current thread without blocking. Returns true if able to obtain the mutex, false otherwise.

 -- Function: release-mutex mutex
     Release the mutex object MUTEX (which must have previously been obtained by the current thread). Returns true if the mutex has no new owner.

Thread Implementation Notes
---------------------------

The threads used by librep are software threads. This means that they are currently implemented by manually switching in and out thread context (i.e. the call stack) as required. There are a number of disadvantages to this method:

  * blocking I/O blocks all threads, not just the thread doing the I/O,

  * only a single processor is used, thereby avoiding any true parallelism on multi-processor systems.

The main advantage is the ease of implementation, especially when retrofitting threads into the previously single-threaded interpreter.

Loading
=======

In Lisp, programs (also called modules, or libraries) are stored in files. Each file is a sequence of Lisp forms (known as top-level forms). Most of the top-level forms in a program will be definitions (i.e. function, macro or variable definitions) since generally each library is a system of related functions and variables.

Before the program can be used it has to be loaded into the editor's workspace; this involves reading and evaluating each top-level form in the file, i.e. instantiating all function definitions, or whatever.

Load Function
-------------

 -- Function: load program #!optional no-error no-path no-suffix
     This function loads the file containing the program called PROGRAM; first the file is located then each top-level form contained by the file is read and evaluated in order.

     Each directory named by the variable load-path is searched until the file containing PROGRAM is found. In each directory three different file names are tried,

      1. PROGRAM with .jlc appended to it. Files with a .jlc suffix are usually compiled Lisp files. See Compiled Lisp.

      2. PROGRAM with .jl appended, most uncompiled Lisp programs are stored in files with names like this.

      3. PROGRAM with no modifications.

     If none of these gives a result the next directory is searched in the same way, when all directories in load-path have been exhausted and the file still has not been found an error is signalled.

     Next the file is opened for reading and Lisp forms are read from it one at a time, each form is evaluated before the next form is read. When the end of the file is reached the file has been loaded and this function returns true.

     The optional arguments to this function are used to modify its behaviour,

     no-error
          When this argument is true no error is signalled if the file can not be located. Instead the function returns false.

     no-path
          The variable load-path is not used, PROGRAM must point to the file from the current working directory.

     no-suffix
          When true no .jlc or .jl suffixes are applied to the PROGRAM argument when locating the file.

     If a version of the program whose name ends in .jlc is older than a .jl version of the same file (i.e. the source code is newer than the compiled version) a warning is displayed and the .jl version is used.

     If no Lisp file can be found matching PROGRAM, then each directory in the variable dl-load-path is searched for a libtool shared library called PROGRAM.la (see Shared Libraries).

 -- Variable: load-filename
     Whilst loading a Lisp library, this variable is bound to the name of the file being loaded.

 -- Variable: load-path
     A list of strings, each element is the name of a directory which is prefixed to the name of a program when Lisp program files are being searched for.

          load-path
              => ("/usr/local/lib/rep/1.0/lisp/"
                  "/usr/local/lib/rep/site-lisp/" "")

     The element "" refers to the current directory, note that directory names should have an ending / (or whatever) so that when concatenated with the name of the file they make a meaningful filename.

 -- Variable: dl-load-path
     A list of strings defining all directories to search for shared libraries.

 -- Variable: lisp-lib-directory
     The name of the directory in which the standard Lisp files are stored.

          lisp-lib-dir
              => "/usr/local/lib/rep/1.0/lisp/"

 -- Variable: after-load-alist
     An association list of elements of the format (FILE FORMS ...). When the library FILE is loaded, all FORMS are executed. However, note that FILE must exactly match the PROGRAM argument to the load function.

 -- Function: eval-after-load library form
     Arrange for FORM to be evaluated immediately after the Lisp library of LIBRARY has been read by the load function. Note that LIBRARY must exactly match the PROGRAM argument to load.

Autoloading
-----------

Obviously, not all features of the librep environment are always used. Autoloading allows libraries to only be loaded when they are first required. This speeds up the initialisation process and may save memory.

Functions which may be autoloaded have a special form in their symbol's function cell---an autoload form. This is a special kind of closure. When the function call dispatcher finds one of these forms it loads the program file specified in the form then re-evaluates the function call. The true function definition will then have been loaded and therefore the call may proceed as normal.

Autoload stubs may be created through the autoload function.

 -- Function: autoload symbol file #!optional is-command
     Installs an autoload form into the symbol SYMBOL. It marks that when SYMBOL is called as a function the lisp library FILE should be loaded to provided the actual definition of SYMBOL.

It is not necessary to call the autoload function manually. Simply prefix the definitions of all the functions that may be autoloaded (i.e. the entry points to your module; not all the internal functions.) with the magic comment ;;;###autoload. Then load the file into the Jade editor and invoke the add-autoloads command, creating all the necessary calls to the autoload function in the autoloads.jl Lisp file (this file which lives in the Lisp library directory is loaded when the environment is initialised).

Meta-x add-autoloads
     Scans the current buffer for any autoload definitions. Functions with the comment ;;;###autoload preceding them have autoload forms inserted into the autoloads.jl file. Simply save this file's buffer and the new autoloads will be used the next time Jade is initialised.

     It is also possible to mark arbitrary forms for inclusion in the autoloads.jl file: put them on a single line which starts with the comment ;;;###autoload call the command.

     The unsaved autoloads.jl buffer will become the current buffer.

          ;;;###autoload
          (defun foo (bar)                ;foo is to be autoloaded
            ...

          ;;;###autoload (setq x y)       ;Form to eval on initialisation

Meta-x remove-autoloads
     Remove all autoload forms from the autoloads.jl file which are marked by the ;;;###autoload comment in the current buffer.

     The unsaved autoloads.jl buffer will become the current buffer.

XXX these editor commands don't really belong here, but they'll do for now...

Features
--------

Features correspond to libraries of Lisp code. Each feature is loaded separately. Each feature has a name, when a certain feature is required its user asks for it to be present (with the require function), the feature may then be used as normal.

When a feature is loaded one of the top-level forms evaluated is a call to the provide function. This names the feature and installs it into the list of present features.

 -- Variable: features
     A list of the features currently present (that is, loaded) in the current module. Each feature is represented by a symbol. Usually the print name of the symbol (the name of the feature) is the same as the name of the file it was loaded from, minus any .jl or .jlc suffix.

          features
              => (info isearch fill-mode texinfo-mode lisp-mode xc)

 -- Function: featurep feature
     Returns true if the feature FEATURE has been loaded into the current module.

 -- Function: provide feature
     Adds FEATURE (a symbol) to the list of loaded features. A call to this function is normally one of the top-level forms in a file.

          ;;;; maths.jl -- the maths library

          (provide 'maths)
          ...

 -- Function: require feature #!optional file
     Show that the caller is planning to use the feature FEATURE (a symbol). This function will check the features variable to see if FEATURE is already loaded, if so it will return immediately.

     If FEATURE is not present it will be loaded. If FILE is given it specifies the first argument to the load function, else the print name of the symbol FEATURE is used, with any . characters replaced by the operating system's directory separator (see Module Loading).

          ;;;; physics.jl -- the physics library

          (require 'maths)                ;Need the maths library
          (provide 'physics)
          ...

     When called interactively the symbol FEATURE is prompted for.

Features may also be provided by modules, for more details see Module Loading.

Compiled Lisp
=============

librep contains a Lisp compiler as well as an interpreter; this takes a Lisp form or program and compiles it into a byte-code object. This byte-code object is a string of characters representing virtual machine instructions, a vector of constants and some other meta-information. The system also contains a byte-code interpreter; this takes the compiled byte-codes and executes them by simulating the virtual machine. This simulation will have exactly the same effect as interpreting the original form or program.

One of the main reasons for compiling programs is to increase their efficiency. Compiled functions are likely to be more efficient than interpreted counterparts in all areas (space and time). For example:

     user> (define (fib n) (if (<= n 2) 1 (+ (fib (- n 1)) (fib (- n 2)))))
     user> ,time (fib 30)
     832040
     Elapsed: 17.05572 seconds
     user> ,compile
     user> ,time (fib 30)
     832040
     Elapsed: 1.479007 seconds

---the compiled function is over an order of magnitude faster than the interpreted version.

Compilation Functions
---------------------

 -- Function: compile-form form
     This function compiles the Lisp form FORM into a byte-code form which is returned.

          (compile-form '(setq foo bar))
              => (run-byte-code "F!" [bar foo] 2)

 -- Command: compile-function function
     This function replaces the uncompiled body of the function FUNCTION (a symbol) with a compiled version, then returns FUNCTION.

 -- Command: compile-file file-name
     This function compiles the file called FILE-NAME into a file of compiled Lisp forms whose name is FILE-NAME with c appended to it (i.e. if FILE-NAME is foo.jl it will be compiled to foo.jlc).

     If an error occurs while the file is being compiled any semi-written file will be deleted.

     When called interactively this function will ask for the value of FILE-NAME.

 -- Command: compile-directory directory #!optional force exclude
     Compiles all the Lisp files in the directory called DIRECTORY which either haven't been compiled or whose compiled version is older than the source file (Lisp files are those ending in .jl).

     If the optional argument FORCE is true all Lisp files will be recompiled whatever the status of their compiled version.

     The EXCLUDE argument may be a list of filenames, these files will not be compiled.

     When this function is called interactively it prompts for the directory.

 -- Command: compile-module module-name
     Compiles all uncompiled function definitions in the module named MODULE-NAME (a symbol).

     When called interactively the module name will be prompted for.

 -- Function: run-byte-code byte-codes constants stack
     Interprets the string of byte instructions BYTE-CODES with the vector of constants CONSTANTS.

     This function should never be called by hand. The compiler will produce calls to this function when it compiles a form or a function.

There is a second form that byte-code objects can take: a vector whose read syntax includes a preceding # character is a byte-code subr. These objects represent compiled Lisp functions and macros.

 -- Function: bytecodep arg
     Returns true if ARG is a byte-code subroutine.

Compiler Declarations
---------------------

It is often useful to be able to give the compiler extra knowledge about the program forms that it is compiling. The language includes special declaration forms that have no effect when interpreted, but are meaningful to the compiler as it traverses the program.

 -- Macro: declare clause...
     Offer the information contained in the CLAUSE... forms to the compiler, which it may or may not use when compiling the program.

     Each CLAUSE is a list, the first element of each clause is a symbol defining the type of declaration, the interpretation of any other elements in the clause depends upon the declaration type.

     The following table lists the syntax of all currently supported declaration types:

     (bound VARIABLES...)
          This declaration tells the compiler that all symbols VARIABLES have lexical bindings for the extent of the current lexical scope. This is often useful to prevent spurious compiler warnings.

     (special VARIABLES...)
          This tells the compiler that all symbols VARIABLES have special (dynamic) bindings for the extent of the current lexical scope.

          (It is important that the compiler is able to distinguish special bindings from lexical bindings, since different instruction sequences must be generated to access the different types of binding.)

     (unused VARIABLES...)
          Directs the compiler not to warn about bindings for VARIABLES... being unreferenced.

     (inline NAMES...)
          Tells the compiler that it should consider inlining calls to the functions called NAMES.... Inlining will only occur if these functions are declared in the same module as, and after, the declaration itself.

     (in-module MODULE-NAME)
          This declaration should occur at the top-level of a program; it tells the compiler that the forms in the program will be evaluated within the context of the module called MODULE-NAME (a symbol).

     (language MODULE)
          Explicitly specifies the particular language dialect that the current module or file body is written for. Language dialects included with the librep distribution include rep, scheme and unscheme. These are also the names of the modules that should be imported to use a particular dialect.

          By default, the rep dialect is assumed for code outside module definitions. For code inside a module definition the list of imported modules is scanned for a known language dialect (i.e. if the module imports rep, then the rep language dialect is compiled for).

     (unsafe-for-call/cc)
          Tell the compiler that it may register-allocate variables, even if it can't prove that doing so wouldn't produce incorrect results if call/cc causes a function call to return more than once (see Continuations). This declaration applies to the entire file that it occurs in.

          Without this declaration, the compiler will only register-allocate bindings if the following conditions are met:

            * the binding is not accessed from any inner closures, and,

            * the binding is never modified after being initialized (actually, the binding may be modified between being intialized and the next function call)

          this declaration is often useful where call/cc isn't used, and there is a lot of side effecting of local variables.

     Declaration forms always evaluate to false.

A second type of declaration is the eval-when-compile form, it allows Lisp forms to be evaluated only at compile-time.

 -- Macro: eval-when-compile form
     This form tells the system that FORM should only be evaluated when the containing code is being compiled.

     The compiler knows to recognize FORMs of the pattern (eval-when-compile (require 'FEATURE)) as marking that FEATURE should be imported at compile-time. Any other FORMs are simply evaluated in an unspecified environment.

     When interpreted, eval-when-compile forms alway evaluate to false, when compiled they evaluate to the result of evaluating the FORM at compile-time.

Compilation Tips
----------------

Here are some tips for making compiled code run fast(er):

  * Instead of using while loops to traverse lists, use mapc or tail recursion.

     For example you might code a function to scan a list using iteration through a while loop:

          (defun scan-list (lst elt)
            "Search the LST for an element similar to ELT.
          Return it if one is found."
            (catch 'return
              (while (consp lst)
                (when (equal (car lst) elt)
                  (throw 'return elt))
                (setq lst (cdr lst)))))

     As well as obscuring what is actually happening, this will probably be fairly slow to execute. A more elegant solution is to use tail-recursion:

          (defun scan-list (lst elt)
            (if (equal (car lst) elt)
                elt
              (scan-list (cdr lst) elt)))

     An alternative idiom is to map an anonymous function over the list using the mapc function:

          (defun scan-list (lst elt)
            (catch 'return
              (mapc (lambda (x)
                      (when (equal x elt)
                        (throw 'return elt)))
                    lst)
              nil))

     In fact, the compiler knows that calls to mapc with a constant lambda expression can be open-coded, so it will code the list traversal directly using the virtual machine stack.

     However, in most cases the execution time differences are likely to negligible.

  * In some cases the functions member, memq, assoc, etc... can be used to search lists. Since these are primitives written in C they will probably execute several times faster than an equivalent Lisp function.

     So the above scan-list example can again be rewritten, this time as:

          (defun scan-list (lst elt)
            (car (member elt lst)))

  * All conditional structures are equivalent when compiled (they are all translated to cond statements), so use whichever is the easiest to understand.

  * A certain amount of constant folding is performed. If a function is known to be free of side effects, and all its arguments are constants, then it is evaluated at compile-time, and the result folded into the program in its place. For example

          (logor (lsh 1 6) x)
              ==> (logor 32 x)

  * Careful use of named constants (see Defining Variables) can increase the speed of some programs. For example, in the Lisp compiler itself all the opcode values (small integers) are defined as constants.

     It must be stressed that in some cases constants may not be suitable; they may drastically increase the size of the compiled program (when the constants are `big' objects, i.e. long lists) or even introduce subtle bugs (since two references to the same constant may not be eq whereas two references to the same variable are always eq).

  * Many primitives have corresponding byte-code instructions; these primitives will be quicker to call than those that don't (and incur a normal function call). Currently, the functions which have byte-code instructions (apart from all the special forms) are:

     cons, car, cdr, rplaca, rplacd, nth, nthcdr, aset, aref, length, eval, +, *, /, %, mod, lognot, not, logior, logand, logxor, equal, eq, =, /=, >, <, >=, <=, 1+, 1-, -, set, lsh, zerop, null, atom, consp, listp, numberp, stringp, vectorp, throw, boundp, symbolp, get, put, signal, return, reverse, nreverse, assoc, assq, rassoc, rassq, last, mapcar, mapc, member, memq, delete, delq, delete-if, delete-if-not, copy-sequence, sequencep, functionp, special-form-p, subrp, eql, max, min, filter, macrop, bytecodep, bind-object.

  * When a file is being compiled each top-level form it contains is inspected to see if it should be compiled into a byte-code form. Different types of form are processed in different ways:

       * Function and macro definitions have their body forms compiled into a single byte-code form. The doc-string and interactive declaration are not compiled.

       * If the form is a list form (see List Forms) and the symbol which is the car of the list is one of:

          if, cond, when, unless, let, let*, catch, unwind-protect, error-protect, with-buffer, with-window, progn, prog1, prog2, while, and, or, case.

          then the form is compiled. Otherwise it is just written to the output file in its uncompiled state.

     If your program contains a lot of top-level forms which you know will not be compiled automatically, consider putting them in a progn block to make the compiler coalesce them into one byte-code form.

Disassembly
-----------

It is possible to disassemble byte-code forms; originally this was so I could figure out why the compiler wasn't working but if you're curious about how the compiler compiles a form it may be of use to you.

Naturally, the output of the disassembler is a listing in the assembly language of the librep virtual machine---it won't take a byte-code form and produce the equivalent Lisp code!

 -- Command: disassemble-fun function #!optional stream
     This function disassembles the compile Lisp function FUNCTION. It writes a listing to the output stream STREAM (normally the value of the standard-output variable).

     When called interactively it will prompt for a function to disassemble.

When reading the output of the disassembler bear in mind that librep simulates a stack machine for the code to run on. All calculations are performed on the stack, the value left on the stack when the piece of code ends is the value of the byte-code form.

Here is a small example. Consider the fib function given at the start of this section:

     (define (fib n)
       (if (<= n 2)
           1
         (+ (fib (- n 1))
            (fib (- n 2)))))

After compilation and disassembly, the following is produced (but without the annotations):

     Disassembly of #<closure fib>:

     21 bytes, 1 constants, and (5,0,1) stack slots

     0    required-arg       ;requires a single parameter
     1    dup
     2    slot-set #0        ;store it in register 0 (r0)
     3    pushi 2
     4    le
     5    jn 10              ;unless r0 <= 2, goto 10
     8    pushi 1
     9    return             ;else, return 1
     10   refg [0] fib
     11   slot-ref #0
     12   dec
     13   call #1            ;push result of (fib (1- n))
     14   refg [0] fib
     15   slot-ref #0
     16   pushi 2
     17   sub
     18   call #1            ;push (fib (- n 2))
     19   add
     20   return             ;return the sum of the two calls

Datums
======

Datums are the mechanism by which librep allows Lisp programs to define new data types, such that these types are completely distinct from the built-in data types (i.e. they match none of the standard type predicates).

They also provide encapsulation, in that the data objects they provide are completely opaque, unless a pre-defined value is known (which was specified when the object was created, and is typically known only by the object's creator).

 -- Function: make-datum value key
     Create and return a new datum object. It has the value VALUE associated with it, and has type KEY.

 -- Function: datum-ref arg key
     If ARG has type KEY, then return the value associated with it. Otherwise, an error is signalled.

 -- Function: datum-set arg key value
     If ARG has type KEY, then set the value associated with it to be VALUE. Otherwise, an error is signalled.

 -- Function: has-type-p arg key
     Return true if ARG has type KEY.

 -- Function: define-datum-printer key printer
     Associate the function PRINTER with all datum objects of type KEY. When any such object is printed, PRINTER is applied to two arguments, the datum and the stream to which it should be printed (see Output Streams).

Queues
======

A queue is an ordered set of objects, such that objects enter at one end of the queue (the tail), and leave from the other end of the queue (the head). The acts of entering and leaving a queue are often called enqueing and dequeueing.

librep provides a straightforward queue implementation, implemented by the rep.data.queues module (see Modules).

 -- Function: make-queue
     Create and return a new queue object. The queue will initially be empty.

 -- Function: enqueue q arg
     Add the object ARG to the tail of the queue Q.

 -- Function: dequeue q
     Remove the object at the head of the queue Q, and return it. If Q is empty, an error is signalled.

 -- Function: queue-empty-p q
     Return true if the queue Q is not empty.

 -- Function: queuep arg
     Return true if the object ARG is a queue.

 -- Function: queue->list q
     Return a list of objects representing the contents of the queue Q, with objects ordered from head to tail. Modifying the list structure causes undefined effects to the queue itself.

 -- Function: queue-length q
     Return the number of objects stored in the queue Q.

 -- Function: delete-from-queue q arg
     Removes any occurrences of the object ARG from the queue Q.

Records
=======

librep provides a convenient means of defining structured data types, these types are known as records. Each record is a distinct data type, meaning that there will only be a single type-predicate matching objects of any individual record type.

All definitions documented in this section are provided by the rep.data.records module (see Modules).

Record types are defined using the define-record-type macro, this in turn defines a number of functions implementing the type. These functions include a constructor, a type predicate, and a user-defined set of field-accessor and -modifier functions.

 -- Macro: define-record-type type (constructor fields...) [predicate] (field accessor [modifier])...

     This macro creates a new record type storing an opaque object identifying the type in the variable named TYPE.

     It then defines a function CONSTRUCTOR with parameter list as specified by the FIELDS..., and a predicate function called PREDICATE if PREDICATE is given.

     The fields of the record are defined by the sequence of (FIELD ACCESSOR [MODIFIER]) forms, each form describes a single field (named FIELD, which may match one of the constructor arguments).

     For each field a function ACCESSOR will be defined that when applied to an argument of the record type, returns the value stored in the associated FIELD. If the MODIFIER name is defined a function will be defined of that name, that when applied to a record and an object, stores the object into the associated field of the record.

     Note that the FIELDS... may include all the standard lambda-list features (see Lambda Expressions), including keyword parameters and default values.

Here is an example record definition:

     (define-record-type :pare
       (kons x y)                         ; constructor
       pare?                              ; predicate
       (x kar set-kar!)                   ; fields w/ optional accessors
       (y kdr))                           ;and modifiers

the variable :pare is bound to the record type. Following this definition, the record type could be used as follows:

     (define x (kons 1 2))

     (pare? x)
         => t

     (kar x)
         => 1

     (set-kar! x 42)

     (kar x)
         => 42

By default record objects print as the name of their type in angle brackets, e.g. for the above pare type, each object would print as the string #<:pare>. This may be redefined using the define-record-discloser function.

 -- Function: define-record-discloser type discloser
     Associate the function DISCLOSER with the record type TYPE. When any record of this type is printed, DISCLOSER is applied to the object, it should return the value that will actually be printed.

For the above example, the following could be used:

     (define-record-discloser :pare (lambda (x) `(pare ,(kar x) ,(kdr x))))

     (kons 'a 'b)
         => (pare a b)

Constructors for records with large numbers of fields often benefit from using keyword parameters. For example the kons record above could be defined as follows (though this would make more sense if it had more than two fields):

     (define-record-type :pare
       (kons #!key (kar 1) (kdr 2))
       pare?
       (kar kar set-kar!)
       (kdr kdr set-kdr!))

     (kons #:kar 42) => (pare 42 2)
     (kons #:kdr 42) => (pare 1 42)

Hash Tables
===========

The rep.data.tables module provides a flexible hash table implementation (see Modules). Each hash table is represented by a lisp object satisfying the tablep predicate:

 -- Function: tablep arg
     Return true if ARG is a hash table.

Hash tables may be created by using the make-table and make-weak-table functions:

 -- Function: make-table hash-fun compare-fun
     Create and return a new hash table. When storing and referencing keys it will use the function HASH-FUN to map keys to hash codes (positive fixnums), and the predicate function COMPARE-FUN to compare two keys (should return true if the keys are considered equal).

 -- Function: make-weak-table hash-fun compare-fun
     Similar to make-table, except that key-value pairs stored in the table are said to be ``weakly keyed''. That is, they are only retained in the table as long the key has not been garbage collected.

     Unlike with tables created by the make-table function, the fact that the key is stored in the table is not considered good enough to prevent it being garbage collected.

 -- Function: table-ref table key
     Return the value stored in hash table TABLE indexed by object KEY. Returns false if no such value exists.

 -- Function: table-bound-p table key
     Returns true if the hash table TABLE contains a value associated with KEY.

 -- Function: table-set table key value
     Associate the value VALUE with KEY in hash table TABLE. Returns value.

 -- Function: table-unset table key
     Remove any value stored in TABLE associated with KEY.

 -- Function: table-walk function table
     Call function FUNCTION for every key-value pair stored in hash table TABLE. For each pair, the function is called with arguments (KEY VALUE).

Several hash functions are also provided:

 -- Function: string-hash string
     Return an integer representing the string STRING.

 -- Function: symbol-hash symbol
     Call (string-hash (symbol-name SYMBOL)).

 -- Function: eq-hash arg
     Return a hash value representing object ARG. The hash is generated from the address of the object.

 -- Function: equal-hash arg
     Return a hash value representing object ARG. The hash is generated from the contents of the object.

Guardians
=========

A guardian is a lisp object used to control when other data objects are recycled by the garbage collector (see Garbage Collection). (Guardians were first described in a paper by R. Kent Dybvig, Carl Bruggeman, and David Eby: "Guardians in a Generation-Based Garbage Collector", ACM SIGPLAN Conference on Programming Language Design and Implementation, June 1993.) The usual behaviour of the collector is to recycle objects as soon as they have no remaining references.

Guardians allow the programmer to detect when a specified object would be freed by the garbage collector, and to implement their own allocation policy. This can be useful, for example, with objects that have a high creation-overhead, and thus need to be cached for performance reasons.

 -- Function: make-guardian
     This function allocates and returns a new guardian. Each guardian has a list of data objects associated with it; some of which may have been proved to have no remaining references to them (except from the guardian system).

     Calling the guardian object with a single argument, adds that value to the list of objects associated with that guardian. Calling the guardian with no arguments has one of two effects:

       * If objects are associated with the guardian that have been proved to be inaccessible, then return one of those objects, and remove it from the list of objects associated with the guardian.

       * If none of the associated objects have been proved to be inaccessible, then return the value false.

Note the use of the word ``prove'' in the above description, objects are only moved into a guardian's inaccessible set by the garbage collector.

Here is an example use of the guardian system:

     ;; create a new guardian object
     (setq G (make-guardian))

     ;; create a lisp object
     (setq x (cons 'a 'b))
        => (a . b)

     ;; protect the object using the guardian
     (G x)

     ;; remove the sole reference to the object
     (setq x nil)
        => ()

     ;; invoke the garbage collector, this will
     ;; prove that the value added to the
     ;; guardian is no longer accessible
     (garbage-collect)

     ;; call the guardian to retrieve the
     ;; inaccessible value
     (G)
        => (a . b)

     ;; no more inaccessible values available
     (G)
        => ()

Streams
=======

A stream is a Lisp object which is either a data sink (an output stream) or a data source (an input stream). All streams produce or consume sequences of 8-bit characters.

Streams are very flexible, functions using streams for their input and output do not need to know the type of stream being accessed. For example the Lisp reader (the read function) takes an input stream as its sole argument, it then reads characters from this stream until it has parsed a whole object. This stream could be a file, a function, or even a string; the read function does not need to differentiate.

 -- Function: streamp arg
     Return true if ARG is a stream.

 -- Function: input-stream-p arg
     Return true if ARG is an input stream.

 -- Function: output-stream-p arg
     Return true if ARG is an output stream.

Input Streams
-------------

These are the possible types of input stream, for the functions which use them see Input Functions.

FILE
     Characters are read from the file object FILE, for the functions which manipulate file objects see Files.

FUNCTION
     Each time an input character is required the FUNCTION is called with no arguments. It should return the character read (an integer) or false if for some reason no character is available.

     FUNCTION should also be able to `unread' one character. When this happens the function will be called with one argument---the value of the last character read. The function should arrange it so that the next time it is called it returns this character. A possible implementation could be,

          (defvar ms-unread-char nil
            "If true the character which was pushed back.")

          (defun my-stream (#!optional unread-char)
            (if unread-char
                (setq ms-unread-char unread-char)
              (if ms-unread-char
                  (prog1
                    ms-unread-char
                    (setq ms-unread-char nil))
                ;; Normal case -- read and return a character from somewhere
                ...

nil
     Read from the stream stored in the variable standard-input.

It is also possible to use a string as an input stream. The string to be read from must be applied to the make-string-input-stream function and the result from this function used as the input stream.

 -- Function: make-string-input-stream string #!optional start
     Returns an input stream which will supply the characters of the string STRING in order starting with the character at position START (or from position zero if this argument is undefined).

          (read (make-string-input-stream "(1 . 2)"))
              => (1 . 2)

 -- Variable: standard-input
     The input stream which is used when no other is specified or is false.

Applications that embed librep, or dynamically loaded extensions, may provide further input stream types.

Output Streams
--------------

These are the different types of output stream, for the functions which use them see Output Functions.

FILE
     Writes to the file object FILE. See Files.

FUNCTION
     The function FUNCTION is called with one argument, either a string or a character. This should be used as the circumstances dictate. If the function returns a number it is the number of characters actually used, otherwise it is assumed that all the characters were successful.

PROCESS
     Writes to the standard input of the process object PROCESS. If PROCESS isn't running an error is signalled. See Processes.

t
     Appends the character(s) to the end of the status line message.

()
     Write to the stream stored in the variable standard-output.

It is also possible to store the characters sent to an output stream in a string.

 -- Function: make-string-output-stream
     Returns an output stream. It accumulates the text sent to it for the benefit of the get-output-stream-string function.

 -- Function: get-output-stream-string string-output-stream
     Returns a string consisting of the text sent to the STRING-OUTPUT-STREAM since the last call to GET-OUTPUT-STREAM-STRING (or since this stream was created by make-string-output-stream).

          (setq stream (make-string-output-stream))
              => ("" . 0)
          (prin1 keymap-path stream)
              => ("(lisp-mode-keymap global-keymap)" . 64)
          (get-output-stream-string stream)
              => "(lisp-mode-keymap global-keymap)"

 -- Variable: standard-output
     This variable contains the output stream which is used when no other is specified (or when the given output stream is false).

 -- Variable: standard-error
     This variable contains the output stream which is used when an error message is being reported.

Applications that embed librep, or dynamically loaded extensions, may provide further output stream types.

Input Functions
---------------

 -- Function: read-char stream
     Read and return the next character from the input stream STREAM. If the end of the stream is reached false is returned.

 -- Function: read-line stream
     This function reads one line of text from the input stream STREAM, a string containing the line (including the newline character which terminates the line).

     If the end of stream is reached before any characters can be read false is returned, if the end of stream is reached but some characters have been read (but not the newline) these characters are made into a string and returned.

     Note that unlike the Common Lisp function of the same name, the newline character is not removed from the returned string.

 -- Function: read stream
     This function is the function which encapsulates the Lisp reader (see The Lisp Reader). It reads as many characters from the input stream STREAM as required to form the read syntax of a single Lisp object (see Read Syntax), this object is then returned.

 -- Function: read-from-string string #!optional start
     Reads one Lisp object from the string STRING, the first character is read from position START (or position zero).

          (read-from-string STRING START)
          ==
          (read (make-string-input-stream STRING START))

Output Functions
----------------

 -- Function: write stream data #!optional length
     Writes the specified character(s) to the output stream STREAM. DATA is either the character or the string to be written. If DATA is a string the optional argument LENGTH may specify how many characters are to be written. The value returned is the number of characters successfully written.

          (write standard-output "Testing 1.. 2.. 3..")
              -| Testing 1.. 2.. 3..
              => 19

 -- Function: copy-stream input-stream output-stream
     This function copies all characters which may be read from INPUT-STREAM to OUTPUT-STREAM. The copying process is not stopped until the end of the input stream is read. Returns the number of characters copied.

     Be warned, if you don't choose the streams carefully you may get a deadlock which only an interrupt signal can break!

 -- Function: print object #!optional stream
     Outputs a newline character to the output stream STREAM, then writes a textual representation of OBJECT to the stream.

     If possible, this representation will be the read syntax of OBJECT.

     OBJECT is returned.

          (print '(1 2 3))
              -|
              -| (1 2 3)
              => (1 2 3)

 -- Function: prin1 object #!optional stream
     Similar to print but no initial newline is output.

          (prin1 '(1 2 3))
              -| (1 2 3)
              => (1 2 3)

          (prin1 '|(xy((z]|)              ;A strange symbol
              -| \(xy\(\(z\]
              => \(xy\(\(z\]

 -- Function: prin1-to-string object
     Returns a string containing the characters that prin1 would output when it prints OBJECT.

          (prin1-to-string '(1 2 3))
              => "(1 2 3)"

 -- Function: princ object #!optional stream
     Prints a textual representation of OBJECT to the output stream STREAM. No steps are taken to create output that read can parse; in particular, no double-quote characters surround strings.

          (princ "foo")
              -| foo
              => "foo"

          (princ '|(xy((z]|)
              -| (xy((z]
              => \(xy\(\(z\]

Several variables may be used to control how objects are printed.

 -- Variable: print-escape
     This defines which control characters print and prin1 will escape (using backslashes). Possible values are:

     ()
          Only escape double-quote and backslash characters.

     newlines
          Only escape double-quote, backslash, newline, TAB, and formfeed characters.

     t
          Escape double-quote, backslash, and all control characters (anything with a numeric value less than 32, or greater than 126).

 -- Variable: print-length
     This variable, if true, limits the number of elements printed from lists.

 -- Variable: print-level
     This variable, if true, limits the recursion depth when printing lists.

Formatted Output
----------------

 -- Function: format stream template #!rest values
     Writes to a stream, STREAM, a string constructed from the format string, TEMPLATE, and list of arguments VALUES.

     If STREAM is false the resulting string will be returned, not written to a stream.

     TEMPLATE is a template for the output string, any % characters introduce a substitution, using the next unused argument. The substitutions have the following syntax,

          %[INDEX$][FLAGS][FIELD-WIDTH]CONVERSION

     INDEX is an optional decimal number specifying exactly which of the VALUES this conversion refers to (with the first at position one), and is usually used when translating messages; by default the next value is used.

     FIELD-WIDTH is a positive decimal integer, defining the size in characters of the substitution output.

     CONVERSION is a character defining how to convert the corresponding argument value to text. The default options are:

     s
          Write the printed representation of the value without quoting (as if from the princ function).

     S
          Write the printed representation with quoting enabled (like the prin1 function).

     d
          Output the value as a decimal number.

     o
          Write the value in octal.

     x
     X
          In hexadecimal.

     c
          Write the character specified by the value.

     %
          Print a literal percent character. None of the VALUES are used.

     FLAGS is a sequence of zero or more of the following characters,

     _
          Left justify the substitution within the field.

     ^
          Truncate the substitution at the size of the field.

     0
          Pad the field with zeros instead of spaces.

     +
          For d, x, and o conversions, output a leading plus sign if the argument is positive.

       (a space)
          For d, x, and o conversions, if the result doesn't start with a plus or minus sign, output a leading space.

     The list of CONVERSIONS can be extended through the format-hooks-alist variable; the strings created by these extra conversions are formatted as if by the `s' conversion.

     Note that the FIELD-WIDTH and all flags currently have no effect on the S conversion, (or the s conversion when the argument isn't a string).

     If STREAM isn't false (in which case the created string is returned) the value of STREAM is returned.

          (format nil "foo %S bar 0x%x" '(x . y) 255)
              => "foo (x . y) bar 0xff"

          (format standard-output "The %2$s is %1$s!" "purple" "dog")
              -| The dog is purple!
              => #<buffer *jade*>

 -- Variable: format-hooks-alist
     This variable is an association-list, each element being (CHAR . FUNCTION), defining extra conversions for the format function.

     If a conversion %X is given, and the alist contains an element whose car is the character X, the the associated function is called with one value, the next argument to be formatted. It should return the string to be inserted.

Hooks
=====

A hook allows you to wedge your own pieces of Lisp code into the operation of other functions, enable the extension of that functionality. These pieces of code are evaluated via the hook and the result is available to the hook's caller. One hook has already been encountered, the format-hooks-alist variable (see Formatted Output).

Functions As Hooks
------------------

Some hooks only allow a single piece of code to be hooked in. Usually a normally-undefined function is used; to install your hook defined a function with the name of the hook. When the hook is to be evaluated the function is called.

Generally the name of the hook's function will end in -function.

An alternative scheme is to use a variable to store the hook, its value should be the function to call.

Normal Hooks
------------

This is the standard type of hook, it is a variable whose value is a list of functions. When the hook is evaluated each of the functions will be called in turn.

The names of hooks of this type will normally end in -hook.

These functions are exported by the rep.system module.

 -- Function: add-hook hook function #!optional at-end
     This function adds a new function FUNCTION to the list of functions installed in the (list) hook HOOK (a symbol).

     If AT-END is true the new function is added at the end of the hook's list of functions (and therefore will be called last when the hook is evaluated), otherwise the new function is added to the front of the list.

          text-mode-hook
              => (#<closure fill-mode-on>)
          (add-hook 'text-mode-hook my-function)
              => (#<closure my-function> #<closure fill-mode-on>)

 -- Function: remove-hook hook function
     This function removes the function FUNCTION from the list of functions stored in the (list) hook HOOK (a symbol).

     All instances of FUNCTION are deleted from the hook.

There are actually three calling conventions for this type of hook, differing in how many of the functions in the list actually get called. In this simplest form, all functions are called. In an and type hook, functions are only invoked while all others have returned true. As soon as the first function in the hook returns false, no others will be called. Finally, an or type hook aborts when a function returns a true result.

 -- Function: call-hook hook arg-list #!optional type

     Call the hook named by the symbol HOOK, passing all functions the arguments in the list ARG-LIST. Note that HOOK may also be the actual list of functions to call.

     TYPE defines how the return values of each function in the hook are treated. If TYPE is false they are ignored, if TYPE is the symbol and the hook aborts after a function returns false, if TYPE is or the hook aborts when a function returns true.

     In all cases the value returned by the last-evaluated function is returned.

Files
=====

librep allows you to manipulate files in the operating system's filing system; a special type of Lisp object, a file object, is used to represent files which have been opened for reading or writing (through the streams mechanism, see Streams).

Names of files are represented by strings, the syntax of file names is defined by the underlying operating system: librep simply treats it as a string.

Unless otherwise stated, all functions and variables described in the following sections are exported by the rep.io.files module.

File Names
----------

A file name is a string identifying an individual file (or directory) in the filing system (i.e. the disk). The exact syntax of file names depends on the operating system. There are several functions for manipulating file names.

 -- Function: file-name-absolute-p file-name
     Returns true when FILE-NAME is not specified relative to the current directory.

 -- Function: file-name-directory file-name
     This function returns the directory part of the file name string FILE-NAME. This is the substring of FILE-NAME defining the directory containing the file.

          (file-name-directory "/tmp/foo")
              => "/tmp/"

          (file-name-directory "foo")
              => ""

          (file-name-directory "foo/bar/")
              => "foo/bar/"

 -- Function: file-name-nondirectory file-name
     Returns the substring of the file name FILE-NAME which is not the directory part.

          (file-name-nondirectory "/tmp/foo")
              => "foo"

          (file-name-nondirectory "foo")
              => "foo"

          (file-name-nondirectory "foo/bar/")
              => ""

 -- Function: file-name-as-directory file-name
     Returns a string through which the item in the file system named by FILE-NAME can be referred to as a directory.

          (file-name-as-directory "./foo")
              => "./foo/"

          (file-name-as-directory "./foo/")
              => "./foo/"

 -- Function: directory-file-name directory-name
     Returns a string through which the directory named by DIRECTORY-NAME can be referred to as a file.

          (directory-file-name "./foo/")
              => "./foo"

          (directory-file-name "./foo")
              => "./foo"

 -- Function: expand-file-name file-name #!optional base-dir
     Expands FILE-NAME assuming that it specifies a file relative to BASE-DIR. If BASE-DIR is undefined it is taken as the current value of the default-directory variable. While expanding the file name, any obvious simplifications will be performed (e.g. on Unix the removal of "." and ".." where possible).

     Note that the returned file name will only be absolute if one of the following conditions is met:

      1. BASE-DIR (or default-directory) is absolute,

      2. FILE-NAME is already absolute.

          (expand-file-name "foo" "./bar")
              => "bar/foo"

     Note for file handler implementors: when a handler is called for the expand-file-name operation, it will only ever receive one argument, the already expanded file name. The only action that may be need to be taken is to simplify the file name (e.g. removing . and .. entries or whatever).

 -- Function: canonical-file-name file-name
     This function returns the canonical name of the file referred to by the string FILE-NAME. The canonical name of a file is defined such that two files can be compared simply by comparing their canonical names; if the names match, they refer to the same file.

     (Note that the opposite isn't always true, if two canonical names don't match the files could still be the same, for example via hard links. On most operating systems, symbolic links will be expanded where possible.

          (canonical-file-name "foo")
              => "/home/john/src/librep/man/foo"

 -- Function: local-file-name file-name
     librep supports extensible file handling (see File Handlers), so file names may refer to files not residing in the system's local file structure, and thus which are unavailable to other programs.

     This function returns either the absolute name of the file FILE-NAME, if it is found in the local system, or false, if the file does not.

          (local-file-name "foo")
              => "/home/john/src/librep/man/foo"

          (local-file-name "/john@tango:foo")
              => ()

 -- Function: make-temp-name
     This function returns the name of a file which, when created, may be used for temporary storage. Each time this function is called a unique name is computed.

          (make-temp-name)
              => "/tmp/00088aaa"

          (make-temp-name)
              => "/tmp/00088baa"

 -- Variable: default-directory
     This variable names the current working directory. All relative file names are interpreted starting from this location in the file system.

File Objects
------------

A file object is a Lisp object which represents an open file in the filing system. Any file object may be used as a stream (either input or output) to access the contents of the file (see Streams).

 -- Function: filep object
     This function returns true when its argument is a file object.

Creating File Objects
.....................

 -- Function: open-file file-name mode
     This function opens the file called FILE-NAME (see File Names) and returns the new file object.

     The MODE argument is a symbol defining the access modes used to open the file with, the options are:

     read
          Open an existing file for reading only.

     write
          Open the file for writing only, if the file exists it is truncated to zero length. Otherwise a new file is created.

     append
          Open the file for appending to, i.e. writing to the end of the file. If the file doesn't exist it is created.

The three standard I/O streams are also available as file handles.

 -- Function: stdin-file
     Return a file object representing the interpreters standard input.

 -- Function: stdout-file
     Return a file object representing the interpreters standard output.

 -- Function: stderr-file
     Return a file object representing the interpreters standard error.

Attempting to close any of these files will result in the associated stream being connected to /dev/null.

Destroying File Objects
.......................

The easiest way to close a file is simply to eliminate all references to it, subsequently the garbage collector will close it for you. It is better to close files explicitly though since only a limited number of files may be opened concurrently.

 -- Function: close-file file-object
     This function closes the file pointed to by the file object FILE-OBJECT.

     Subsequently, any stream accesses FILE-OBJECT are illegal and will signal an error.

Functions on File Objects
.........................

 -- Function: seek-file file #!optional offset where
     When called as (seek-file FILE), returns the distance in bytes from the start of the file that the next character would be read from.

     When called as (seek-file FILE OFFSET [WHERE]) alters the position from which the next byte will be read. WHERE can be one of the values:

     ()
          OFFSET bytes after the current position.

     start
          OFFSET bytes after the beginning of the file.

     end
          OFFSET bytes before the end of the file.

     Note that not all files may be seekable; if (seek-file FILE) returns false, indicating that the current position is unknown, any attempts to set the current position will also fail.

 -- Function: flush-file file
     This function flushes any buffered output to the file object FILE to disk.

 -- Function: file-binding file
     Returns the name of the file which the file object FILE is currently bound to. Returns false if the file is currently unbound (i.e. close-file was called on it).

The next three functions are used when non-local files are being accessed. See File Handlers for more details.

 -- Function: file-bound-stream file
     If the file object FILE doesn't refer to a file in the local filing system, return the stream that it is bound to (allowing the file's contents to be accessed), this may or may not be another file object.

 -- Function: file-handler-data file
     Return the file-handler-specific data associated with the file object FILE.

 -- Function: set-file-handler-data file data
     Set the handler-specific data of file object FILE to DATA. This should only be done by the handler owning the file.

It's also possible to register a callback function to be invoked when input is available on a file,

 -- Function: set-input-handler local-file function
     Arrange for FUNCTION to be called whenever pending input is available on LOCAL-FILE, a file object bound to a file in the local file space.

     Note that this makes LOCAL-FILE subsequently do non-blocking input.

     This function is normally only useful when LOCAL-FILE represents a pipe or socket.

File Information
----------------

A number of functions exist which when given the name of a file return one of the attributes relating to that file.

 -- Function: file-exists-p file-name
     Returns true when a file FILE-NAME exists.

 -- Function: file-regular-p file-name
     Returns true when the file FILE-NAME is a `normal' file. This means that it isn't a directory, device, symbolic link or whatever.

 -- Function: file-directory-p file-name
     Returns true when the file FILE-NAME is a directory.

 -- Function: file-symlink-p file-name
     Returns true when the file FILE-NAME is a symbolic link.

 -- Function: file-readable-p file-name
     Returns true when the file FILE-NAME is readable.

 -- Function: file-writable-p file-name
     Returns true when the file FILE-NAME is writable.

 -- Function: file-owner-p file-name
     Returns true when the ownership of the file FILE-NAME is the same as that of any files written by the editor.

 -- Function: file-size file-name
     Returns the number of bytes stored in the file named FILE-NAME.

 -- Function: file-nlinks file-name
     Returns the number of hard links pointing to the file FILE-NAME. If FILE-NAME has only one name the number will be one.

 -- Function: file-modes file-name
     This function returns the access permissions of the file FILE-NAME. This will be an integer whose format is undefined; it differs from operating system to operating system.

 -- Function: file-modes-as-string file-name
     Returns a ten-character string describing the attibutes of the file called FILE-NAME

          (file-modes-as-string ".")
              => "drwxr-sr-x"

 -- Function: set-file-modes file-name modes
     This function sets the access permissions of the file FILE-NAME to the integer MODES (as returned by the file-modes function).

 -- Function: file-modtime file-name
     Returns the system time at the last modification to the file FILE-NAME, this will be in the usual timestamp format, see Timestamps.

 -- Function: file-newer-than-file-p file-name1 file-name2
     This function returns true if the file FILE-NAME1 was modified more recently than the file FILE-NAME2 was.

Manipulating Files
------------------

 -- Command: delete-file file-name
     This function deletes the file called FILE-NAME. When called interactively FILE-NAME is prompted for.

 -- Command: rename-file file-name new-name
     This function attempts to change the name of the file NEW-NAME to NEW-NAME.

     This won't work from one file system to another or if a file called NEW-NAME already exists, in these cases an error is signalled.

     This prompts for its arguments when called interactively.

 -- Command: copy-file file-name destination-name
     Creates a new copy of the file FILE-NAME with the name DESTINATION-NAME.

     The access modes of the new file will be the same as those of the original file.

     The arguments are prompted for when this function is called interactively.

Manipulating Directories
------------------------

 -- Command: make-directory directory-name
     Create a new directory called DIRECTORY-NAME.

 -- Command: delete-directory directory-name
     Delete the directory called DIRECTORY-NAME. This only succeeds if the directory in question is empty, otherwise an error is signalled.

 -- Function: directory-files directory-name
     Returns a list of the names of all items in the directory whose name is DIRECTORY-NAME. The names in the list will be relative to the directory DIRECTORY-NAME.

          (directory-files "/tmp/foo"
              => ("bar" "subdir" "xyz" "." "..")

Manipulating Symbolic Links
---------------------------

 -- Function: make-symlink name contents
     Create a symbolic link called NAME, containing the string CONTENTS.

 -- Function: read-symlink name
     Return the string that is the contents of the symbolic link called NAME. Signals an error if no such link exists.

File Handlers
-------------

As noted earlier, librep supports virtual files; that is it allows file names to be accessed that don't reside on the local filing system, or aren't normally valid as file names. This is achieved through the use of file handlers, Lisp functions that have signalled that they should be used to redirect all accesses to files whose names match a particular regular expression (see Regular Expressions).

For example, there is a convention under Unix that a user's home directory can be accessed via the file name ~, even though there is no such support from the operating system itself. So a file handler can be (and has been) written that recognises all file names starting with a tilde, translating them to the actual file in the file system.

 -- Variable: file-handler-alist
     This variable contains a list of file handler declarations, each one of the form (REGEXP . FUNCTION). Whenever a file operation is performed on a file whose name matches REGEXP, FUNCTION is invoked to perform the action. The function is called as (FUNCTION OPERATION ARGS...), where OPERATION and ARGS are from the original call.

     For example if the file-handler-alist contains the entry ("^~" . tilde-file-handler), then all file operations on files starting with a tilde are redirected to the tilde-file-handler function.

     Thus if a form (file-exists-p "~/foo") is executed, it would result in a call to tilde-file-handler as (tilde-file-handler 'file-exists-p "~/foo").

The list of operations that may be redirected to a file handler is: file-name-absolute-p, expand-file-name, local-file-name, canonical-file-name, file-name-nondirectory, file-name-directory, file-name-as-directory, directory-file-name, open-file, close-file, flush-file, seek-file, write-buffer-contents, read-file-contents, insert-file-contents, delete-file, rename-file, copy-file, copy-file-to-local-fs, copy-file-from-local-fs, make-directory, delete-directory, file-exists-p, file-regular-p, file-readable-p, file-writable-p, file-directory-p, file-symlink-p, file-owner-p, file-nlinks, file-size, file-modes, file-modes-as-string, set-file-modes, file-modtime, directory-files, make-symlink, read-symlink.

There are several undefined functions in this list. The write-buffer-contents, read-file-contents, and insert-file-contents pertain to the Jade text editor. The other two are defined as follows.

 -- Operation: copy-file-to-local-fs file-name local-name
     Called when copying files between file handlers, this operation should copy the file matching the handler FILE-NAME, to the file on the local file system LOCAL-NAME.

 -- Operation: copy-file-from-local-fs local-name file-name
     Called when copying files between file handlers, this operation should copy the local file FILE-NAME to the file matching the handler FILE-NAME.

To prevent infinite recursion, while a particular operation is being processed by a file handler, that operation may not be passed back to the same handler.

To allow file handlers to handle the open-file operation, it is possible to create file handles from arbitrary streams.

 -- Function: make-file-from-stream file-name stream handler
     Return a new file object that refers to the logical file called FILE-NAME, that is not in the local filing system. All access to the file object will be directed through the stream object STREAM, and the file handler function HANDLER.

An alternative method of opening remote files is to use a temporary file in the local file system with either one (read or write modes), or two (append mode) synchronisations with the remote system. This is the method used by the FTP remote file backend (see the next section). It has the advantage of simplifying the seek-file operation.

Remote files
------------

Since one of the intentions for file handlers is to allow remote files to be accessed, a common method of providing new methods of doing this has been implemented, in the remote.jl Lisp library.

Accessing a file name matching the regular expression:

     ^/(([a-zA-Z0-9._-]+)@)?([a-zA-Z0-9._-]+):

for example /john@host.com:file refers to a file called file owned by the user john, on the system host.com.

If no username is specified explicitly, two variables are used to select the user:

 -- Variable: remote-host-user-alist
     An alist mapping host regexps to the default user name to use for remote file connections to that host.

 -- Variable: remote-default-user
     User name to use for remote file connections when otherwise unspecified. By default the current user name on the local system.

Two variables control how individual hosts are matched to methods of accessing files.

 -- Variable: remote-auto-backend-alist
     An alist of (HOST-REGEXP . BACKEND-TYPE) mapping host names to methods of accessing remote files.

 -- Variable: remote-default-backend
     A symbol defining the method to use for otherwise unselected hosts.

A method of accessing files, or a backend is a symbol whose remote-backend property names a function to call when files need to be accessed. For example the ftp backend is initialised as:

     (put 'ftp 'remote-backend remote-ftp-handler)

The backend function is called as (FUNCTION SPLIT-NAME OPERATION ARGS). The SPLIT-NAME is a three-element list, (USER-OR-NIL HOST FILE) defining the file to be accessed. The other options are as usual. Further details can be found in the remote.jl, remote-ftp.jl and remote-rcp.jl Lisp source files.

The ftp backend is currently the most well-developed, several functions and variables may be used to customise its behaviour.

 -- Function: remote-ftp-add-passwd user host passwd
     Add the string PASSWD as the password for the FTP session connecting to USER@HOST.

 -- Variable: remote-ftp-show-messages
     When true (the default), messages are displayed as FTP commands are executed.

 -- Variable: remote-ftp-display-progress
     When true (the default) display progress of FTP transfers.

 -- Variable: remote-ftp-anon-users
     A regular expression matching the user names for ``anonymous'' FTP sessions.

 -- Variable: remote-ftp-anon-passwd
     The string to send as the passwd of an anonymous FTP session. By default the current uses email address.

There is a problem with the ftp backend however; due to limitations in the FTP protocol, not all librep file operations are supported, with the most obvious exception being the make-symlink function.

When this is a problem it may be possible to use rep's custom file transfer protocol. If it is possible to use rsh to connect to the remote host in question, then the rep backend may be used.

The rep-remote program distributed with librep must exist on the remote host, this is executed via rsh and provides a protocol for executing all of librep's file operations on that host. See the lisp/remote-rep.jl file in the distribution for more details.

Processes
=========

When running on a Unix-style operating system librep allows you to launch and control an arbitrary number of subprocesses. These subprocesses can run either synchronously or asynchronously in respect to the Lisp system; data can be sent to the stdin channel and any output from the process is automatically written to a specified Lisp output stream.

Unless otherwise stated, all functions and variables described in the following sections are exported by the rep.io.processes module.

Process Objects
---------------

A process object is a type of Lisp object used to provide a link between a `physical' process running in the operating system and the Lisp system. Each process object consists of a number of values (references to other Lisp objects); these values are used when the object is used to run a subprocess.

Process objects which aren't currently being used to run a subprocess store the exit value of the last subprocess which was run on that object.

 -- Function: processp object
     This function returns true when its argument is a process object.

The programmer-accessible components of a process object are,

Output stream
     A normal Lisp output stream (see Output Streams), all data which the subprocess outputs to its stdout channel is copied to this output stream. See Process I/O.

Error stream
     A normal Lisp output stream (see Output Streams), all data which the subprocess outputs to its stderr channel is copied to this output stream. Unless explicitly specified error output goes to the stdout stream. See Process I/O.

State change function
     A Lisp function, called each time the state of the subprocess being run on the object changes. See Process States.

Program name
     The name of the program (a string) to execute when the subprocess is created.

Program arguments
     A list of strings defining the arguments which the program executed is given.

Directory
     When a subprocess is started its current working directory is set to the directory named by this component of its process object.

Connection type
     Asynchronous subprocesses (see Asynchronous Processes) use this component to decide how to connect to the I/O channels of the subprocess. Current options include pseudo-terminals and pipes.

 -- Function: make-process #!optional output-stream state-function directory program args
     This functions creates and returns a new process object. No subprocess will be started.

     The optional arguments are used to define the values of the components of the new process object, any undefined components will be set to default or null values.

For each component of a process object two functions exist; one to read the component's value in a specific process object, the other to set the component's value.

 -- Function: process-prog process
     Returns the value of the program name component of the process object PROCESS.

 -- Function: set-process-prog process prog-name
     Sets the value of the program name component of the process object PROCESS to the string PROG-NAME, then returns PROG-NAME.

 -- Function: process-args process
     Returns the value of the program arguments component of the process object PROCESS.

 -- Function: set-process-args process arg-list
     Sets the value of the program arguments component of the process object PROCESS to the list ARG-LIST, then returns ARG-LIST.

 -- Function: process-dir process
     Returns the value of the directory component of the process object PROCESS.

 -- Function: set-process-dir process directory
     Sets the value of the directory component of the process object PROCESS to the string DIRECTORY, then returns DIRECTORY.

 -- Variable: process-environment
     This is a list of environment variable definitions, as well as being used by the setenv and getenv functions (see Environment Variables), it also provides the environment of all started subprocesses.

          (car process-environment)
              => "LOGNAME=john"

 -- Function: active-processes
     Returns a list containing all active (i.e. running or stopped) process objects.

Asynchronous Processes
----------------------

An asynchronous process is one that runs in parallel with Lisp evaluation, basically this means that once the subprocess has been started (by the start-process function) librep will carry on as normal.

The event loop checks for output from asynchronous processes, any found is copied to the process' output stream, and calls the the process' state change function when necessary (see Process States). Alternatively the accept-process-output function can be called to explicitly allow output to be processed.

When using asynchronous processes you have a choice as to the Unix mechanism used to connect the stdin, stdout and stderr streams of the subprocess to librep's process.

The two options currently available are pipes or pseudo-terminals; in general pseudo-terminals should only be used to provide a direct interface between the user and a process (i.e. the *shell* buffer) since they allow job control to work properly. At other times pipes will be more efficient and are used by default. However, there are cases where the buffering characteristics of pipes mean that ptys must be used.

 -- Function: start-process #!optional process program #!rest args
     This function starts an asynchronous subprocess running on the process object PROCESS. If PROCESS is undefined a new process object is created (by calling the function make-process with all arguments undefined).

     The function always returns the process object which the subprocess has been started on. If for some reason the subprocess can't be created an error of type process-error is signalled.

     The optional argument PROGRAM is a string defining the name of the program to execute, it will be searched for in all the directories in the PATH environment variable. The ARGS are strings to pass to the subprocess as its arguments.

     When defined, the optional arguments overrule the values of the related components of the process object.

     The following example runs the ls program asynchronously, its output is sent to the standard-output stream.

          (let
              ((process (make-process standard-output)))
            (start-process process "ls" "-s"))

Note that when librep exits it kills all of its asynchronous subprocesses which are still running without warning.

 -- Function: process-connection-type process
     Returns the value of the connection type component of the process object PROCESS. See the documentation of the set-process-connection-type function for the values this may take.

 -- Function: set-process-connection-type process symbol
     Sets the value of the connection type component of the process object PROCESS to SYMBOL, then returns SYMBOL.

     SYMBOL should be one of the following symbols,

     pty
          Use pseudo-terminals to connect to subprocesses running asynchronously on this process object.

     pipe
          Use standard Unix pipes to connect, this is the default value of this component.

     socketpair
          Uses a connected pair of sockets.

Note that currently only the pipe connection type allows the normal and error output streams of the process to be separated.

Synchronous Processes
---------------------

When a synchronous process is started librep waits for it to terminate before continuing; they are usually used when a Lisp program must invoke an external program as part of its function, i.e. the auto-compression feature runs the compression program gzip synchronously when it needs to compress a buffer.

Unlike asynchronous processes their is no choice between pipes and pseudo-terminals for connecting to a subprocess. Instead, it is possible to link the stdin channel of a synchronous process to a named file.

 -- Function: call-process #!optional process input-file-name program #!rest args
     This function starts a process running on the process object PROCESS. If PROCESS is undefined a new process object is created by calling the make function.

     If defined, the string INPUT-FILE-NAME names the file to connect to the standard input of the subprocess, otherwise the subprocess' input comes from the null device (/dev/null on UNIX).

     The optional arguments PROGRAM and ARGS define the name of the program to invoke and any arguments to pass to it. The program will be searched for in all directories listed in the process-environment variable.

     If any of the optional parameters are unspecified they should have been set in the PROCESS-OBJECT prior to calling this function.

     After successfully creating the new subprocess, this function simply copies any output from the process to the output stream defined by the output stream component of the process object. When the subprocess exits its exit-value is returned (an integer). Note that the exit-value is the value returned by the process-exit-value function, see Process Information.

     If, for some reason, the new subprocess can't be created an error of type process-error is signalled.

The following function definition is taken from the gzip.jl file, it shows how the call-process function can be used to uncompress a file into a buffer (for Jade).

     ;; Uncompress FILE-NAME into the current buffer
     (defun gzip-uncompress (file-name)
       (let
           ((proc (make-process (current-buffer))))
         (message (concat "Uncompressing `" file-name "'") t)
         ;; gunzip can do .Z files as well
         (unless (zerop (call-process proc nil "gunzip" "-c" file-name))
           (signal 'file-error (list "Can't gunzip file" file-name)))))

The user is able to interrupt synchronous subprocesses (for example if they seem to have got wedged somehow). Each time a user-interrupt is received by librep (i.e. the INT signal), a stronger signal is sent to the subprocess. First an interrupt signal, then a termination signal, before finally a non-ignoreable quit signal is sent.

Process I/O
-----------

It is only possible for lisp programs to explicitly send input data to asynchronous processes (by the time it's possible to call a function to send data to a synchronous process, the process will already have terminated!). Simply use the process object which an asynchronous process is running on as a normal Lisp input stream, any strings or characters written to the stream will immediately be copied to the stdin channel of the subprocess.

With synchronous processes, the only control over input data possible is by giving the call-process function the name of a file containing the subprocess' input data.

Output data from subprocesses is handled the same way by both asynchronous and synchronous processes: it is simply copied to the stream defined by the output stream component of the subprocess' process object.

 -- Function: process-output-stream process
     Returns the value of the output stream component of the process object PROCESS.

 -- Function: set-process-output-stream process stream
     Sets the value of the output stream component of the process object PROCESS to the stream STREAM, then returns STREAM.

By default the stdout and stderr streams are combined, use the set-process-error-stream function to separate them. (Note that this currently only works with pipe connection types.)

 -- Function: process-error-stream process
     Returns the value of the error stream component of the process object PROCESS.

 -- Function: set-process-error-stream process stream
     Sets the value of the error stream component of the process object PROCESS to the stream STREAM, then returns STREAM.

Output from asynchronous subprocesses (this includes changes of state as well as stream output) is only propagated at well-defined times. Either when in the read stage of the read-eval-print, or input, loop, or when the accept-process-output or sit-for functions are called.

 -- Function: accept-process-output #!optional seconds milliseconds
     Wait SECONDS plus MILLISECONDS for output from any asynchronous subprocesses. If any arrives, process it, then return false. Otherwise return true. If either of the arguments is undefined, they count as zero in the addition.

 -- Function: sit-for #!optional seconds milliseconds
     Wait for input to arrive and be processed. No more than SECONDS seconds plus MILLISECONDS milliseconds will be waited. If at the end of this time no input has arrived, return true. Otherwise return false if input was found.

     Note that this function is only distinct to accept-process-output when librep is embedded in another application, or an extension has been loaded that provides an event loop (such as the gtk binding). In this case other input forms, such as user input, for example, can preempt the timeout.

     This function is exported by the rep.system module.

See Streams.

Process States
--------------

Each process object has a state associated with it; this depends on the status of the subprocess currently running on the process object (or not as the case may be).

The possible states are,

running
     This state means that the subprocess using this process object is currently running, i.e. it hasn't been stopped.

stopped
     Means that the subprocess has been temporarily suspended from running.

unused
     This means that the process object is free to have a new subprocess created on it.

Predicates exist which test whether a given process object is in one of these states.

 -- Function: process-running-p process-object
     Returns true when PROCESS-OBJECT is in the running state.

 -- Function: process-stopped-p process-object
     Returns true when PROCESS-OBJECT is in the stopped state.

 -- Function: process-in-use-p process-object
     Returns true when PROCESS-OBJECT is not in the unused state.

The following two functions are used to stop and then subsequently continue a process running.

 -- Function: stop-process process-object #!optional whole-group
     This function suspends execution of the subprocess running on the process object PROCESS-OBJECT.

     If WHOLE-GROUP is true all subprocesses in the process group of PROCESS-OBJECT are stopped.

 -- Function: continue-process process-object #!optional whole-group
     Use this function to continue a subprocess executing after it has been stopped (by the stop-process function).

     If WHOLE-GROUP is true all subprocesses in the process group of PROCESS-OBJECT are continued.

The state change function component of a process object defines a function which will be called each time the state of the process object changes. If your program needs to be informed when an asynchronous process terminates this function is the way to do it.

 -- Function: process-function process
     Returns the value of the state change function component of the process object PROCESS.

 -- Function: set-process-function process function
     Sets the value of the state change function component of the process object PROCESS to the function FUNCTION, then returns FUNCTION.

Signalling Processes
--------------------

 -- Function: interrupt-process process-object #!optional whole-group
     Sends the SIGINT signal to PROCESS-OBJECT.

 -- Function: kill-process process-object #!optional whole-group
     Sends the SIGKILL signal to the PROCESS-OBJECT.

Note that the functions stop-process and continue-process also send signals to the subprocess.

 -- Function: signal-process process signal #!optional whole-group
     Send the signal SIGNAL to the process PROCESS; if WHOLE-GROUP is true the signal is also sent to all processes in the process group of PROCESS.

     PROCESS may be either a Lisp process object, or an integer defining the pid of the process to signal (not necessarily started by librep).

     SIGNAL may either be an integer defining the actual signal number, or a symbol naming the signal. All names are as usual but with the preceding SIG removed, for example the SIGINT signal would be sent by using the symbol INT. If a named signal doesn't exist on the current operating system, an error is raised.

     Returns true if the signal was sent successfully.

As with the UNIX kill system call, signal-process may also be used to test whether a process with a particular pid is currently active, by using a signal with value zero.

Process Information
-------------------

 -- Function: process-id process-object
     This function returns the operating-system identifier associated with the subprocess currently running on the process object PROCESS-OBJECT.

 -- Function: process-exit-value process-object
     Returns the integer representing the return code of the last subprocess to be run on PROCESS-OBJECT.

     If no subprocess has been run on PROCESS-OBJECT, PROCESS-OBJECT is currently in the running state or the last subprocess exited abnormally (i.e. from a terminal signal) false is returned.

 -- Function: process-exit-status process-object
     This function returns the integer that was the exit status of the last subprocess which was run on the process object PROCESS-OBJECT.

     Note that the exit status is not the value given to the exit function in a C program, use the process-exit-value to access this value.

     If no process has been run on PROCESS-OBJECT, or the process is currently in the running state false is returned.

Regular Expressions
===================

Regular expressions (or regexps) are a powerful method of matching patterns in strings. librep uses the regexp(3) implementation by Henry Spencer, with some modifications that I have made. It comes with this banner:

     Copyright (c) 1986 by University of Toronto.

     Written by Henry Spencer.  Not derived from licensed software.

     Permission is granted to anyone to use this software for any purpose on any computer system, and to redistribute it freely, subject to the following restrictions:

      1. The author is not responsible for the consequences of use of this software, no matter how awful, even if they arise from defects in it.

      2. The origin of this software must not be misrepresented, either by explicit claim or by omission.

      3. Altered versions must be plainly marked as such, and must not be misrepresented as being the original software.

Regular Expression Syntax
-------------------------

The syntax of a regular expression is as follows (this is adapted from the manual page):

A regular expression is zero or more branches, separated by |. It matches anything that matches one of the branches.

A branch is zero or more pieces, concatenated. It matches a match for the first, followed by a match for the second, etc.

A piece is an atom possibly followed by *, +, or ?. An atom followed by * matches a sequence of 0 or more matches of the atom. An atom followed by + matches a sequence of 1 or more matches of the atom. An atom followed by ? matches a match of the atom, or the null string.

An atom is a regular expression in parentheses (matching a match for the regular expression), a range (see below), . (matching any single character), ^ (matching the null string at the beginning of the input string), $ (matching the null string at the end of the input string), one of the strings \s, \S, \w, \W, \d, \D, \b, \B, or a \ followed by a single character (matching that character), or a single character with no other significance (matching that character).

A range is a sequence of characters enclosed in []. It normally matches any single character from the sequence. If the sequence begins with ^, it matches any single character not from the rest of the sequence. If two characters in the sequence are separated by -, this is shorthand for the full list of ASCII characters between them (e.g. [0-9] matches any decimal digit). To include a literal ] in the sequence, make it the first character (following a possible ^). To include a literal -, make it the first or last character.

Also, any of the *, + or ? operators can be suffixed by a ? character (i.e. *?, +?, ??). The meaning of the operator remains the same but it becomes non-greedy. This means that it will match the smallest number of characters satisfying the regular expression, instead of the default behaviour which is to match the largest.

The backslash-introduced atoms have the following meanings:

\s
     Match any whitespace character.

\S
     Match any non-whitespace character.

\w
     Match any alphanumeric or underscore character.

\W
     Match any non-(alphanumeric or underscore) character.

\d
     Match any numeric character.

\D
     Match any non-numeric character.

\b
     Match the null string between two adjacent \w and \W characters (in any order).

\B
     Match the null string that is not between two adjacent \w and \W characters.

Some example legal regular expressions could be:

ab*a+b
     Matches an a followed by zero or more b characters, followed by one or more a characters, followed by a b. For example, aaab, abbbab, etc...

(one|two)_three
     Matches one_three or two_three.

^cmd_[0-9]+
^cmd_\d+
     Matches cmd_ followed by one or more digits, it must start at the beginning of the line.

Regexp Functions
----------------

These functions are exported by the rep.regexp module.

 -- Function: quote-regexp string
     Return a version of STRING, such that when used as a regexp, it will match the original contents of STRING verbatim, and nothing else. This involves quoting regexp meta-characters.

          (quote-regexp "abc")
              => "abc"

          (quote-regexp "a+c")
              => "a\\+c"

 -- Function: string-match regexp string #!optional start ignore-case
     Returns true if the string STRING matches the regular expression REGEXP. The string matches if executing the regexp at any position in the string succeeds.

     When defined, START is the index of the first character to start matching at (counting from zero). When IGNORE-CASE is true the case of matched strings are ignored. Note that character classes are still case-significant.

          (string-match "ab+c" "abbbc")
              => t

          (string-match "ab+c" "xxxabbbcyyy")
              => t

 -- Function: string-looking-at regexp string #!optional start ignore-case
     Similar to string-match, but only returns true if STRING matches REGEXP starting at the character at index START in the string (or the first character if START is undefined).

          (string-looking-at "ab+c" "abbbc" 0)
              => t

          (string-looking-at "ab+c" "xxxabbbcyyy" 0)
              => ()

          (string-looking-at "ab+c" "xxxabbbcyyy" 3)
              => t

 -- Function: match-start #!optional n
     Returns the position at which the N'th parenthesised expression started in the last successful regexp match. If N is false or zero the position of the start of the whole match is returned instead.

     When matching strings, all positions are integers, with the first character in the string represented by zero. However, extensions that allow regexps to be matched against other textual inputs may return different position values.

          (string-match "x*(foo|bar)y" "xxxbary")
              => t

          (match-start 1)
              => 3

 -- Function: match-end #!optional n
     Similar to match-start, but returns the position of the character following the matched item.

          (string-match "x*(foo|bar)y" "xxxbary")
              => t

          (match-end 1)
              => 6

A common use of regular expressions is to match a string, then replace certain portions of the string with other text.

 -- Function: expand-last-match template
     Expand the TEMPLATE substituting the parenthesised expressions from the most recent successfully matched regular expression.

     TEMPLATE may contain the following substitution-inducing escape sequences:

     \0
     \&
          Substitute the whole string matched by the last regexp

     \N
          Substitute the N'th parenthensised expression, where 1 <= N <= 9.

     \\
          Substitute a single backslash character.

          (string-match "x*(foo|bar)y" "xxxbary")
              => t

          (expand-last-match "test-\\1-ing")
              => "test-bar-ing"

     Note that double backslashes are required due to the read syntax of strings (see Strings).

 -- Function: string-replace regexp template string
     Returns the string created by replacing all matches of REGEXP in STRING with the result of expanding TEMPLATE using the expand-last-match function.

          (string-replace "-" "_" "foo-bar-baz")
              => "foo_bar_baz"

          (string-replace "&(optional|rest)" "#!\\1" "(a &optional b &rest c)")
              => "(a #!optional b #!rest c)"

Time and Date
=============

This section describes how time and date values are handled in librep.

Timestamps
----------

As in UNIX, librep measures time as the number of seconds since January 1st, 1970 (known as the epoch). For historical reasons rep stores timestamps as a pair of integers, using a cons cell.

The first integer records the number of whole days since the epoch, the second records the number of seconds since the start of the day (in universal time).

These function are exported by the rep.system module:

 -- Function: current-time
     Return the number of seconds since the epoch, in a cons-cell.

          (current-time)
              => (10744 . 61063)

 -- Function: fix-time timestamp
     Ensure that the two parts of TIMESTAMP (a pair or integers) are consistent, simply that the number of seconds is less than the number of seconds in a whole day. If not, the timestamp is adjusted to meet this constraint.

 -- Function: time-later-p timestamp-1 timestamp-2
     Returns true if TIMESTAMP-1 is later than TIMESTAMP-2.

On the plus side, this scheme won't wrap around as quickly as UNIX's time_t will ;-)

The rep.util.time module also provides some functions for manipulating timestamps:

 -- Function: time->seconds timestamp
     Convert TIMESTAMP to an integer, the number of seconds since the epoch that it represents.

 -- Function: seconds->time seconds
     Convert from an integer SECONDS to a timestamp object.

 -- Function: time- timestamp-1 timestamp-2
     Return the number of seconds difference between TIMESTAMP-1 and TIMESTAMP-2.

 -- Constant: seconds-per-day
     The number of seconds in a 24-hour day.

Formatting Dates
----------------

Given a timestamp value it is possible to format it as a string, in many different formats.

 -- Function: current-time-string #!optional timestamp format
     Return a string defining TIMESTAMP according to the string FORMAT. If TIMESTAMP is undefined, the current time is used.

     The FORMAT string may include any of the formatting characters from the C library's strftime(3) function. If undefined a standard, fixed-width, format is used:

          (current-time-string)
              => "Wed Jun  2 18:07:53 1999"

     Some of the possible formatting substitutions include (this is copied from the GNU libc manual, see (libc)Formatting Date and Time):

     %a
          The abbreviated weekday name according to the current locale.

     %A
          The full weekday name according to the current locale.

     %b
          The abbreviated month name according to the current locale.

     %B
          The full month name according to the current locale.

     %c
          The preferred date and time representation for the current locale.

     %d
          The day of the month as a decimal number (range 01 to 31).

     %H
          The hour as a decimal number, using a 24-hour clock (range 00 to 23).

     %I
          The hour as a decimal number, using a 12-hour clock (range 01 to 12).

     %j
          The day of the year as a decimal number (range 001 to 366).

     %m
          The month as a decimal number (range 01 to 12).

     %M
          The minute as a decimal number.

     %p
          Either am or pm, according to the given time value; or the corresponding strings for the current locale.

     %S
          The second as a decimal number.

     %U
          The week number of the current year as a decimal number, starting with the first Sunday as the first day of the first week.

     %W
          The week number of the current year as a decimal number, starting with the first Monday as the first day of the first week.

     %w
          The day of the week as a decimal number, Sunday being 0.

     %x
          The preferred date representation for the current locale, but without the time.

     %X
          The preferred time representation for the current locale, but with no date.

     %y
          The year as a decimal number, but without a century (range 00 to 99).

     %Y
          The year as a decimal number, including the century.

     %Z
          The time zone or name or abbreviation (empty if the time zone can't be determined).

     %%
          A literal % character.

          (current-time-string nil "%Y-%m-%d")
              => "1999-06-02"

Parsing Dates
-------------

The date Lisp library provides rudimentary support for parsing date and time descriptions to their individual components, and to timestamps. Evaluate the form (require 'date) to load this library.

 -- Function: parse-date string #!optional start
     Returns a vector encoding the date described by STRING. If START is defined, it specifies the index of the character in the string to start parsing from.

     Each element of the vector contains a separate component of the overall point in time described by the string. The indices of these elements are defined by the following constants:

     date-vec-day-abbrev
          The abbreviated name of the day of the week.

     date-vec-day
          The numeric day of the month, counting from one.

     date-vec-month-abbrev
          The abbreviated name of the month.

     date-vec-month
          The numeric month of the year, counting from January equals one.

     date-vec-year
          The numeric year.

     date-vec-hour
          The numeric hour of the day.

     date-vec-minute
          The numeric minute of the hour.

     date-vec-second
          The numeric second of the minute.

     date-vec-timezone
          If true, a string defining the timezone.

     date-vec-epoch-time
          The timestamp (see Timestamps), including the effects of the timezone, if given.

          (current-time-string)
              => "Wed Jun  2 18:37:17 1999"

          (parse-date (current-time-string))
              => ["Wed" 2 "Jun" 6 1999 18 37 17 0 (10744 . 67037)]

          (parse-date "1999-06-02")
              => ["Tue" 2 "Jun" 6 1999 0 0 0 0 (10744 . 0)]

          (parse-date "June 6, 1999")
              => ["" 0 "Jun" 6 1999 0 0 0 0 (10742 . 0)]

          (aref (parse-date "June 6, 1999") date-vec-epoch-time)
              => (10742 . 0)

XXX provide more information on accepted formats, outputs for incomplete descriptions, etc...

Internationalisation
====================

librep has support for internationalisation (or i18n) of text messages, using the GNU gettext implementation (see Overview (The GNU gettext Manual)), a run-time library managing the mapping between text strings in the programmer's native language and in the language of the end user.

Three functions are provided to access the message catalogues maintained by GNU gettext. Import the rep.i18n.gettext module to load them.

 -- Function: _ string
     Attempt to find a native language equivalent of STRING. If no equivalent is found the original string is returned.

     Note that this function is always defined, even if the gettext module hasn't been required. In this case it always returns the original string.

 -- Function: bindtextdomain domain directory
     Tell gettext that message catalogues for message domain DOMAIN (a string) can be found under the directory called DIRECTORY.

 -- Function: textdomain domain
     Note that any strings that are to be translated in the future (until the next call to textdomain) are in the domain called DOMAIN (a string).

The usual method of constructing message catalogue templates (.pot files) is to run xgettext on the C source files of the program (that have been annotated for i18n). librep provides the rep-xgettext program to perform the same task for files of Lisp code.

System Information
==================

These definitions are all exported by the rep.system module.

 -- Variable: operating-system
     A symbol naming the current operating system. The only current option is unix.

 -- Function: system-name
     This function returns a string naming the host that the interpreter is running on. When possible this be a fully-qualified name (i.e. including the domain)

 -- Variable: rep-build-id
     A string describing the environment under which librep was built. This will always have the format DATE by USER@HOST, for ARCH..

          rep-build-id
              => "Mon May 17 1999 by john@tizer.dcs.warwick.ac.uk, for sparc-sun-solaris2.6."

 -- Variable: rep-version
     A string describing the current release of librep.

          rep-version
              => "1.0"

User Information
================

These functions are exported by the rep.system module.

 -- Function: user-login-name
     This function returns a string containing the login name of the user.

          (user-login-name)
              => "john"

 -- Function: user-full-name #!optional real-name
     This function returns a string containing the `real' name of the user; the format of the string will depend on the host system.

     If REAL-NAME is a string, it defines the name that will be returned by subsequent calls to this function.

          (user-full-name)
              => "John Harper"

 -- Function: user-home-directory #!optional user
     This function returns the home directory of the user whose login name is USER, or the current user if USER is undefined. The returned string will be as returned by file-name-as-directory (i.e. terminated by a / character under UNIX)

          (user-home-directory)
              => "/home/john/"

Environment Variables
=====================

These functions are exported by the rep.system module.

 -- Function: getenv variable-name
     This function returns the value (a string) of the environment variable called VARIABLE-NAME. If the specified variable doesn't exist false is returned.

          (getenv "OSTYPE")
              => "Linux"

 -- Function: setenv variable-name new-value
     This function sets the value of the environment variable called VARIABLE-NAME to NEW-VALUE. NEW-VALUE can either be a string containing the new contents of the variable or false, in which case the environment variable is deleted.

 -- Function: unsetenv variable-name
     Deletes any variable in process-environment named VARIABLE-NAME.

See also Process Objects for the description of the process-environment variable.

String Functions
================

 -- Function: translate-string string map
     Applies the MAP to each character in the STRING. MAP is also string, each character represents the translation for an ASCII character of that characters position in the string. If the string is less than 256 chars long any undefined characters will remain unchanged.

     For example, if STRING contains the character A, with ASCII code 65, then it would be replaced by the 65th character in the string MAP.

     Note that the STRING really is modified, no copy is made

 -- Variable: upcase-table
     A translate-string compatible translation map to convert lowercase characters to uppercase characters.

 -- Variable: downcase-table
     A map to convert uppercase characters to lowercase.

 -- Variable: flatten-table
     A translation map to convert newline characters to spaces.

     (translate-string "Foo" upcase-table)
         => "FOO"

     (translate-string "Foo" downcase-table)
         => "foo"

 -- Function: complete-string template list #!optional ignore-case
     Return a string whose beginning matches the string TEMPLATE, and is unique in the set of all strings in LIST which also match TEMPLATE. If IGNORE-CASE is true, all matching ignores case of characters.

          (complete-string "foo" '("bar" "foobar" "forbarf" "foobat"))
              => "fooba"

 -- Function: string-head-eq string-1 string-2
     Returns t if STRING-2 matches the beginning of STRING-1.

          (string-head-eq "foobar" "foo")
              => t

          (string-head-eq "foo" "foobar")
              => ()

 -- Function: string-upper-case-p string
     Return true if STRING contains no lower case characters.

 -- Function: string-lower-case-p string
     Return true if STRING contains no upper case characters.

 -- Function: string-capitalized-p string
     Return true if the first character of STRING is upper case.

 -- Function: string-upcase string
     Return a new string, an upper case copy of STRING.

 -- Function: string-downcase string
     Return a new string, a lower case copy of STRING.

 -- Function: capitalize-string string
     Return a new string, a copy of STRING with the first character in upper case.

 -- Function: mapconcat function sequence separator
     Call FUNCTION for each member of SEQUENCE, concatenating the results. Between each pair of results, insert SEPARATOR. Return the resulting string.

Sleeping
========

 -- Function: sleep-for seconds #!optional milliseconds
     Pause for a SECONDS (plus the optional MILLISECONDS component) long period of time. Input becoming available will not break the sleep (see Process I/O).

     This function is exported by the rep.system module.

Beeping
=======

Use this function to attract the user's attention.

 -- Function: beep
     Ring a bell somewhere.

Messages
========

The message function will show the user a small message (typically no more than a single column of text). In graphical applications it won't bring up a separate window, only displaying the text in a status bar or something similar. In a console-based environment, the message will be printed to the stderr stream, followed by a line break.

 -- Function: message #!optional display-now
     Displays a one-line message, the string MESSAGE. If DISPLAY-NOW, every effort will be made to display the message as soon as possible, possibly before the next scheduled screen update (if applicable).

     This function is exported by the rep.system module.

Command Line Options
====================

As noted earlier any unused command line arguments are made available to scripts through the command-line-args variable (see Invocation).

 -- Variable: command-line-args
     The list of unused command line arguments, in their original order.

The get-command-line-option function may be used to scan this list of arguments. The rep.system module exports this function.

 -- Function: get-command-line-option option #!optional requires-arg
     Returns t if OPTION was specified on the command line (OPTION is typically a phrase beginning with --).

     If REQUIRES-ARG is true, the option requires a parameter, the value of which is returned. If a parameter isn't supplied an error is signalled.

     (setq command-line-args '("--foo" "bar"))
         => ("--foo" "bar")
     (get-command-line-option "--foo" t)
         => "bar"
     command-line-args
         => ()

     (setq command-line-args '("--foo=bar"))
         => ("--foo=bar")
     (get-command-line-option "--foo" t)
         => "bar"
     command-line-args
         => ()

Executing Shell Commands
========================

The subprocess handling of librep provides a comprehensive interface to starting and controlling external processes (see Processes). However it can be overkill when all that is required is to invoke a shell command, with its I/O going to the same places as the interpreter's.

 -- Function: system command
     Execute the shell command COMMAND synchronously, returning its exit status. An error will be signalled if the shell process could not be started.

     The stdin, stdout and stderr streams of the shell are left as in the interpreter process.

     The subprocesses environment is copied from the current value of the process-environment variable.

Note that the exit status is not the same as the return code of the command. It depends on the operating system, but under UNIX the return code can be found through right-shifting the exit status by eight bits. Low non-zero values represent that the process was killed by a signal.

It is possible to interrupt a running shell process in the same way as with a normal synchronous process (see Synchronous Processes). Interrupt the interpreter, it will send progressively harder-to-ignore signals to the child each interrupt, until it is eventually terminated.

Asynchronous Timers
===================

The rep.io.timers module (see Modules) allows a Lisp program to create multiple asynchronous timers, each of which is primed to call a specified function after a specified period of time. These functions only work when the Lisp event loop is being used (i.e. at least one recursive-edit is currently in progress).

 -- Function: make-timer function #!optional seconds milliseconds
     Create and return a new timer object. It will be set to call the Lisp function FUNCTION after SECONDS seconds plus MILLISECONDS milliseconds. FUNCTION will be called with a single argument, the timer object that has just fired.

     If both SECONDS and MILLISECONDS are undefined, or zero, the timer will be created but won't call FUNCTION.

     After the time interval has passed, and FUNCTION has been called, the timer will not be restarted. Use the set-timer function to reset it.

 -- Function: delete-timer timer
     Prevent the timer object TIMER from calling the Lisp function associated with it. Use the set-timer function to reset it.

 -- Function: set-timer timer #!optional seconds milliseconds
     Reset the timer object TIMER. If either/both of SECONDS and MILLISECONDS are defined the interval of the timer will be set to the specified time period. If neither are defined then the current interval of the timer is preserved.

Debugging
=========

When you have written a Lisp program you will have to debug it (unless all your programs work first time?). There are two main classes of errors; syntax errors and semantic errors.

Syntax errors occur when the text you've typed out to represent your program is not a valid representation of a Lisp object (since a program is simply an ordered set of Lisp objects). When you try to load your program the Lisp reader will find the syntax error and tell you about, unfortunately though it probably won't be able to tell you exactly where the error is.

The most common source of syntax errors is too few or too many parentheses; the Jade or Emacs Ctrl-Meta-f and Ctrl-Meta-b commands can be used to show the structure of the program as the Lisp reader sees it.

Semantic errors are what we normally call bugs---errors in logic, the program is syntactically correct but doesn't do what you want it to. For these types of errors librep provides hooks to allow interactive debugging. The debugger supplied with librep uses these hooks to implement a simple command line debugger; programs using librep as an extension language may provide their own debugger interface.

There are several ways to enter the Lisp debugger; functions can be marked so that they cause the debugger to be entered when they are called, breakpoints can be written in functions or it can be called explicitly with a form to step through.

 -- Command: trace symbol
     This command marks the symbol SYMBOL so that each time its value is dereferenced the debugger is entered when the next form is evaluated. This can be used to set breakpoints on functions (or variables).

     When called interactively SYMBOL is prompted for.

 -- Command: untrace symbol
     The opposite of trace---unmarks the symbol.

 -- Function: break
     This function causes the debugger to be entered immediately. By putting the form (break) at suitable points in your program simple breakpoints can be created.

 -- Command: step form
     This function invokes the debugger to step through the form FORM.

     When called interactively FORM is prompted for.

 -- Function: backtrace #!optional stream
     Prints a description of the current Lisp function call stack to STREAM (or standard-output if STREAM is undefined).

          (backtrace (current-buffer))
               -| #<subr backtrace> ((current-buffer)) nil
               -| #<closure eval-and-print> ((backtrace (current-buffer))) t
               => t

     Each line represents a stack frame, the first item is the called function, the second is the list of arguments applied to it. The third item is true if the list of arguments as displayed has already been evaluated.

Whenever the Lisp debugger is entered the form waiting to be evaluated is printed, preceded by the current depth of execution in angular brackets. At this point the special debugger commands available are,

step
s
     Step into the current form; this means that in a list form the debugger is used to evaluated each argument in turn.

next
n
     Continue evaluating forms normally until the next form at the current level is entered, then re-enter the debugger.

continue
c
     Continue execution normally. Note that this command is the one to use when an error has been trapped.

return FORM
r FORM
     Evaluate FORM then return this value as the result of the current form.

print FORM
p FORM
     Evaluate FORM, then print its value.

form
f
     Print the form being debugged.

backtrace
b
     Print a backtrace of the current Lisp call stack.

Entering a null string repeats the previous next, step, or continue command.

After the form has been evaluated (i.e. after you've typed one of the commands above) the value of the form is printed in the buffer, prefixed by the string => .

Note that it is also possible to make certain types of errors invoke the debugger immediately they are signalled, see Errors. Also note that the debugger is unable to step through compiled Lisp code.

Tips
====

This section of the manual gives advice about programming in librep.

For advice on getting the most out of the compiler, see Compilation Tips.

Comment Styles
--------------

As already described, single-line comments in Lisp are introduced by a semi-colon (;) character. By convention a different number of semi-colons is used to introduce different types of comments,

;
     A comment referring to the line of Lisp code that it occurs on, comments of this type are usually indented to the same depth, on the right of the Lisp code. When editing in Jade's Lisp mode the command Meta-; can be used to insert a comment of this type.

     For example,

          (defconst op-call #x08)         ;call (stk[n] stk[n-1] ... stk[0])
                                          ; pops n values, replacing the
                                          ; function with the result.
          (defconst op-push #x10)         ;pushes constant # n

;;
     Comments starting with two semi-colons are written on a line of their own and indented to the same depth as the next line of Lisp code. They describe the following lines of code.

     For example,

          (let
              ((fname (concat file-name ?c)))
            ;; Be sure to remove any partially written dst-file.
            (when (file-exists-p fname)
              (delete-file fname)))

     Comments of this type are also placed before a function definition to describe the function. This saves wasting memory with a documentation string in a module's internal functions.

     For example,

          ;; Compile a form which occurred at the `top-level' into a
          ;; byte code form.
          ;; defuns, defmacros, defvars, etc... are treated specially.
          ;; require forms are evaluated before being output uncompiled;
          ;; this is so any macros are brought in before they're used.
          (defun comp-compile-top-form (form)
            ...

;;;
     This type of comment always starts in the first column of the line, they are used to make general comments about a program and don't refer to any function or piece of code in particular.

     For example,

          ;;; Notes:

          ;;; Instruction Encoding
          ;;; ====================
          ;;; Instructions which get an argument (with opcodes of zero up to
          ...

;;;;
     Each program should have a comment of this type as its first line, the body of the comment is the name of the file, two dashes and a brief description of what the program does. They always start in the first column.

     For example,

          ;;;; compiler.jl -- Simple compiler for Lisp files/forms

If you adhere to these standards the indentation functions provide by the Lisp mode will indent your comments to the correct depth.