A parser generator in Python

# A parser generator in Python: kwParsing

> Aaron Watters

> This is the documentation for the kjParsing package, an experimental parser generator implemented in Python which generates parsers implemented in Python. It won't serve as a complete reference on programming language syntax and interpretation, but it will review terminology for the knowledgable and I hope it will pique the interest of the less experienced.

## Introduction

The `kjParsing` package is a parser generator written in Python which generates parsers for use in Python.

These modules and their documentation and demo files may be of use for classes on parsing, compiling, or formal languages, and may also be helpful to people who like to create experimental interpreters or translators or compilers.

The package consists of three Python modules: `kjParser, kjParseBuild,` and `kjSet`. Together these modules are called the `kjParsing` package. The package also includes some documentation and demo files and a `COPYRIGHT` file which explains the conditions for copying and propagating this code and the fact that the author assumes no responsibility for any difficulties resulting from the use of this package by anyone (including himself).

## What a Parser Does

Parsers can be part of a lot of different things: compilers, interpreters, translators, or code generators, among others. Nevertheless, at an abstract level parsers all translate expressions of a language into computational actions.

Parsers generated by the `kjParseBuild` module may do three different sorts of actions:

Value Computation
The parser may build a data structure as the result of the expression. For example the silly `LispG` grammar from the file ```DLispShort.py''` can construct integers, strings and lists from string representations.
    >>> from DLispShort import LispG, Context
    >>> LispG.DoParse1( ' ("list with string and int" 23) ', Context)
    ['list with string and int', 23]
    >>>
Environment Modification
The parser may modify the context of the computation. For example the `LispG` grammar allows the assignment of values to internal variable names.
    >>> LispG.DoParse1( '(setq Variable (4 5 9))', Context)
    [4, 5, 9]
    >>> Context['Variable']
    [4, 5, 9]
    >>>
(Here the second result indicates that the string `'Variable'` has been associated with the value `[4,5,9]` in the `Context` structure, which in this case is a simple python dictionary.)
External Side Effects
The parser may also perform external actions. For example the `LispG` grammar has the ability to print values to the terminal.
    >>> LispG.DoParse1( '( (print Variable) (print "bye bye")  )', Context )
    [4, 5, 9]
    bye bye
    [[4, 5, 9], 'bye bye']
    >>>
(Here the first two lines are the results of printing and the last is the value of the expression.)

More realistic parsers will perform more interesting actions, of course.

To implement a parser using `kjParseBuild` you must define the grammar to parse and associate each rule and terminal of the grammar with an action which defines the computational meaning of each language construct.

The grammar generation process consists of two phases

Generation
During this phase you must define the syntax of the language and function bindings that define the semantics of the language. When you've debugged the syntax and semantics you can dump the grammar object representing the syntax only to a grammar file which can be reloaded without re-analyzing the language syntax. For large grammars each regeneration may require significant time and computational resources.
Use
During this phase you may load the grammar file without re-analyzing the grammar on each use. However, the semantics functions must still be rebound on each load. The reloaded grammar object augmented with interpretation functions may be used to parse strings of the language.

Note that the functions that define the semantics of the language are must be bound in both phases.

               # from file DLispShort.py (with small differences)
         ( 1)  def GrammarBuild():
         ( 2)      import kjParseBuild
         ( 3)      LispG = kjParseBuild.NullCGrammar()
         ( 4)      LispG.SetCaseSensitivity(0)
         ( 5)      DeclareTerminals(LispG)
         ( 6)      LispG.Keywords("setq print")
         ( 7)      LispG.punct("().")
         ( 8)      LispG.Nonterms("Value ListTail")
         ( 9)      LispG.comments([LISPCOMMENTREGEX])
         (10)      LispG.Declarerules(GRAMMARSTRING)
         (11)      LispG.Compile()
                   print "dumping as binary to TESTLispG.mar"
         (12)      outfile = open("TESTLispG.mar", "w")
         (13)      LispG.MarshalDump(outfile)
         (14)      outfile.close()
         (15)      BindRules(LispG)
         (16)      return LispG
A function for building simple grammar (GrammarBuild)

## Defining a Grammar

A programming language grammar is conventionally divided into several components:

Keywords
These are special strings that ``highlight'' a language construct. Familiar keywords from Python and Pascal and C are ``if'', ``else'', and ``while''.
Terminals
These are special patterns of characters that indicate a value in the language. For example many programming languages will classify the string `123` as an instance of the integer nonterminal and the string `snark` (not contained in quotes) as an instance of the nonterminal identifier or variable. Terminals are usually restricted to very simple constructs like identifiers, numbers, and strings. More complex things (such as a ``date'' data type) might be better handled by nonterminals and rules.
Nonterminals
These are ``place holders'' for language constructs of the grammar. They represent parts of the grammar which sometimes expand to great size and complexity. For instance the C language grammar presented by Kernigan and Ritchie has a nonterminal translationUnit which represents a complete C language module, a nonterminal conditionalExpression which represents a truth valued expression of the language.
Punctuations
These are special characters or strings which are recognized as separate entities even if they aren't physically separated from other strings by white space. For example, most languages would ``see'' the string `if0` as a single token (probably an identifier) even if `if` is a keyword, whereas `if(0)` would be recognized the same as `if ( 0 )` because parentheses are normally considered punctuations. Except for the special treatment at recognition, punctuations are similar to keywords.

The syntax of a language describes how to recognize the components of the language. To define a language syntax using `kjParseBuild` you must create a null compilable grammar object to contain the grammar (in Figure GrammarBuild this is done on line 3 using the class constructor `kjParseBuild.NullCGrammar()` creating the grammar object `LispG`) and define the components of the grammar and the rules for recognizing the components. The component definitions and rule declarations, as well as the specification of case sensitivity and comment patterns, are performed on lines 4 through 10 of Figure GrammarBuild for the `LispG` grammar.

### Declaring Case Sensitivity and Comments

There are two nuances to parsing not yet mentioned: case sensitivity and comments.

Some grammars are not case sensitive in recognizing keywords or identifiers. For example ANSI standard SQL (which is not case sensitive for keywords or identifiers) recognizes `Select, select, SELECT,` and `SeLect` all as the keyword `SELECT`. To specify the case sensitivity of the grammar for keywords only use

         GRAMMAROBJECT.SetCaseSensitivity(TrueOrFalse)

where `TrueOrFalse` is 0 for no case sensitivity or 1 for case sensitivity. This must be done before any keyword declarations for the grammar. All other syntax declarations may be done in any order before the compilation of the grammar object. In Figure GrammarBuild the `LispG` grammar object is declared to be case insensitive on line 4.

Comments are patterns in the input string which are ignored (or more precisely interpreted as white space) by the language. To declare a sequence of regular expressions to be interpreted as a comment in a grammar use

        GRAMMAROBJECT.comments(LIST_OF_COMMENT_REGULAR_EXPR_STRINGS)

For example, line 9 or Figure GrammarBuild declares the constant string previously declared as

        LISPCOMMENTREGEX = ";.*"

to represent a comment of the grammar `LispG`. For the syntax of regular expression strings you must look elsewhere, but as a hint `";.*"` represents any string commencing with a semicolon, followed by any sequence of characters up to, but not including, a newline.

### Declaring Keywords, Punctuations, and Terminals

To declare keywords for your grammar use

          GRAMMAROBJECT.Keywords( STRING )

where `STRING` is a white space separated string of keywords. Line 6 of Figure GrammarBuild declares `setq` and `print` as keywords of `LispG`.

To declare nonterminals for your grammar, similarly, use

          GRAMMAROBJECT.Nonterms( STRING )

where `STRING` is a white space separated string of nonterminal names. Line 8 of Figure GrammarBuild declares `Value` and `ListTail` as nonterminals of the `LispG`.

Similarly, use

          GRAMMAROBJECT.punct( STRING )

to declare a sequence of punctuations for the grammar, except that in this case the string must not contain any white space. Line 7 of Figure GrammarBuild declares parentheses and dot to be punctuations of the `LispG`.

If you have a lot of keywords, punctuations, or nonterminals you can make many separate calls to the appropriate declaration methods with different strings.

These declarations will cause the grammar to recognize the declared keyword strings (when separated from other strings by white space or punctuations) and punctuations as special tokens of the grammar at the lowest level of parsing. The parsing process derives nonterminals of the grammar at a higher level as discussed below.

A small difficulty with `kjParseBuild` is that the strings `@R, ::, >>,` and `##` cannot be used as names of keywords for the grammar because they are used to specify rule syntax in the ``metagrammar''. If you need these in your grammar they may be implemented as ``trivial'' terminals. For example,

`Grammar.Addterm("poundpound", "##", echo)`

I'm unsure whether this patch is good enough. Does anyone have any advice for me? If this is a bad problem for some grammar the keywords of the meta grammar can be changed of course, but this is a hack.

### Declaring Terminals

            # from DLispShort.py
            def DeclareTerminals(Grammar):
       (1)      Grammar.Addterm("int", INTREGEX, intInterp)
       (2)      Grammar.Addterm("str", STRREGEX, stripQuotes)
       (3)      Grammar.Addterm("var", VARREGEX, echo)
Defining the terminals of a grammar. (TermDef)

Figure TermDef shows the declarations for installing the `int, str,` and `var` terminals in the grammar. This is given as a separate function because the declarations define both the syntax and semantics for the terminals, and therefore must be called both during grammar generation and after loading the generated grammar object. To declare a terminal for a grammar use

          GRAMMAROBJECT.Addterm(NAMESTR, REGEXSTR, FUNCTION)

This declaration associates both a regular expression string `REGEXSTR` and an interpretation function `FUNCTION` to the terminal of the grammar named by the string `NAMESTR`. The `FUNCTION` defines the semantics of the terminal as describe below and the `REGEXSTR` specifies a regular expression for recognizing the string. For example on line 2 of Figure TermDef the `var` terminal is associated with the regular expression string

          STRREGEX = '"[^\n"]*"'

which matches any string starting with double quotes and ending with double quotes which contains neither double quotes nor a newline.

### Declaring Rules of the Grammar

    # from DLispShort.py
    GRAMMARSTRING ="""
           Value ::  ## indicates Value is the root nonterminal for the grammar
             @R SetqRule :: Value >> ( setq var Value )
             @R ListRule :: Value >> ( ListTail
             @R TailFull :: ListTail >> Value ListTail
             @R TailEmpty :: ListTail >> )
             @R Varrule :: Value >> var
             @R Intrule :: Value >> int
             @R Strrule :: Value >> str
             @R PrintRule :: Value >> ( print Value )
    """
A grammar definition string.

To declare the rules of a grammar use the simple rule definition language which comes with `kjParseBuild`, for example as shown in Figure GramStr. Line 10 of Figure GrammarBuild uses the string defined in Figure GramStr to associate the rules with the grammar using

          GRAMMAROBJECT.DeclareRules(RULE_DEFINITION_STRING)

This declaration does not analyse the string; analysis and syntax/semantics errors are reported by `*.Compile()` described below.

The rule definition language allows you to identify the root nonterminal of your grammar and specify a sequence of named derivation rules for the grammar. It also allows comments which start with `##` and end with a newline. An acceptible string for the rule definition language looks like

RootNonterminalName `::` NamedRule1 NamedRule2 ...

Here the Root nonterminal name should be the nonterminal that ``stands for'' any complete string of the language. Furthermore, each named rule looks like

`@R` NameString `::` GoalNonterm `>>` RuleBody

where the name string for the rule is a string without whitespace, the goal nonterminal is the nonterminal that the rule derives, and the rule body is a sequence of keywords, punctuations and nonterminals separated by white space. Rule names are used for mapping rules to semantic interpretations and should be unique.

Note that punctuations for the grammar you are defining are not punctuations for the rule definition language (which has none), so they must be separated from other tokens by white space. The keyword for the rule definition language `@R, ::, >>` must also be separated from other tokens by whitespace in the rule definition string. Furthermore, all punctuations, keywords, nonterminals, and terminals used in the rules must be declared for the grammar before the grammar is compiled (if one isn't the compilation will fail with an error).

As a bit of sugar you may break up the declarations of rules.

        LispG.DeclareRules("Value::\n")
        LispG.DeclareRules("  @R SetqRule :: Value >> ( setq var Value )\n")
        LispG.DeclareRules("  @R ListRule :: Value >> ( ListTail\n")
        ...

This might be useful for larger grammars.

### A Brief Discussion of Derivations

The rules for a grammar don't really describe how to parse a string of the language, they actually describe how to derive a string of the grammar. For this reason it is possible to create a grammar which derives the same string in two different ways; such grammars are termed ambiguous. If you try to generate a parser for an ambiguous grammar the parse generation process will cause the parser generation process to complain.

For a more precise definition of the derivation of a language string from a grammar see the ``further readings'' below. For illustrative purposes, and to help explain how to define semantics functions, consider the following derivation of the string

`( 123 ( setq x "this" ) )`

using the rules declared in Figure GramStr.

Derivation | Rule used
`Value1 >> ( ListTail1` | `ListRule`
`ListTail1 >> Value2 ListTail2` | `TailFull`
`Value2 >> [int = 123]` | `Intrule`
`ListTail2 >> Value3 ListTail3` | `TailFull`
`Value3 >> (setq [var='x'] Value4)` | `SetqRule`
`Value4 >> [string='this']` | `StrRule`
`ListTail3 >> )` | `TailEmpty`

To obtain the string derived we simply substitute the representations derived for each of the numbered nonterminals and terminals of the derivation as shown in Figure Derive.

    (1)    Value1
    (2)    ( ListTail1                        (ListRule)
    (3)    ( Value2 ListTail2                 (TailFull)
    (4)    ( 123 ListTail2                    (Intrule)
    (5)    ( 123 Value3 ListTail3             (TailFull)
    (6)    ( 123 ( setq x Value4 ) ListTail3  (SetqRule)
    (7)    ( 123 ( setq x "this" ) ListTail3  (StrRule)
    (8)    ( 123 ( setq x "this" ) )          (TailEmpty)
Right-to-left derivation steps for `(123 (setq x "this"))`. (Derive)

### Compiling the Grammar Syntax, and Storing the Compilation

Once you have defined all the keywords, comments, terminals, nonterminals, punctuations, and rules of your grammer you may create the datastructures needed for parsing by compiling the grammar using

          GRAMMAROBJECT.Compile()

Line 11 of Figure GrammarBuild performs the compilation for the LispG grammar.

If the compilation succeeds you may use

          GRAMMAROBJECT.MarshalDump( OUTPUTFILE )

to store the compiled grammar structure to a file that may be later loaded without recompiling the grammar. Here `MarshalDump` will create a binary ``marshalled'' representation for the grammar in the `OUTPUTFILE`. For example line 13 of figure GrammarBuild marshalls a representation for `LispG` to the file `TESTLispG.mar`. `TESTLisp.GRAMMAR()` will then reconstruct the internal structure of LispG as a grammar object and return the grammar object as the result of the function.

Nevertheless, compilation of the grammar by itself does not yeild a grammar that will do any useful parsing [Actually, it will do ``parsing'' using default actions (implemented as a function which simply return the list argument).] Rules must be associated with computational actions before useful parsing can be done.

## Defining a Semantics

Two sorts of objects require semantic actions that define their meaning: rules and terminals. All semantic actions must be defined as Python functions and bound in the grammar before parsing can be performed.

Before you can define the semantics of your language in Python you better have a pretty good idea of what the components of the language are supposed to represent, of course. Using your intuitive understanding of the language you can:

Decide what the context of the computation should be and how it should be implemented as a Python structure. If the process of Parsing must modify the context, then then the context structure must be a ``mutable'' python structure. In the case of `LispG` the context is simply a structure that maps ``internal'' variable names to values, implemented as a simple Python dictionary mapping name strings to the appropriate value.
Decide what kind of Python value each terminal of the grammar represents. In the case of `LispG`
str
should represent a string value corresponding to the string recognized (minus the surrounding quotes).
int
should represent an integer value corresponding to the string recognized.
var
should represent the string representing the variable name recognized (the name must be translated to a corresponding value at a higher level since the terminal interpretation functions don't have access to the context structure).
Decide what kind of Python structure or value each nonterminal represents. In the case of the `LispG` grammar:
Value
represents a Python integer, string, or list.
ListTail
represents a Python list containing the members of the tail of a list.

Decide how each rule should derive a structure corresponding to the Goal (left hand side) of the rule based on the values corresponding to the terminals and nonterminals on the right hand side of the rule. In the case of the `LispG` grammar (refer to Figure GramStr for rule definitions):

SetqRule
should return whatever the `Value` terminal in the body represents.
ListRule
should return the list represented by the `ListTail` nonterminal of the body.
TailFull
should return the result of adding the value corresponding to the `Value` nonterminal of the list to the front of the list corresponding to the `Listtail` nonterminal of the body.
Varrule
should return the value from the computational context that corresponds to the variable name represented by the `var` terminal of the body.
Intrule
should return the integer corresponding to the `int` terminal of the body.
Strrule
should return the string corresponding to the `str` terminal of the body.
PrintRule
should return the value represented by the `Value` nonterminal of the body.

Decide what side effects, if any, each rule should have on the computational context or externally. In the case of the `LispG` grammar:

SetqRule
should associate the variable name represented by `var` to the value represented by `Value` in the body.
PrintRule
should print the value corresponding to the `Value` nonterminal to the screen.

The other rules of `LispG` should have no internal or external side effects.
More complex languages may require much more complex contexts, values and side effects, including function definitions, modules, database table accesses, user authorization verifications, and/or file creation, among other possibilities.

Having determined the intuitive semantics of the language you may now specify implement the semantic functions and bind them in your grammar.

### Semantics for Terminals

To define the meaning of a terminal you must create a Python function that translates a string (which the parser has recognized as an instance of the terminal) into an appropriate value. For instance, when the LispG grammar recognizes a string

          "this is a string"

the interpretation function should translate the recognized string into the Python string it represents: namely, the same string but with the double quotes stripped off. The following ``string intepretation function'' will perform this simple interpretation.

            # from DLispShort.py
            def stripQuotes( str ):
                return str[1:len(str)-1]

Similarly, when the parser recognizes a string as an integer, the associated interpretation function should translate the string into a Python integer.

The binding of interpretation functions to terminal names is performed by the `Addterm` method previously mentioned. For example, line 2 of Figure TermDef associates the `stripQuotes` function to the nonterminal named `str`.

All functions passed to `Addterm` should take a single string argument which represents the recognized string, and return a value which represents the semantic interpretation for the input string.

### Semantics for Rules

The semantics of rules is more interesting since they may have side effects and require the kind of recursive thinking that gives most people headaches. The semantics for rules are specified by functions. To perform the semantic action associated with a rule, the ``reduction function'' should perform any side effects (to the computational context or externally) and return a result value that represents the interpretation for the nonterminal at the head of the rule.

          # from DLispShort.py
          def EchoValue( list, Context ):
              return list[0]

          def VarValue( list, Context ):
              varName = list[0]
              if Context.has_key(varName):
                 return Context[varName]
              else:
                 raise NameError, "no such lisp variable in context "+varName

          def NilTail( list, Context ):
              return []

          def AddToList( list, Context ):
              return [ list[0] ] + list[1]

          def MakeList( list, Context ):
              return list[1]

          def DoSetq( list, Context):
              Context[ list[2] ] = list[3]
              return list[3]

          def DoPrint( list, Context ):
              print list[2]
              return list[2]
The reduction functions for the rules. (RedFun)

            # from DLispShort.py
            def BindRules(LispG):
                LispG.Bind( "Intrule", EchoValue )
                LispG.Bind( "Strrule", EchoValue )
                LispG.Bind( "Varrule", VarValue )
                LispG.Bind( "TailEmpty", NilTail )
                LispG.Bind( "TailFull", AddToList )
                LispG.Bind( "ListRule", MakeList )
                LispG.Bind( "SetqRule", DoSetq )
                LispG.Bind( "PrintRule", DoPrint )
Binding named rules to interpretation functions. (ruleBind)

The Python functions that define the semantics of the rules of `LispG` appear in Figure RedFun and the declarations that bind the rule names to the functions in the grammar object `LispG` appear in Figure ruleBind.

Each ``reduction function'' for a rule must take two arguments: a list representing the body of the rule and a context structure which represents the computational context of the computation. The list argument will have the same length as the body of the rule, counting the keywords and punctuations as well as the terminals and nonterminals.

For example the `SetqRule` has a body with five tokens,

          @R SetqRule :: Value >> ( setq var Value )

so the `DoSetq` function should expect the parser to deliver a Python list argument with five elements of form

          list = [ '(', 'SETQ', VARIABLE_NAME, VALUE_RESULT, ')' ]

note that the ``names'' of keywords and punctuations appear in the appropriate positions (0, 1, and 4) of the `list` corresponding to their positions in `SetqRule`. Furthermore, the position occupied by the terminal `var` in `SetqRule` has been replaced by a string representing a variable name in the `list` and the position occupied by the nonterminal `Value` in `SetqRule` has been replaced by a Python value.

More generally, the parser will call reduction functions for rules with a `list` representing the ``interpreted body of the rule'' where

keywords and punctuations
are interpreted as themselves (i.e., their names), except that letters will be in upper case if the grammar is not case sensitive;
terminals
are interpreted as values previously returned by a call to the appropriate terminal interpretation function; and
nonterminals
are interpreted as values previously returned by a reduction function for a rule that derived this terminal.

Although, the occurrence of the keyword names in the list may seem useless, it may have its purposes. For example, a careful programmer might check them during debugging to make sure the right function was bound to the right rule.

To determine how to implement the semantics of a rule you must refer to the semantic decisions you made earlier. For example, above we specified that the `setq` construct should bind the variable name recieved (`list[2]`) to the value (`list[3]`) in the `Context`, and return the value (`list[3]`) as the result of the expression. Translated into the more concise language of Python this is exactly what `DoSetq` shown in Figure RedFun does.

To bind a rule name to a (previously declared) reduction function use

          GRAMMAROBJECT.Bind( RULENAME, FUNCTION )

where `RULENAME` is the string name for the rule previously declared for the grammar `GRAMMAROBJECT` and `FUNCTION` is the appropriate reduction function for the rule. These bindings for `LispG` are shown in Figure ruleBind.

### A Bit on the Parsing Process

The following is not a precise definition of the actions of a Parser, but it may help you understand how the parsing process works and the order in which rules are recognized and functions are evaluated.

| Tokens seen S | input remaining | rule R and function call
0 | `` | `(123 (setq x "this"))` |
1 | `( 123` | `(setq x "this"))` | Intrule
|  |  | `Value2 = EchoValue([123],C))`
2 | `( Value2 ( setq x "this"` | `))` | StrRule
|  |  | `Value4 = EchoValue(['this'],C)`
3 | `( Value2 ( setq x Value4 )` | `)` | SetqRule
|  |  | `Value3 = DoSetq(['(','SETQ','x',Value4,')'],C)`
4 | `( Value2 Value3 )` |  | TailEmpty
|  |  | `ListTail3 = NilTail([')'],C)`
5 | `( Value2 Value3 ListTail3` |  | TailFull
|  |  | `ListTail2 = AddToList([Value3,ListTail3],C)`
6 | `( Value2 ListTail2` |  | TailFull
|  |  | `ListTail3 = AddToList([Value2,ListTail2],C)`
7 | `( ListTail3` |  | ListRule
|  |  | `Value1 = MakeList(['(',Value1],C)`
8 | `Value1` |  |
Parsing `(123 (setq x "this"))` (Parse)

Technically, each entry of S is tagged with the kind of token it represents (keyword, nonterminal, or terminal) and the name of the token it represents (e.g., `Value`, `str`) as well as the values shown.

Figure Parse illustrates the sequence of reduction actions performed by `LispG` when parsing the input string `(123 (setq x "this"))`. We can think of this parse as ``reversing'' the derivation process shown in Figure Derive using the rule reduction functions to obtain semantic interpretations for the nonterminals.

At the lowest level of parsing a lexical analyser examines the unread portion of the input string tries to match a prefix of the input string with a keyword or a regular expression for a terminal (ignoring comments and whitespace, except as separators). The analyser ``passes'' the recognized token to the higher level parser together with its interpreted value. The interpreted value of a terminal is determined by using the appropriate interpretation function and the interpreted value of a keyword is simply its name (in upper case, if the grammer is not case sensitive). For example the `LispG` lexical analyser recognizes `'('` as a keyword with the value `'('` and `"this"` as an instance of the nonterminal `str` with the value `'this'`.

The higher level parser accepts tokens T from the lexical analyser and does one of two things with them

If the most recent token values V the parser has saved on its ``tokens seen'' stack S ``looks like'' the body B of a rule R and the current token is a token that could follow the nonterminal N at the head of R, then the parser evaluates the reduction function F associated with R, using the values V from the stack S that match the body B together with the computational context C. The resulting value F(V,C) replaces the values V
Otherwise the current token is shifted onto the ``tokens seen'' stack S and the parser moves on to the next token.

The above is a lie. Actually, the parsing process is much smarter than this, but from a users perspective this simplification may be helpful.

Figure Parse shows ``reduction'' steps and not the ``shifts'', and glosses over the lexical analysis and other nuances, but it illustrates the idea of the parsing process nonetheless. For example at step 2 the parse recognizes the last token on the stack S (an instance of the `"str"` terminal with value `"this"`) as matching the body of `StrRule`, and replaces it with the an instance of the nonterminal `Value` with value determined by the reduction of `StrRule`. In this case `StrRule` is associated with the reduction function `EchoValue`, so the result of the reduction is given by `EchoValue( 'this', C )` where C is the context structure for the Parse.

At Step 3 the most recent entries of S

          V = ['(', 'SETQ', 'x', Value4, ')']

match the body of the rule `SetqRule`, so they are replaced on S by an instance of the `Value` nonterminal with value determined by

          Value3 = DoSet( V, C )

Finally, at step 8, the interpretation associated with `Value1` (an instance of the root nonterminal for `LispG`) is considered the result of the computation.

## Parsing with a Grammar

Before you can perform a parse you probably must create a computational context for the parse. In the case of `LispG` the context is simply a dictionary so we may initialize

          Context = {}

To create a context for Parsing.

There are two methods which provide the primary interfaces for the parsing process for a grammar.

          RESULT = GRAMMAROBJECT.Parse1(STRING, CONTEXT)
          (RESULT, CONTEXT) = GRAMMAROBJECT.Parse(STRING, CONTEXT)

The second allows you to make explicit in code that uses parsing the possibility that a parse may alter the context of the parse -- aside from that the two functions are identical. Example usage for `Parse1` using `LispG` were given earlier.

## Storing and Reloading a Grammar Object

The process of compiling a grammar may take significant time and consume significant quantities of memory. To free up memory from structures in a compilable grammar object that aren't needed after compilation use `GRAMMAR.CleanUp()`.

Once you have debugged the syntax and semantics of your grammar you may store syntactic information for the grammar using the `Reconstruct` method already mentioned. The declarations created by `Reconstruct` only defines the syntax for the grammar. The semantics must be rebound separately. But it is much better to use UnMarshalGram as shown below, which stores the grammar in a binary format.

For example, lines 12 through 14 of Figure GrammarBuild create a file `TESTLispG.py` containing a function `GRAMMAR()` which will reconstruct the syntax for the `LispG` grammar.

            # from DLispShort.py

    def unMarshalLispG():
        import kjParser
        infile = open("TESTLispG.mar", "r")
        LispG = kjParser.UnMarshalGram(infile)
        infile.close()
        DeclareTerminals(LispG)
        BindRules(LispG)
        return LispG
This function can then be used in another file, provided `DLispShort.GrammarBuild()` has been executed at some point in the past, thusly:
            import DLispShort
            LGrammar = DLispShort.unMarshalLispG()
Loading a precomputed grammar object.

Figure Load shows a function `LoadLispG` that reloads the syntactic portion of `LispG` from `TESTLispG.mar`. To rebind the semantics as well the function re-declares the terminals and re-binds the rules to make the reconstructed `LispG` suitable for use in parsing.

## Errors raised

You may see the following errors:

LexTokenError
This usually means the lowest level of the parser ran into a string it couldn't recognize.
BadPunctError
You tried to make a whitespace character a punctuation. This is not currently allowed.
EOFError, SyntaxError
You tried to parse a string that is not valid for the grammar.
TokenError
During parser generation you used a string in the rule definitions that wasn't previously registered as a terminal, nonterminal, or punctuation.
NotSLRError
You attempted to build a grammar that is not ``SLR'' according to the definition of Aho and Ullman. Either the grammar is ambiguous, or it doesn't have a derivation for the root nonterminal, or it is too tricky for the generator.

Furthermore `NondetError, ReductError, FlowError, ParseInitError, UnkTermError` or errors raised by other modules shouldn't happen. If an error that shouldn't happen happens there are two possibilities (1) you have fiddled with the code or data structures and you broke something, or (2) there is a serious bug in the module.

## Possible Gotchas

This package has a number of known deficiencies, and there are probably many that are yet to be discovered.

Syntax errors are not reported nicely. Sorry.

Currently, there is no way to to resolve grammar ambiguities. For example a C construct

    if (x)
       if (y)
          x = 0;
       else
          y = 1;

could have the `else` associated with either the first or second if; the grammar doesn't indicate which. This is normally resolved by informing the parser generator to prefer one binding or the other. No method for providing a preference is implemented here, yet. Let me know if you need such a method or if you have any suggestions.

Keywords of the meta-grammar cannot name tokens of the object grammar (see footnote above).

If you want keywords to be recognized without case sensitivity you must declare `G.SetCaseSensitivity(0)` before any keyword declarations.

Name and regular expression collisions are not always checked and reported. If you name two rules the same, for example, you may get undefined behavior.

The lexical analysis implementation is not as fast as it could be (of course). It also sees all white space as a `single space' so, for example, if indentation is significant in your grammar (as in Python) you'll need a different lexical analyzer. Also if `x=+y` means something different from `x = + y` (as it did in the original C, I believe) you may have trouble. Happily the lexical component can be easily ``plug replaced'' by another implementation if needed.

Also, the system currently only handles SLR grammars (as defined by Aho and Ullman), as mentioned above. If you get a `NonSLRError` during grammar compilation you need a better parser generator. I may provide one, if I have motivation and time.

I know of no outright bugs. Trust me, they're there. Please find them for me and tell me about them. I'm not a big expert on parsing so I'm sure I've made some errors, particularly at the lexical level.

## Further Reading

A standard reference for parsing and compiler, interpreter, and translator implementation is Principles of Compiler Design, by Aho and Ullman (Addison Wesley).

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