Expressions

## 6 Expressions

In this chapter, all valid Erlang expressions are listed. When writing Erlang programs, it is also allowed to use macro- and record expressions. However, these expressions are expanded during compilation and are in that sense not true Erlang expressions. Macro- and record expressions are covered in separate chapters: Macros and Records.

### 6.1 Expression Evaluation

All subexpressions are evaluated before an expression itself is evaluated, unless explicitly stated otherwise. For example, consider the expression:

    Expr1 + Expr2

`Expr1` and `Expr2`, which are also expressions, are evaluated first - in any order - before the addition is performed.

Many of the operators can only be applied to arguments of a certain type. For example, arithmetic operators can only be applied to numbers. An argument of the wrong type will cause a `badarg` run-time error.

### 6.2 Terms

The simplest form of expression is a term, that is an integer, float, atom, string, list or tuple. The return value is the term itself.

### 6.3 Variables

A variable is an expression. If a variable is bound to a value, the return value is this value. Unbound variables are only allowed in patterns.

Variables start with an uppercase letter or underscore (_) and may contain alphanumeric characters, underscore and @. Examples:

    X
    Name1
    PhoneNumber
    Phone_number
    _
    _Height

Variables are bound to values using pattern matching. Erlang uses single assignment, a variable can only be bound once.

The anonymous variable is denoted by underscore (_) and can be used when a variable is required but its value can be ignored. Example:

    [H|_] = [1,2,3]

Variables starting with underscore (_), for example `_Height`, are normal variables, not anonymous. They are however ignored by the compiler in the sense that they will not generate any warnings for unused variables. Example: The following code

    member(_, []) ->
        [].

can be rewritten to be more readable:

    member(Elem, []) ->
        [].

This will however cause a warning for an unused variable `Elem`, if the code is compiled with the flag `warn_unused_vars` set. Instead, the code can be rewritten to:

    member(_Elem, []) ->
        [].

Note that since variables starting with an underscore are not anonymous, this will match:

    {_,_} = {1,2}

But this will fail:

    {_N,_N} = {1,2}

The scope for a variable is its function clause. Variables bound in a branch of an `if`, `case`, or `receive` expression must be bound in all branches to have a value outside the expression, otherwise they will be regarded as 'unsafe' outside the expression.

For the `try` expression introduced in Erlang 5.4/OTP-R10B, variable scoping is limited so that variables bound in the expression are always 'unsafe' outside the expression. This will be improved.

### 6.4 Patterns

A pattern has the same structure as a term but may contain unbound variables. Example:

    Name1
    [H|T]
    {error,Reason}

Patterns are allowed in clause heads, `case` and `receive` expressions, and match expressions.

#### 6.4.1 Match Operator = in Patterns

If `Pattern1` and `Pattern2` are valid patterns, then the following is also a valid pattern:

    Pattern1 = Pattern2

When matched against a term, both `Pattern1` and `Pattern2` will be matched against the term. The idea behind this feature is to avoid reconstruction of terms. Example:

    f({connect,From,To,Number,Options}, To) ->
        Signal = {connect,From,To,Number,Options},
        ...;
    f(Signal, To) ->
        ignore.

can instead be written as

    f({connect,_,To,_,_} = Signal, To) ->
        ...;
    f(Signal, To) ->
        ignore.

#### 6.4.2 String Prefix in Patterns

When matching strings, the following is a valid pattern:

    f("prefix" ++ Str) -> ...

This is syntactic sugar for the equivalent, but harder to read

    f([$p,$r,$e,$f,$i,$x | Str]) -> ...

#### 6.4.3 Expressions in Patterns

An arithmetic expression can be used within a pattern, if it uses only numeric or bitwise operators, and if its value can be evaluated to a constant at compile-time. Example:

    case {Value, Result} of
        {?THRESHOLD+1, ok} -> ...

This feature was added in Erlang 5.0/OTP R7.

### 6.5 Match

    Expr1 = Expr2

Matches `Expr1`, a pattern, against `Expr2`. If the matching succeeds, any unbound variable in the pattern becomes bound and the value of `Expr2` is returned.

If the matching fails, a `badmatch` run-time error will occur.

Examples:

    1> {A, B} = {answer, 42}.
    {answer,42}
    2> A.
    answer
    3> {C, D} = [1, 2].
    ** exited: {{badmatch,[1,2]},[{erl_eval,expr,3}]} **

### 6.6 Function Calls

    ExprF(Expr1,...,ExprN)
    ExprM:ExprF(Expr1,...,ExprN)

`ExprM` should evalutate to a module name and `ExprF` to a function name or a fun.

When including the module name, the function is said to be called by using the fully qualified function name. This is often referred to as a remote or external function call. Example:

    lists:keysearch(Name, 1, List)

The module name can be omitted, if `ExprF` evaluates to the name of a local function, an imported function, or an auto-imported BIF. Then the function is said to be called by using the implicitly qualified function name. Examples:

    handle(Msg, State)
    spawn(m, init, [])

To avoid possible ambiguities, the fully qualified function name must be used when calling a function with the same name as a BIF, and the compiler does not allow defining a function with the same name as an imported function.

Note that when calling a local function, there is a difference between using the implicitly or fully qualified function name, as the latter always refer to the latest version of the module. See Compilation and Code Loading.

If `ExprF` evaluates to a fun, only the format `ExprF(Expr1,...,ExprN)` is correct. Example:

    Fun1 = fun(X) -> X+1 end
    Fun1(3)
    => 4

    Fun2 = {lists,append}
    Fun2([1,2],[3,4])
    => [1,2,3,4]

See also the chapter about Function Evaluation.

### 6.7 If

    if
        GuardSeq1 ->
            Body1;
        ...;
        GuardSeqN ->
            BodyN
    end

The branches of an `if`-expression are scanned sequentially until a guard sequence `GuardSeq` which evaluates to true is found. Then the corresponding `Body` (sequence of expressions separated by ',') is evaluated.

The return value of `Body` is the return value of the `if` expression.

If no guard sequence is true, an `if_clause` run-time error will occur. If necessary, the guard expression `true` can be used in the last branch, as that guard sequence is always true.

Example:

    is_greater_than(X, Y) ->
        if
            X>Y ->
                true;
            true -> % works as an 'else' branch
                false
        end

### 6.8 Case

    case Expr of
        Pattern1 [when GuardSeq1] ->
            Body1;
        ...;
        PatternN [when GuardSeqN] ->
            BodyN
    end

The expression `Expr` is evaluated and the patterns `Pattern` are sequentially matched against the result. If a match succeeds and the optional guard sequence `GuardSeq` is true, the corresponding `Body` is evaluated.

The return value of `Body` is the return value of the `case` expression.

If there is no matching pattern with a true guard sequence, a `case_clause` run-time error will occur.

Example:

    is_valid_signal(Signal) ->
        case Signal of
            {signal, _What, _From, _To} ->
                true;
            {signal, _What, _To} ->
                true;
            _Else ->
                false
        end.

### 6.9 Send

    Expr1 ! Expr2

Sends the value of `Expr2` as a message to the process specified by `Expr1`. The value of `Expr2` is also the return value of the expression.

`Expr1` must evaluate to a pid, a registered name (atom) or a tuple `{Name,Node}`, where `Name` is an atom and `Node` a node name, also an atom.

- If `Expr1` evaluates to a name, but this name is not registered, a `badarg` run-time error will occur.
- Sending a message to a pid never fails, even if the pid identifies a non-existing process.
- Distributed message sending, that is if `Expr1` evaluates to a tuple `{Name,Node}` (or a pid located at another node), also never fails.

### 6.10 Receive

    receive
        Pattern1 [when GuardSeq1] ->
            Body1;
        ...;
        PatternN [when GuardSeqN] ->
            BodyN
    end

Receives messages sent to the process using the send operator (!). The patterns `Pattern` are sequentially matched against the first message in time order in the mailbox, then the second, and so on. If a match succeeds and the optional guard sequence `GuardSeq` is true, the corresponding `Body` is evaluated. The matching message is consumed, that is removed from the mailbox, while any other messages in the mailbox remain unchanged.

The return value of `Body` is the return value of the `receive` expression.

`receive` never fails. Execution is suspended, possibly indefinitely, until a message arrives that does match one of the patterns and with a true guard sequence.

Example:

    wait_for_onhook() ->
        receive
            onhook ->
                disconnect(),
                idle();
            {connect, B} ->
                B ! {busy, self()},
                wait_for_onhook()
        end.

It is possible to augment the `receive` expression with a timeout:

    receive
        Pattern1 [when GuardSeq1] ->
            Body1;
        ...;
        PatternN [when GuardSeqN] ->
            BodyN
    after
        ExprT ->
            BodyT
    end

`ExprT` should evaluate to an integer. The highest allowed value is 16#ffffffff, that is, the value must fit in 32 bits. `receive..after` works exactly as `receive`, except that if no matching message has arrived within `ExprT` milliseconds, then `BodyT` is evaluated instead and its return value becomes the return value of the `receive..after` expression.

Example:

    wait_for_onhook() ->
        receive
            onhook ->
                disconnect(),
                idle();
            {connect, B} ->
                B ! {busy, self()},
                wait_for_onhook()
        after
            60000 ->
                disconnect(),
                error()
        end.

It is legal to use a `receive..after` expression with no branches:

    receive
    after
        ExprT ->
            BodyT
    end

This construction will not consume any messages, only suspend execution in the process for `ExprT` milliseconds and can be used to implement simple timers.

Example:

    timer() ->
        spawn(m, timer, [self()]).

    timer(Pid) ->
        receive
        after
            5000 ->
                Pid ! timeout
        end.

There are two special cases for the timeout value `ExprT`:

`infinity`
  The process should wait indefinitely for a matching message -- this is the same as not using a timeout. Can be useful for timeout values that are calculated at run-time.
0
  If there is no matching message in the mailbox, the timeout will occur immediately.

### 6.11 Term Comparisons

    Expr1 op Expr2

Term Comparison Operators.
op | Description
== | equal to
/= | not equal to
=< | less than or equal to
< | less than
>= | greater than or equal to
> | greater than
=:= | exactly equal to
=/= | exactly not equal to

The arguments may be of different data types. The following order is defined:

    number < atom < reference < fun < port < pid < tuple < list < binary

Lists are compared element by element. Tuples are ordered by size, two tuples with the same size are compared element by element.

All comparison operators except =:= and =/= are of type coerce: When comparing an integer and a float, the integer is first converted to a float. In the case of =:= and =/=, there is no type conversion.

Returns the Boolean value of the expression, `true` or `false`.

Examples:

    1> 1==1.0.
    true
    2> 1=:=1.0.
    false
    3> 1 > a.
    false

### 6.12 Arithmetic Expressions

    op Expr
    Expr1 op Expr2

Arithmetic Operators.
op | Description | Argument type
+ | unary + | number
- | unary - | number
+ |  | number
- |  | number
* |  | number
/ | floating point division | number
bnot | unary bitwise not | integer
div | integer division | integer
rem | integer remainder of X/Y | integer
band | bitwise and | integer
bor | bitwise or | integer
bxor | arithmetic bitwise xor | integer
bsl | arithmetic bitshift left | integer
bsr | bitshift right | integer

Examples:

    1> +1.
    1
    2> -1.
    -1
    3> 1+1.
    2
    4> 4/2.
    2.00000
    5> 5 div 2.
    2
    6> 5 rem 2.
    1
    7> 2#10 band 2#01.
    0
    8> 2#10 bor 2#01.
    3

### 6.13 Boolean Expressions

    op Expr
    Expr1 op Expr2

Logical Operators.
op | Description
not | unary logical not
and | logical and
or | logical or
xor | logical xor

Examples:

    1> not true.
    false
    2> true and false.
    false
    3> true xor false.
    true

### 6.14 Short-Circuit Boolean Expressions

    Expr1 orelse Expr2
    Expr1 andalso Expr2

Boolean expressions where `Expr2` is evaluated only if necessary. That is, `Expr2` is evaluated only if `Expr1` evaluates to `false` in an `orelse` expression, or only if `Expr1` evaluates to `true` in an `andalso` expression. Returns the Boolean value of the expression, that is `true` or `false`.

As of Erlang 5.5/OTP R11B, short-circuit boolean expressions are allowed in guards. In guards, however, evaluation is always short-circuited since guard tests are known to be free of side effects.

Example 1:

    case A >= -1.0 andalso math:sqrt(A+1) > B of

This will work even if `A` is less than `-1.0`, since in that case, `math:sqrt/1` is never evaluated.

Example 2:

    OnlyOne = is_atom(L) orelse
             (is_list(L) andalso length(L) == 1),

This feature was added in Erlang 5.1/OTP R8.

### 6.15 List Operations

    Expr1 ++ Expr2
    Expr1 -- Expr2

The list concatenation operator `++` appends its second argument to its first and returns the resulting list.

The list subtraction operator `--` produces a list which is a copy of the first argument, subjected to the following procedure: for each element in the second argument, the first occurrence of this element (if any) is removed.

Example:

    1> [1,2,3]++[4,5].
    [1,2,3,4,5]
    2> [1,2,3,2,1,2]--[2,1,2].
    [3,1,2]

### 6.16 Bit Syntax Expressions

    <<>>
    <<E1,...,En>>

Each element `Ei` specifies a segment of the binary. Each element `Ei` is a value, followed by an optional size expression and an optional type specifier list.

    Ei = Value |
         Value:Size |
         Value/TypeSpecifierList |
         Value:Size/TypeSpecifierList

Used in a binary construction, `Value` is an expression which should evaluate to an integer, float or binary. If the expression is something else than a single literal or variable, it should be enclosed in parenthesis.

Used in a binary matching, `Value` must be a variable, or an integer, float or string.

Note that, for example, using a string literal as in `<<"abc">>` is syntactic sugar for `<<$a,$b,$c>>`.

Used in a binary construction, `Size` is an expression similar to `Value`, which should evaluate to an integer.

Used in a binary matching, `Size` must be an integer, or a variable bound to an integer.

The value of `Size` specifies the size of the segment in units (see below). The default value depends on the type (see below). For `integer` it is 8, for `float` it is 64, for `binary` it is all of the binary. In matching, this default value is only valid for the very last element. All other binary elements in the matching must have a size specification.

`TypeSpecifierList` is a list of type specifiers, in any order, separated by hyphens (-). Default values are used for any omitted type specifiers.

`Type` = `integer` | `float` | `binary`
  The default is `integer`.
`Signedness` = `signed` | `unsigned`
  Only matters for matching. The default is `unsigned`.
`Endianness` = `big` | `little` | `native`
  Native-endian means that the endian will be resolved at load time to be either big-endian or little-endian, depending on what is native for the CPU that the Erlang machine is run on. The default is `big`.
`Unit` = `unit:IntegerLiteral`
  The allowed range is 1..256. Defaults to 1 for `integer` and `float`, and to 8 for `binary`.

The value of `Size` multiplied with the unit gives the number of bits for the segment. Each segment can consist of zero or more bits, but the total number of bits must be divisible by 8, or a `badarg` run-time error will occur. Also, a segment of type `binary` must have a size evenly divisble by 8.

Examples:

    1> Bin1 = <<1,17,42>>.
    <<1,17,42>>
    2> Bin2 = <<"abc">>.
    <<97,98,99>>
    3> Bin3 = <<1,17,42:16>>.
    <<1,17,0,42>>
    4> <<A,B,C:16>> = <<1,17,42:16>>.
    <<1,17,0,42>>
    5> C.
    42
    6> <<D:16,E,F>> = <<1,17,42:16>>.
    <<1,17,0,42>>
    7> D.
    273
    8> F.
    42
    9> <<G,H/binary>> = <<1,17,42:16>>.
    <<1,17,0,42>>
    10> H.
    <<17,0,42>>

Note that binary patterns cannot be nested.

Note also that "`B=<<1>>`" is interpreted as "`B =< <1>>`" which is a syntax error. The correct way is to write a space after '=': "`B= <<1>>`.

More examples can be found in Programming Examples.

### 6.17 Fun Expressions

    fun
        (Pattern11,...,Pattern1N) [when GuardSeq1] ->
            Body1;
        ...;
        (PatternK1,...,PatternKN) [when GuardSeqK] ->
            BodyK
    end

A fun expression begins with the keyword `fun` and ends with the keyword `end`. Between them should be a function declaration, similar to a regular function declaration, except that no function name is specified.

Variables in a fun head shadow variables in the function clause surrounding the fun expression, and variables bound in a fun body are local to the fun body.

The return value of the expression is the resulting fun.

Examples:

    1> Fun1 = fun (X) -> X+1 end.
    #Fun<erl_eval.6.39074546>
    2> Fun1(2).
    3
    3> Fun2 = fun (X) when X>=5 -> gt; (X) -> lt end.
    #Fun<erl_eval.6.39074546>
    4> Fun2(7).
    gt

The following fun expressions are also allowed:

    fun Name/Arity
    fun Module:Name/Arity

In `Name/Arity`, `Name` is an atom and `Arity` is an integer. `Name/Arity` must specify an existing local function. The expression is syntactic sugar for:

    fun (Arg1,...,ArgN) -> Name(Arg1,...,ArgN) end

In `Module:Name/Arity`, `Module` and `Name` are atoms and `Arity` is an integer. A fun defined in this way will refer to the function `Name` with arity `Arity` in the latest version of module `Module`.

When applied to a number N of arguments, a tuple `{Module,FunctionName}` is interpreted as a fun, referring to the function `FunctionName` with arity N in the module `Module`. The function must be exported. This usage is deprecated. See Function Calls for an example.

More examples can be found in Programming Examples.

### 6.18 Catch and Throw

    catch Expr

Returns the value of `Expr` unless an exception occurs during the evaluation. In that case, the exception is caught. For exceptions of class `error`, that is run-time errors: `{'EXIT',{Reason,Stack}}` is returned. For exceptions of class `exit`, that is the code called `exit(Term)`: `{'EXIT',Term}` is returned. For exceptions of class `throw`, that is the code called `throw(Term)`: `Term` is returned.

`Reason` depends on the type of error that occurred, and `Stack` is the stack of recent function calls, see Errors and Error Handling.

Examples:

    1> catch 1+2.
    3
    2> catch 1+a.
    {'EXIT',{badarith,[...]}}

Note that `catch` has low precedence and catch subexpressions often needs to be enclosed in a block expression or in parenthesis:

    3> A = catch 1+2.
    ** 1: syntax error before: 'catch' **
    4> A = (catch 1+2).
    3

The BIF `throw(Any)` can be used for non-local return from a function. It must be evaluated within a `catch`, which will return the value `Any`. Example:

    5> catch throw(hello).
    hello

If `throw/1` is not evaluated within a catch, a `nocatch` run-time error will occur.

### 6.19 Try

    try Expr
    catch
        [Class1:]ExceptionPattern1 [when ExceptionGuardSeq1] ->
            ExceptionBody1;
        [ClassN:]ExceptionPatternN [when ExceptionGuardSeqN] ->
            ExceptionBodyN
    end

This is an enhancement of catch that appeared in Erlang 5.4/OTP-R10B. It gives the possibility do distinguish between different exception classes, and to choose to handle only the desired ones, passing the others on to an enclosing `try` or `catch` or to default error handling.

Note that althought the keyword `catch` is used in the `try` expression there is not a `catch` expression whithin the `try` expression.

Returns the value of `Expr` unless an exception occurs during the evaluation. In that case the exception is caught and the patterns `ExceptionPattern` with the right exception class `Class` are sequentially matched against the caught exception. An omitted `Class` is shorthand for `throw`. If a match succeeds and the optional guard sequence `ExceptionGuardSeq` is true, the corresponding `ExceptionBody` is evaluated to become the return value.

If an exception occurs during evaluation of `Expr` but there is no matching `ExceptionPattern` of the right `Class` with a true guard sequence, the exception is passed on as if `Expr` had not been enclosed in a `try` expression.

If an exception occurs during evaluation of `ExceptionBody` it is not caught.

The `try` expression can have an `of` section:

    try Expr of
        Pattern1 [when GuardSeq1] ->
            Body1;
        ...;
        PatternN [when GuardSeqN] ->
            BodyN
    catch
        [Class1:]ExceptionPattern1 [when ExceptionGuardSeq1] ->
            ExceptionBody1;
        ...;
        [ClassN:]ExceptionPatternN [when ExceptionGuardSeqN] ->
            ExceptionBodyN
    end

If the evaluation of `Expr` succedes without an exception, the value is then matched against the patterns `Pattern` in the same way as for a case expression to produce the return value, except that if the matching fails, a `try_clause` run-time error will occur.

If an exception occurs during evaluation of `Body` it is not caught.

The `try` expression can also be augumented with an `after` section, intended to be used for cleanup with side effects:

    try Expr of
        Pattern1 [when GuardSeq1] ->
            Body1;
        ...;
        PatternN [when GuardSeqN] ->
            BodyN
    catch
        [Class1:]ExceptionPattern1 [when ExceptionGuardSeq1] ->
            ExceptionBody1;
        ...;
        [ClassN:]ExceptionPatternN [when ExceptionGuardSeqN] ->
            ExceptionBodyN
    after
        AfterBody
    end

The `AfterBody` is evaluated after either `Body` or `ExceptionBody` no matter which one. The evaluated value of the `AfterBody` is lost; the return value of the `try` expression is the same with an `after` section as without.

Even if an exception occurs during evaluation of `Body` or `ExceptionBody`, the `AfterBody` is evaluated. In this case the exception is caught and passed on after the `AfterBody` has been evaluated, so the exception from the `try` expression is the same with an `after` section as without.

If an exception occurs during evaluation of `AfterBody` itself it is not caught, so if the `AfterBody` is evaluated due to an exception in `Expr`, `Body` or `ExceptionBody`, that exception is lost and masked by the new exception.

The `of`, `catch` and `after` sections, are all optional in the `try` expression, as long as there is at least a `catch` or an `after` section, so the following are also valid `try` expressions:

    try Expr of
        Pattern when GuardSeq ->
            Body
    after
        AfterBody
    end

    try Expr
    catch
        ExpressionPattern ->
            ExpressionBody
    after
        AfterBody
    end

    try Expr after AfterBody end

Example of using `after`, this code will close the file even in the event of exceptions in `file:read/2` or in `binary_to_term/1`, and exceptions will be the same as without the `try`...`after`...`end` expression:

    termize_file(Name) ->
        {ok,F} = file:open(Name, [read,binary]),
        try
            {ok,Bin} = file:read(F, 1024*1024),
            binary_to_term(Bin)
        after
            file:close(F)
        end.

Example: Using `try` to emulate `catch Expr`

    try Expr
    catch
        throw:Term -> Term;
        exit:Reason -> {'EXIT',Reason}
        error:Reason -> {'EXIT',{Reason,erlang:get_stacktrace()}}
    end

### 6.20 Parenthesized Expressions

    (Expr)

Parenthesized expressions are useful to override operator precedences, for example in arithmethic expressions:

    1> 1 + 2 * 3.
    7
    2> (1 + 2) * 3.
    9

### 6.21 Block Expressions

    begin
       Expr1,
       ...,
       ExprN
    end

Block expressions provide a way to group a sequence of expressions, similar to a clause body. The return value is the value of the last expression `ExprN`.

### 6.22 List Comprehensions

List comprehensions are a feature of many modern functional programming languages. Subject to certain rules, they provide a succinct notation for generating elements in a list.

List comprehensions are analogous to set comprehensions in Zermelo-Frankel set theory and are called ZF expressions in Miranda. They are analogous to the `setof` and `findall` predicates in Prolog.

List comprehensions are written with the following syntax:

    [Expr || Qualifier1,...,QualifierN]

`Expr` is an arbitrary expression, and each `Qualifier` is either a generator or a filter.

- A generator is written as:
  `Pattern <- ListExpr`.
  `ListExpr` must be an expression which evaluates to a list of terms.
- A filter is an expression which evaluates to `true` or `false`.

The variables in the generator patterns shadow variables in the function clause surrounding the list comprehensions.

A list comprehension returns a list, where the elements are the result of evaluating `Expr` for each combination of generator list elements for which all filters are true.

Example:

    1> [X*2 || X <- [1,2,3]].
    [2,4,6]

More examples can be found in Programming Examples.

### 6.23 Guard Sequences

A guard sequence is a sequence of guards, separated by semicolon (;). The guard sequence is true if at least one of the guards is true.
`Guard1;...;GuardK`

A guard is a sequence of guard expressions, separated by comma (,). The guard is true if all guard expressions evaluate to `true`.
`GuardExpr1,...,GuardExprN`

The set of valid guard expressions (sometimes called guard tests) is a subset of the set of valid Erlang expressions. The reason for restricting the set of valid expressions is that evaluation of a guard expression must be guaranteed to be free of side effects. Valid guard expressions are:

- the atom `true`,
- other constants (terms and bound variables), all regarded as false,
- calls to the BIFs specified below,
- term comparisons,
- arithmetic expressions,
- boolean expressions, and
- short-circuit boolean expressions.

Type Test BIFs.
`is_atom/1`
`is_binary/1`
`is_constant/1`
`is_float/1`
`is_function/1`
`is_function/2`
`is_integer/1`
`is_list/1`
`is_number/1`
`is_pid/1`
`is_port/1`
`is_reference/1`
`is_tuple/1`
`is_record/2`
`is_record/3`

Note that each type test BIF has an older equivalent, without the `is_` prefix. These old BIFs are retained for backwards compatibility only and should not be used in new code. They are also only allowed at top level. For example, they are not allowed in boolean expressions in guards.

Other BIFs Allowed in Guard Expressions.
`abs(Number)`
`element(N, Tuple)`
`float(Term)`
`hd(List)`
`length(List)`
`node()`
`node(Pid|Ref|Port)`
`round(Number)`
`self()`
`size(Tuple|Binary)`
`tl(List)`
`trunc(Number)`

### 6.24 Operator Precedence

Operator precedence in falling priority:

Operator Precedence.
: |
# |
Unary + - bnot not |
/ * div rem band and | Left associative
+ - bor bxor bsl bsr or xor | Left associative
++ -- | Right associative
== /= =< < >= > =:= =/= | -
andalso |
orelse |
= ! | Right associative
catch |

When evaluating an expression, the operator with the highest priority is evaluated first. Operators with the same priority are evaluated according to their associativity. Example: The left associative arithmethic operators are evaluated left to right:

    6 + 5 * 4 - 3 / 2 evaluates to
    6 + 20 - 1.5 evaluates to
    26 - 1.5 evaluates to
    24.5

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