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# Expressions

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In this section, 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 sections:

- Preprocessor
- Records

## 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 causes a `badarg` runtime error.

## Terms

The simplest form of expression is a term, that is one of `integer/0`, `float/0`, `atom/0`, `string/0`, `list/0`, `map/0`, or `tuple/0`. The return value is the term itself.

## 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 (`_`). Variables can contain alphanumeric characters, underscore, and `@`.

Examples:

    X
    Name1
    PhoneNumber
    Phone_number
    _
    _Height
    name@node

Variables are bound to values using pattern matching. Erlang uses single assignment, that is, 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. However, they are ignored by the compiler in the sense that they do not generate warnings.

Example:

The following code:

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

can be rewritten to be more readable:

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

This causes a warning for an unused variable, `Elem`. To avoid the warning, the code can be rewritten to:

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

Notice that since variables starting with an underscore are not anonymous, the following example matches:

    {_,_} = {1,2}

But this example fails:

    {_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 are regarded as unsafe outside the expression.

For the `try` expression variable scoping is limited so that variables bound in the expression are always unsafe outside the expression.

## Patterns

A pattern has the same structure as a term but can contain unbound variables.

Example:

    Name1
    [H|T]
    {error,Reason}

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

### The Compound Pattern Operator

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

    Pattern1 = Pattern2

When matched against a term, both `Pattern1` and `Pattern2` are 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.

The compound pattern operator does not imply that its operands are matched in any particular order. That means that it is not legal to bind a variable in `Pattern1` and use it in `Pattern2`, or vice versa.

### 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]) -> ...

### Expressions in Patterns

An arithmetic expression can be used within a pattern if it meets both of the following two conditions:

- It uses only numeric or bitwise operators.
- Its value can be evaluated to a constant when complied.

Example:

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

## The Match Operator

The following matches `Pattern` against `Expr`:

    Pattern = Expr

If the matching succeeds, any unbound variable in the pattern becomes bound and the value of `Expr` is returned.

If multiple match operators are applied in sequence, they will be evaluated from right to left.

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

Examples:

    1> {A, B} = T = {answer, 42}.
    {answer,42}
    2> A.
    answer
    3> B.
    42
    4> T.
    {answer,42}
    5> {C, D} = [1, 2].
    ** exception error: no match of right-hand side value [1,2]

Because multiple match operators are evaluated from right to left, it means that:

    Pattern1 = Pattern2 = . . . = PatternN = Expression

is equivalent to:

    Temporary = Expression,
    PatternN = Temporary,
       .
       .
       .,
    Pattern2 = Temporary,
    Pattern = Temporary

## The Match Operator and the Compound Pattern Operator

#### Note

This is an advanced section, which references to topics not yet introduced. It can safely be skipped on a first reading.

The `=` character is used to denote two similar but distinct operators: the match operator and the compound pattern operator. Which one is meant is determined by context.

The compound pattern operator is used to construct a compound pattern from two patterns. Compound patterns are accepted everywhere a pattern is accepted. A compound pattern matches if all of its constituent patterns match. It is not legal for a pattern that is part of a compound pattern to use variables (as keys in map patterns or sizes in binary patterns) bound in other sub patterns of the same compound pattern.

Examples:

    1> fun(#{Key := Value} = #{key := Key}) -> Value end.
    * 1:7: variable 'Key' is unbound
    2> F = fun({A, B} = E) -> {E, A + B} end, F({1,2}).
    {{1,2},3}
    3> G = fun(<<A:8,B:8>> = <<C:16>>) -> {A, B, C} end, G(<<42,43>>).
    {42,43,10795}

The match operator is allowed everywhere an expression is allowed. It is used to match the value of an expression to a pattern. If multiple match operators are applied in sequence, they will be evaluated from right to left.

Examples:

    1> M = #{key => key2, key2 => value}.
    #{key => key2,key2 => value}
    2> f(Key), #{Key := Value} = #{key := Key} = M, Value.
    value
    3> f(Key), #{Key := Value} = (#{key := Key} = M), Value.
    value
    4> f(Key), (#{Key := Value} = #{key := Key}) = M, Value.
    * 1:12: variable 'Key' is unbound
    5> <<X:Y>> = begin Y = 8, <<42:8>> end, X.
    42

The expression at prompt `2>` first matches the value of variable `M` against pattern `#{key := Key}`, binding variable `Key`. It then matches the value of `M` against pattern `#{Key := Value}` using variable `Key` as the key, binding variable `Value`.

The expression at prompt `3>` matches expression `(#{key := Key} = M)` against pattern `#{Key := Value}`. The expression inside the parentheses is evaluated first. That is, `M` is matched against `#{key := Key}`, and then the value of `M` is matched against pattern `#{Key := Value}`. That is the same evaluation order as in 2; therefore, the parentheses are redundant.

In the expression at prompt `4>` the expression `M` is matched against a pattern inside parentheses. Since the construct inside the parentheses is a pattern, the `=` that separates the two patterns is the compound pattern operator (not the match operator). The match fails because the two sub patterns are matched at the same time, and the variable `Key` is therefore not bound when matching against pattern `#{Key := Value}`.

In the expression at prompt `5>` the expressions inside the block expression are evaluated first, binding variable `Y` and creating a binary. The binary is then matched against pattern `<<X:Y>>` using the value of `Y` as the size of the segment.

## Function Calls

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

In the first form of function calls, `ExprM:ExprF(Expr1,...,ExprN)`, each of `ExprM` and `ExprF` must be an atom or an expression that evaluates to an atom. 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:keyfind(Name, 1, List)

In the second form of function calls, `ExprF(Expr1,...,ExprN)`, `ExprF` must be an atom or evaluate to a fun.

If `ExprF` is an atom, the function is said to be called by using the implicitly qualified function name. If the function `ExprF` is locally defined, it is called. Alternatively, if `ExprF` is explicitly imported from the `M` module, `M:ExprF(Expr1,...,ExprN)` is called. If `ExprF` is neither declared locally nor explicitly imported, `ExprF` must be the name of an automatically imported BIF.

Examples:

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

Examples where `ExprF` is a fun:

    1> Fun1 = fun(X) -> X+1 end,
    Fun1(3).
    4
    2> fun lists:append/2([1,2], [3,4]).
    [1,2,3,4]
    3>

Notice that when calling a local function, there is a difference between using the implicitly or fully qualified function name. The latter always refers to the latest version of the module. See Compilation and Code Loading and Function Evaluation.

### Local Function Names Clashing With Auto-Imported BIFs

If a local function has the same name as an auto-imported BIF, the semantics is that implicitly qualified function calls are directed to the locally defined function, not to the BIF. To avoid confusion, there is a compiler directive available, `-compile({no_auto_import,[F/A]})`, that makes a BIF not being auto-imported. In certain situations, such a compile-directive is mandatory.

#### Change

Before Erlang/OTP R14A (ERTS version 5.8), an implicitly qualified function call to a function having the same name as an auto-imported BIF always resulted in the BIF being called. In newer versions of the compiler, the local function is called instead. This is to avoid that future additions to the set of auto-imported BIFs do not silently change the behavior of old code.

However, to avoid that old (pre R14) code changed its behavior when compiled with Erlang/OTP version R14A or later, the following restriction applies: If you override the name of a BIF that was auto-imported in OTP versions prior to R14A (ERTS version 5.8) and have an implicitly qualified call to that function in your code, you either need to explicitly remove the auto-import using a compiler directive, or replace the call with a fully qualified function call. Otherwise you get a compilation error. See the following example:

    -export([length/1,f/1]).

    -compile({no_auto_import,[length/1]}). % erlang:length/1 no longer autoimported

    length([]) ->
        0;
    length([H|T]) ->
        1 + length(T). %% Calls the local function length/1

    f(X) when erlang:length(X) > 3 -> %% Calls erlang:length/1,
                                      %% which is allowed in guards
        long.

The same logic applies to explicitly imported functions from other modules, as to locally defined functions. It is not allowed to both import a function from another module and have the function declared in the module at the same time:

    -export([f/1]).

    -compile({no_auto_import,[length/1]}). % erlang:length/1 no longer autoimported

    -import(mod,[length/1]).

    f(X) when erlang:length(X) > 33 -> %% Calls erlang:length/1,
                                       %% which is allowed in guards

        erlang:length(X);              %% Explicit call to erlang:length in body

    f(X) ->
        length(X).                     %% mod:length/1 is called

For auto-imported BIFs added in Erlang/OTP R14A and thereafter, overriding the name with a local function or explicit import is always allowed. However, if the `-compile({no_auto_import,[F/A])` directive is not used, the compiler issues a warning whenever the function is called in the module using the implicitly qualified function name.

## If

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

The branches of an `if`-expression are scanned sequentially until a guard sequence `GuardSeq` that evaluates to true is found. Then the corresponding `Body` (a 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 evaluated as true, an `if_clause` run-time error occurs. 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

## 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 occurs.

Example:

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

## Maybe

#### Change

The `maybe` feature was introduced in Erlang/OTP 25. Starting from Erlang/OTP 27 is is enabled by default.

    maybe
        Expr1,
        ...,
        ExprN
    end

The expressions in a `maybe` block are evaluated sequentially. If all expressions are evaluated successfully, the return value of the `maybe` block is `ExprN`. However, execution can be short-circuited by a conditional match expression:

    Expr1 ?= Expr2

`?=` is called the conditional match operator. It is only allowed to be used at the top-level of a `maybe` block. It matches the pattern `Expr1` against `Expr2`. If the matching succeeds, any unbound variable in the pattern becomes bound. If the expression is the last expression in the `maybe` block, it also returns the value of `Expr2`. If the matching is unsuccessful, the rest of the expressions in the `maybe` block are skipped and the return value of the `maybe` block is `Expr2`.

None of the variables bound in a `maybe` block must be used in the code that follows the block.

Here is an example:

    maybe
        {ok, A} ?= a(),
        true = A >= 0,
        {ok, B} ?= b(),
        A + B
    end

Let us first assume that `a()` returns `{ok,42}` and `b()` returns `{ok,58}`. With those return values, all of the match operators will succeed, and the return value of the `maybe` block is `A + B`, which is equal to `42 + 58 = 100`.

Now let us assume that `a()` returns `error`. The conditional match operator in `{ok, A} ?= a()` fails to match, and the return value of the `maybe` block is the value of the expression that failed to match, namely `error`. Similarly, if `b()` returns `wrong`, the return value of the `maybe` block is `wrong`.

Finally, let us assume that `a()` returns `{ok,-1}`. Because `true = A >= 0` uses the match operator `=`, a `{badmatch,false}` run-time error occurs when the expression fails to match the pattern.

The example can be written in a less succient way using nested case expressions:

    case a() of
        {ok, A} ->
            true = A >= 0,
            case b() of
                {ok, B} ->
                    A + B;
                Other1 ->
                    Other1
            end;
        Other2 ->
            Other2
    end

The `maybe` block can be augmented with `else` clauses:

    maybe
        Expr1,
        ...,
        ExprN
    else
        Pattern1 [when GuardSeq1] ->
            Body1;
        ...;
        PatternN [when GuardSeqN] ->
            BodyN
    end

If a conditional match operator fails, the failed expression is matched against the patterns in all clauses between the `else` and `end` keywords. If a match succeeds and the optional guard sequence `GuardSeq` is true, the corresponding `Body` is evaluated. The value returned from the body is the return value of the `maybe` block.

If there is no matching pattern with a true guard sequence, an `else_clause` run-time error occurs.

None of the variables bound in a `maybe` block must be used in the `else` clauses. None of the variables bound in the `else` clauses must be used in the code that follows the `maybe` block.

Here is the previous example augmented with `else` clauses:

    maybe
        {ok, A} ?= a(),
        true = A >= 0,
        {ok, B} ?= b(),
        A + B
    else
        error -> error;
        wrong -> error
    end

The `else` clauses translate the failing value from the conditional match operators to the value `error`. If the failing value is not one of the recognized values, a `else_clause` run-time error occurs.

## 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, an alias (reference), a port, a registered name (atom), or a tuple `{Name,Node}`. `Name` is an atom and `Node` is a node name, also an atom.

- If `Expr1` evaluates to a name, but this name is not registered, a `badarg` run-time error occurs.
- Sending a message to a reference never fails, even if the reference is no longer (or never was) an alias.
- 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.

## Receive

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

Fetches a received message present in the message queue of the process. The first message in the message queue is matched sequentially against the patterns from top to bottom. If no match was found, the matching sequence is repeated for the second message in the queue, and so on. Messages are queued in the order they were received. If a match succeeds, that is, if the `Pattern` matches and the optional guard sequence `GuardSeq` is true, then the message is removed from the message queue and the corresponding `Body` is evaluated. All other messages in the message queue remain unchanged.

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

`receive` never fails. The execution is suspended, possibly indefinitely, until a message arrives that matches 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.

The `receive` expression can be augmented with a timeout:

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

`receive...after` works exactly as `receive`, except that if no matching message has arrived within `ExprT` milliseconds, then `BodyT` is evaluated instead. The return value of `BodyT` then becomes the return value of the `receive...after` expression. `ExprT` is to evaluate to an integer, or the atom `infinity`. The allowed integer range is from 0 to 4294967295, that is, the longest possible timeout is almost 50 days. With a zero value the timeout occurs immediately if there is no matching message in the message queue.

The atom `infinity` will make the process wait indefinitely for a matching message. This is the same as not using a timeout. It can be useful for timeout values that are calculated at runtime.

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 does not consume any messages, only suspends execution in the process for `ExprT` milliseconds. This can be used to implement simple timers.

Example:

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

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

For more information on timers in Erlang in general, see the Timers section of the Time and Time Correction in Erlang ERTS User's guide.

## Term Comparisons

    Expr1 op Expr2

op | Description
--- | ---
`==` | Equal to
`/=` | Not equal to
`=<` | Less than or equal to
`<` | Less than
`>=` | Greater than or equal to
`>` | Greater than
`=:=` | Term equivalence
`=/=` | Term non-equivalence

Table: Term Comparison Operators.

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

    number < atom < reference < fun < port < pid < tuple < map < nil < list < bit string

`nil` in the previous expression represents the empty list (`[]`), which is regarded as a separate type from `list/0`. That is why `nil < list`.

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

Bit strings are compared bit by bit. If one bit string is a prefix of the other, the shorter bit string is considered smaller.

Maps are ordered by size, two maps with the same size are compared by keys in ascending term order and then by values in key order. In maps key order integers types are considered less than floats types.

Atoms are compared using their string value, codepoint by codepoint.

When comparing an integer to a float, the term with the lesser precision is converted into the type of the other term, unless the operator is one of `=:=` or `=/=`. A float is more precise than an integer until all significant figures of the float are to the left of the decimal point. This happens when the float is larger/smaller than +/-9007199254740992.0. The conversion strategy is changed depending on the size of the float because otherwise comparison of large floats and integers would lose their transitivity.

The term equivalence operators, `=:=` and `=/=`, return whether two terms are indistinguishable. While the other operators consider the same numbers equal even when their types differ (`1 == 1.0` is true), the term equivalence operators return whether or not there exists a way to tell the arguments apart.

For example, while the terms `0` and `0.0` represent the same number, we can tell them apart by using the `is_integer/1` function. Hence, `=:=` and `=/=` consider them different.

Furthermore, the terms `0.0` and `-0.0` also represent the same number, but they yield different results when converted to string form through `float_to_list/1`: when given the former it returns a string without a sign, and when given the latter it returns a string with a sign. Therefore, `=:=` and `=/=` consider them different.

The term equivalence operators are useful when reasoning about terms as opaque values, for example in associative containers or memoized functions where using the equal-to operator (`==`) risks producing incorrect results as a consequence of mixing up numbers of different types.

Term comparison operators return the Boolean value of the expression, `true` or `false`.

Examples:

    1> 1 == 1.0.
    true
    2> 1 =:= 1.0.
    false
    3> 0 =:= 0.0.
    false
    4> 0.0 =:= -0.0.
    false
    5> 0.0 =:= +0.0.
    true
    6> 1 > a.
    false
    7> #{c => 3} > #{a => 1, b => 2}.
    false
    8> #{a => 1, b => 2} == #{a => 1.0, b => 2.0}.
    true
    9> <<2:2>> < <<128>>.
    true
    10> <<3:2>> < <<128>>.
    false

#### Note

Prior to OTP 27, the term equivalence operators considered `0.0` and `-0.0` to be the same term.

This was changed in OTP 27 but legacy code may have expected them to be considered the same. To help users catch errors that may arise from an upgrade, the compiler raises a warning when `0.0` is pattern-matched or used in a term equivalence test.

If you need to match `0.0` specifically, the warning can be silenced by writing `+0.0` instead, which produces the same term but makes the compiler interpret the match as being done on purpose.

## Arithmetic Expressions

    op Expr
    Expr1 op Expr2

Operator | Description | Argument Type
--- | --- | ---
`+` | Unary + | Number
`-` | Negation (unary -) | Number
`+` | Addition | Number
`-` | Subtraction | Number
`*` | Multiplication | 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` | Bitwise XOR | Integer
`bsl` | Bitshift left | Integer
`bsr` | Arithmetic bitshift right | Integer

Table: Arithmetic Operators.

Examples:

    1> +1.
    1
    2> -1.
    -1
    3> 1+1.
    2
    4> 4/2.
    2.0
    5> 5 div 2.
    2
    6> 5 rem 2.
    1
    7> 2#10 band 2#01.
    0
    8> 2#10 bor 2#01.
    3
    9> a + 10.
    ** exception error: an error occurred when evaluating an arithmetic expression
         in operator  +/2
            called as a + 10
    10> 1 bsl (1 bsl 64).
    ** exception error: a system limit has been reached
         in operator  bsl/2
            called as 1 bsl 18446744073709551616

## Boolean Expressions

    op Expr
    Expr1 op Expr2

Operator | Description
--- | ---
`not` | Unary logical NOT
`and` | Logical AND
`or` | Logical OR
`xor` | Logical XOR

Table: Logical Operators.

Examples:

    1> not true.
    false
    2> true and false.
    false
    3> true xor false.
    true
    4> true or garbage.
    ** exception error: bad argument
         in operator  or/2
            called as true or garbage

## Short-Circuit Expressions

    Expr1 orelse Expr2
    Expr1 andalso Expr2

`Expr2` is evaluated only if necessary. That is, `Expr2` is evaluated only if:

- `Expr1` evaluates to `false` in an `orelse` expression.

or

- `Expr1` evaluates to `true` in an `andalso` expression.

Returns either the value of `Expr1` (that is, `true` or `false`) or the value of `Expr2` (if `Expr2` is evaluated).

Example 1:

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

This works 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),

`Expr2` is not required to evaluate to a Boolean value. Because of that, `andalso` and `orelse` are tail-recursive.

Example 3 (tail-recursive function):

    all(Pred, [Hd|Tail]) ->
        Pred(Hd) andalso all(Pred, Tail);
    all(_, []) ->
        true.

#### Change

Before Erlang/OTP R13A, `Expr2` was required to evaluate to a Boolean value, and as consequence, `andalso` and `orelse` were not tail-recursive.

## 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 that is a copy of the first argument. The procedure is as follows: 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]

## Map Expressions

### Creating Maps

Constructing a new map is done by letting an expression `K` be associated with another expression `V`:

    #{K => V}

New maps can include multiple associations at construction by listing every association:

    #{K1 => V1, ..., Kn => Vn}

An empty map is constructed by not associating any terms with each other:

    #{}

All keys and values in the map are terms. Any expression is first evaluated and then the resulting terms are used as key and value respectively.

Keys and values are separated by the `=>` arrow and associations are separated by a comma (`,`).

Examples:

    M0 = #{},                 % empty map
    M1 = #{a => <<"hello">>}, % single association with literals
    M2 = #{1 => 2, b => b},   % multiple associations with literals
    M3 = #{k => {A,B}},       % single association with variables
    M4 = #{{"w", 1} => f()}.  % compound key associated with an evaluated expression

Here, `A` and `B` are any expressions and `M0` through `M4` are the resulting map terms.

If two matching keys are declared, the latter key takes precedence.

Example:

    1> #{1 => a, 1 => b}.
    #{1 => b }
    2> #{1.0 => a, 1 => b}.
    #{1 => b, 1.0 => a}

The order in which the expressions constructing the keys (and their associated values) are evaluated is not defined. The syntactic order of the key-value pairs in the construction is of no relevance, except in the recently mentioned case of two matching keys.

### Updating Maps

Updating a map has a similar syntax as constructing it.

An expression defining the map to be updated is put in front of the expression defining the keys to be updated and their respective values:

    M#{K => V}

Here `M` is a term of type map and `K` and `V` are any expression.

If key `K` does not match any existing key in the map, a new association is created from key `K` to value `V`.

If key `K` matches an existing key in map `M`, its associated value is replaced by the new value `V`. In both cases, the evaluated map expression returns a new map.

If `M` is not of type map, an exception of type `badmap` is raised.

To only update an existing value, the following syntax is used:

    M#{K := V}

Here `M` is a term of type map, `V` is an expression and `K` is an expression that evaluates to an existing key in `M`.

If key `K` does not match any existing keys in map `M`, an exception of type `badkey` is raised at runtime. If a matching key `K` is present in map `M`, its associated value is replaced by the new value `V`, and the evaluated map expression returns a new map.

If `M` is not of type map, an exception of type `badmap` is raised.

Examples:

    M0 = #{},
    M1 = M0#{a => 0},
    M2 = M1#{a => 1, b => 2},
    M3 = M2#{"function" => fun() -> f() end},
    M4 = M3#{a := 2, b := 3}.  % 'a' and 'b' was added in `M1` and `M2`.

Here `M0` is any map. It follows that `M1` through `M4` are maps as well.

More examples:

    1> M = #{1 => a}.
    #{1 => a }
    2> M#{1.0 => b}.
    #{1 => a, 1.0 => b}.
    3> M#{1 := b}.
    #{1 => b}
    4> M#{1.0 := b}.
    ** exception error: bad argument

As in construction, the order in which the key and value expressions are evaluated is not defined. The syntactic order of the key-value pairs in the update is of no relevance, except in the case where two keys match. In that case, the latter value is used.

### Maps in Patterns

Matching of key-value associations from maps is done as follows:

    #{K := V} = M

Here `M` is any map. The key `K` must be a guard expression, with all variables already bound. `V` can be any pattern with either bound or unbound variables.

If the variable `V` is unbound, it becomes bound to the value associated with the key `K`, which must exist in the map `M`. If the variable `V` is bound, it must match the value associated with `K` in `M`.

#### Change

Before Erlang/OTP 23, the expression defining the key `K` was restricted to be either a single variable or a literal.

Example:

    1> M = #{"tuple" => {1,2}}.
    #{"tuple" => {1,2}}
    2> #{"tuple" := {1,B}} = M.
    #{"tuple" => {1,2}}
    3> B.
    2.

This binds variable `B` to integer `2`.

Similarly, multiple values from the map can be matched:

    #{K1 := V1, ..., Kn := Vn} = M

Here keys `K1` through `Kn` are any expressions with literals or bound variables. If all key expressions evaluate successfully and all keys exist in map `M`, all variables in `V1 .. Vn` is matched to the associated values of their respective keys.

If the matching conditions are not met the match fails.

Note that when matching a map, only the `:=` operator (not the `=>`) is allowed as a delimiter for the associations.

The order in which keys are declared in matching has no relevance.

Duplicate keys are allowed in matching and match each pattern associated to the keys:

    #{K := V1, K := V2} = M

The empty map literal (`#{}`) matches any map when used as a pattern:

    #{} = Expr

This expression matches if the expression `Expr` is of type map, otherwise it fails with an exception `badmatch`.

Here the key to be retrieved is constructed from an expression:

    #{{tag,length(List)} := V} = Map

`List` must be an already bound variable.

#### Matching Syntax

Matching of literals as keys are allowed in function heads:

    %% only start if not_started
    handle_call(start, From, #{state := not_started} = S) ->
    ...
        {reply, ok, S#{state := start}};

    %% only change if started
    handle_call(change, From, #{state := start} = S) ->
    ...
        {reply, ok, S#{state := changed}};

### Maps in Guards

Maps are allowed in guards as long as all subexpressions are valid guard expressions.

The following guard BIFs handle maps:

- is_map/1 in the `erlang` module
- is_map_key/2 in the `erlang` module
- map_get/2 in the `erlang` module
- map_size/1 in the `erlang` module

## Bit Syntax Expressions

The bit syntax operates on bit strings. A bit string is a sequence of bits ordered from the most significant bit to the least significant bit.

    <<>>  % The empty bit string, zero length
    <<E1>>
    <<E1,...,En>>

Each element `Ei` specifies a segment of the bit string. The segments are ordered left to right from the most significant bit to the least significant bit of the bit string.

Each segment specification `Ei` is a value, whose default type is `integer`, followed by an optional size expression and an optional type specifier list.

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

When used in a bit string construction, `Value` is an expression that is to evaluate to an integer, float, or bit string. If the expression is not a single literal or variable, it is to be enclosed in parentheses.

When used in a bit string matching, `Value` must be a variable, or an integer, float, or string.

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

When used in a bit string construction, `Size` is an expression that is to evaluate to an integer.

When used in a bit string matching, `Size` must be a guard expression that evaluates to an integer. All variables in the guard expression must be already bound.

#### Change

Before Erlang/OTP 23, `Size` was restricted to 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` and `bitstring` it is the whole binary or bit string.

In matching, the default value for a binary or bit string segment is only valid for the last element. All other bit string or binary elements in the matching must have a size specification.

Binaries

A bit string with a length that is a multiple of 8 bits is known as a binary, which is the most common and useful type of bit string.

A binary has a canonical representation in memory. Here follows a sequence of bytes where each byte's value is its sequence number:

    <<1, 2, 3, 4, 5, 6, 7, 8, 9, 10>>

Bit strings are a later generalization of binaries, so many texts and much information about binaries apply just as well for bit strings.

Example:

    1> <<A/binary, B/binary>> = <<"abcde">>.
    * 1:3: a binary field without size is only allowed at the end of a binary pattern
    2> <<A:3/binary, B/binary>> = <<"abcde">>.
    <<"abcde">>
    3> A.
    <<"abc">>
    4> B.
    <<"de">>

For the `utf8`, `utf16`, and `utf32` types, `Size` must not be given. The size of the segment is implicitly determined by the type and value itself.

`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` | `bytes` | `bitstring` | `bits` | `utf8` | `utf16` | `utf32` - The default is `integer`. `bytes` is a shorthand for `binary` and `bits` is a shorthand for `bitstring`. See below for more information about the `utf` types.

- `Signedness`= `signed` | `unsigned` - Only matters for matching and when the type is `integer`. The default is `unsigned`.

- `Endianness`= `big` | `little` | `native` - Specifies byte level (octet level) endianness (byte order). Native-endian means that the endianness is 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. Endianness only matters when the Type is either `integer`, `utf16`, `utf32`, or `float`. The default is `big`.

      <<16#1234:16/little>> = <<16#3412:16>> = <<16#34:8, 16#12:8>>

- `Unit`= `unit:IntegerLiteral` - The allowed range is 1 through 256. Defaults to 1 for `integer`, `float`, and `bitstring`, and to 8 for `binary`. For types `bitstring`, `bits`, and `bytes`, it is not allowed to specify a unit value different from the default value. No unit specifier must be given for the types `utf8`, `utf16`, and `utf32`.

### Integer segments

The value of `Size` multiplied with the unit gives the size of the segment in bits.

When constructing bit strings, if the size `N` of an integer segment is too small to contain the given integer, the most significant bits of the integer are silently discarded and only the `N` least significant bits are put into the bit string. For example, `<<16#ff:4>>` will result in the bit string `<<15:4>>`.

### Float segments

The value of `Size` multiplied with the unit gives the size of the segment in bits. The size of a float segment in bits must be one of 16, 32, or 64.

When constructing bit strings, if the size of a float segment is too small to contain the representation of the given float value, an exception is raised.

When matching bit strings, matching of float segments fails if the bits of the segment does not contain the representation of a finite floating point value.

### Binary segments

In this section, the phrase "binary segment" refers to any one of the segment types `binary`, `bitstring`, `bytes`, and `bits`.

See also the paragraphs about Binaries.

When constructing binaries and no size is specified for a binary segment, the entire binary value is interpolated into the binary being constructed. However, the size in bits of the binary being interpolated must be evenly divisible by the unit value for the segment; otherwise an exception is raised.

For example, the following examples all succeed:

    1> <<(<<"abc">>)/bitstring>>.
    <<"abc">>
    2> <<(<<"abc">>)/binary-unit:1>>.
    <<"abc">>
    3> <<(<<"abc">>)/binary>>.
    <<"abc">>

The first two examples have a unit value of 1 for the segment, while the third segment has a unit value of 8.

Attempting to interpolate a bit string of size 1 into a binary segment with unit 8 (the default unit for `binary`) fails as shown in this example:

    1> <<(<<1:1>>)/binary>>.
    ** exception error: bad argument

For the construction to succeed, the unit value of the segment must be 1:

    2> <<(<<1:1>>)/bitstring>>.
    <<1:1>>
    3> <<(<<1:1>>)/binary-unit:1>>.
    <<1:1>>

Similarly, when matching a binary segment with no size specified, the match succeeds if and only if the size in bits of the rest of the binary is evenly divisible by the unit value:

    1> <<_/binary-unit:16>> = <<"">>.
    <<>>
    2> <<_/binary-unit:16>> = <<"a">>.
    ** exception error: no match of right hand side value <<"a">>
    3> <<_/binary-unit:16>> = <<"ab">>.
    <<"ab">>
    4> <<_/binary-unit:16>> = <<"abc">>.
    ** exception error: no match of right hand side value <<"abc">>
    5> <<_/binary-unit:16>> = <<"abcd">>.
    <<"abcd">>

When a size is explicitly specified for a binary segment, the segment size in bits is the value of `Size` multiplied by the default or explicit unit value.

When constructing binaries, the size of the binary being interpolated into the constructed binary must be at least as large as the size of the binary segment.

Examples:

    1> <<(<<"abc">>):2/binary>>.
    <<"ab">>
    2> <<(<<"a">>):2/binary>>.
    ** exception error: construction of binary failed
            *** segment 1 of type 'binary': the value <<"a">> is shorter than the size of the segment

### Unicode segments

The types `utf8`, `utf16`, and `utf32` specifies encoding/decoding of the Unicode Transformation Formats UTF-8, UTF-16, and UTF-32, respectively.

When constructing a segment of a `utf` type, `Value` must be an integer in the range `0` through `16#D7FF` or `16#E000` through `16#10FFFF`. Construction fails with a `badarg` exception if `Value` is outside the allowed ranges. The sizes of the encoded values are as follows:

- For `utf8`, `Value` is encoded in 1-4 bytes.
- For `utf16`, `Value` is encoded in 2 or 4 bytes.
- For `utf32`, `Value` is encoded in 4 bytes.

When constructing, a literal string can be given followed by one of the UTF types, for example: `<<"abc"/utf8>>` which is syntactic sugar for `<<$a/utf8,$b/utf8,$c/utf8>>`.

A successful match of a segment of a `utf` type, results in an integer in the range `0` through `16#D7FF` or `16#E000` through `16#10FFFF`. The match fails if the returned value falls outside those ranges.

A segment of type `utf8` matches 1-4 bytes in the bit string, if the bit string at the match position contains a valid UTF-8 sequence. (See RFC-3629 or the Unicode standard.)

A segment of type `utf16` can match 2 or 4 bytes in the bit string. The match fails if the bit string at the match position does not contain a legal UTF-16 encoding of a Unicode code point. (See RFC-2781 or the Unicode standard.)

A segment of type `utf32` can match 4 bytes in the bit string in the same way as an `integer` segment matches 32 bits. The match fails if the resulting integer is outside the legal ranges previously mentioned.

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>>
    11> <<G,J/bitstring>> = <<1,17,42:12>>.
    <<1,17,2,10:4>>
    12> J.
    <<17,2,10:4>>

    13> <<1024/utf8>>.
    <<208,128>>

    14> <<1:1,0:7>>.
    <<128>>
    15> <<16#123:12/little>> = <<16#231:12>> = <<2:4, 3:4, 1:4>>.
    <<35,1:4>>

Notice that bit string patterns cannot be nested.

Notice 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 are provided in Programming Examples.

## Fun Expressions

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

A fun expression begins with the keyword `fun` and ends with the keyword `end`. Between them is to be a function declaration, similar to a regular function declaration, except that the function name is optional and is to be a variable, if any.

Variables in a fun head shadow the function name and both shadow variables in the function clause surrounding the fun expression. 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
    5> Fun3 = fun Fact(1) -> 1; Fact(X) when X > 1 -> X * Fact(X - 1) end.
    #Fun<erl_eval.6.39074546>
    6> Fun3(4).
    24

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. `Module`, `Name`, and `Arity` can also be variables. A fun defined in this way refers to the function `Name` with arity `Arity` in the latest version of module `Module`. A fun defined in this way is not dependent on the code for the module in which it is defined.

#### Change

Before Erlang/OTP R15, `Module`, `Name`, and `Arity` were not allowed to be variables.

More examples are provided in Programming Examples.

## Catch and Throw

    catch Expr

Returns the value of `Expr` unless an exception is raised during the evaluation. In that case, the exception is caught. The return value depends on the class of the exception:

- `error` (a run-time error or the code called `error(Term)`) - `{'EXIT',{Reason,Stack}}` is returned.

- `exit` (the code called `exit(Term)`) - `{'EXIT',Term}` is returned.

- `throw` (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 Exit Reasons.

Examples:

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

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

Example:

    3> catch throw(hello).
    hello

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

#### Change

Before Erlang/OTP 24, the `catch` operator had the lowest precedence, making it necessary to add parentheses when combining it with the `match` operator:

    1> A = (catch 42).
    42
    2> A.
    42

Starting from Erlang/OTP 24, the parentheses can be omitted:

    1> A = catch 42.
    42
    2> A.
    42

## Try

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

This is an enhancement of catch. It gives the possibility to:

- Distinguish between different exception classes.
- Choose to handle only the desired ones.
- Passing the others on to an enclosing `try` or `catch`, or to default error handling.

Notice that although the keyword `catch` is used in the `try` expression, there is not a `catch` expression within the `try` expression.

It returns the value of `Exprs` (a sequence of expressions `Expr1, ..., ExprN`) 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. If a match succeeds and the optional guard sequence `ExceptionGuardSeq` is true, the corresponding `ExceptionBody` is evaluated to become the return value.

`Stacktrace`, if specified, must be the name of a variable (not a pattern). The stack trace is bound to the variable when the corresponding `ExceptionPattern` matches.

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

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

It is allowed to omit `Class` and `Stacktrace`. An omitted `Class` is shorthand for `throw`:

    try Exprs
    catch
        ExceptionPattern1 [when ExceptionGuardSeq1] ->
            ExceptionBody1;
        ExceptionPatternN [when ExceptionGuardSeqN] ->
            ExceptionBodyN
    end

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

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

If the evaluation of `Exprs` succeeds without an exception, the patterns `Pattern` are sequentially matched against the result in the same way as for a case expression, except that if the matching fails, a `try_clause` run-time error occurs instead of a `case_clause`.

Only exceptions occurring during the evaluation of `Exprs` can be caught by the `catch` section. Exceptions occurring in a `Body` or due to a failed match are not caught.

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

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

`AfterBody` is evaluated after either `Body` or `ExceptionBody`, no matter which one. The evaluated value of `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`, `AfterBody` is evaluated. In this case the exception is passed on after `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 `AfterBody` is evaluated after an exception in `Exprs`, `Body`, or `ExceptionBody`, that exception is lost and masked by the exception in `AfterBody`.

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

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

    try Exprs
    catch
        ExpressionPattern ->
            ExpressionBody
    after
        AfterBody
    end

    try Exprs after AfterBody end

Next is an example of using `after`. This closes the file, even in the event of exceptions in `file:read/2` or in `binary_to_term/1`. The exceptions are 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.

Next is an example of using `try` to emulate `catch Expr`:

    try Expr
    catch
        throw:Term -> Term;
        exit:Reason -> {'EXIT',Reason};
        error:Reason:Stk -> {'EXIT',{Reason,Stk}}
    end

Variables bound in the various parts of these expressions have different scopes. Variables bound just after the `try` keyword are:

- bound in the `of` section
- unsafe in both the `catch` and `after` sections, as well as after the whole construct

Variables bound in `of` section are:

- unbound in the `catch` section
- unsafe in both the `after` section, as well as after the whole construct

Variables bound in the `catch` section are unsafe in the `after` section, as well as after the whole construct.

Variables bound in the `after` section are unsafe after the whole construct.

## Parenthesized Expressions

    (Expr)

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

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

## 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`.

## Comprehensions

Comprehensions provide a succinct notation for iterating over one or more terms and constructing a new term. Comprehensions come in three different flavors, depending on the type of term they build.

List comprehensions construct lists. They have the following syntax:

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

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

Bit string comprehensions construct bit strings or binaries. They have the following syntax:

    << BitStringExpr || Qualifier1, . . ., QualifierN >>

`BitStringExpr` is an expression that evaluates to a bit string. If `BitStringExpr` is a function call, it must be enclosed in parentheses. Each `Qualifier` is either a generator or a filter.

Map comprehensions construct maps. They have the following syntax:

    #{KeyExpr => ValueExpr || Qualifier1, . . ., QualifierN}

Here, `KeyExpr` and `ValueExpr` are arbitrary expressions, and each `Qualifier` is either a generator or a filter.

#### Change

Map comprehensions and map generators were introduced in Erlang/OTP 26.

There are three kinds of generators.

A list generator has the following syntax:

    Pattern <- ListExpr

where `ListExpr` is an expression that evaluates to a list of terms.

A bit string generator has the following syntax:

    BitstringPattern <= BitStringExpr

where `BitStringExpr` is an expression that evaluates to a bit string.

A map generator has the following syntax:

    KeyPattern := ValuePattern <- MapExpression

where `MapExpr` is an expression that evaluates to a map, or a map iterator obtained by calling `maps:iterator/1` or `maps:iterator/2`.

A filter is an expression that evaluates to `true` or `false`.

The variables in the generator patterns shadow previously bound variables, including variables bound in a previous generator pattern.

Variables bound in a generator expression are not visible outside the expression:

    1> [{E,L} || E <- L=[1,2,3]].
    * 1:5: variable 'L' is unbound

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

A bit string comprehension returns a bit string, which is created by concatenating the results of evaluating `BitStringExpr` for each combination of bit string generator elements for which all filters are true.

A map comprehension returns a map, where the map elements are the result of evaluating `KeyExpr` and `ValueExpr` for each combination of generator elements for which all filters are true. If the key expressions are not unique, the last occurrence is stored in the map.

Examples:

Multiplying each element in a list by two:

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

Multiplying each byte in a binary by two, returning a list:

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

Multiplying each byte in a binary by two:

    1> << <<(X*2)>> || <<X>> <= <<1,2,3>> >>.
    <<2,4,6>>

Multiplying each element in a list by two, returning a binary:

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

Creating a mapping from an integer to its square:

    1> #{X => X*X || X <- [1,2,3]}.
    #{1 => 1,2 => 4,3 => 9}

Multiplying the value of each element in a map by two:

    1> #{K => 2*V || K := V <- #{a => 1,b => 2,c => 3}}.
    #{a => 2,b => 4,c => 6}

Filtering a list, keeping odd numbers:

    1> [X || X <- [1,2,3,4,5], X rem 2 =:= 1].
    [1,3,5]

Filtering a list, keeping only elements that match:

    1> [X || {_,_}=X <- [{a,b}, [a], {x,y,z}, {1,2}]].
    [{a,b},{1,2}]

Combining elements from two list generators:

    1> [{P,Q} || P <- [a,b,c], Q <- [1,2]].
    [{a,1},{a,2},{b,1},{b,2},{c,1},{c,2}]

More examples are provided in Programming Examples.

When there are no generators, a comprehension returns either a term constructed from a single element (the result of evaluating `Expr`) if all filters are true, or a term constructed from no elements (that is, `[]` for list comprehension, `<<>>` for a bit string comprehension, and `#{}` for a map comprehension).

Example:

    1> [2 || is_integer(2)].
    [2]
    2> [x || is_integer(x)].
    []

What happens when the filter expression does not evaluate to a boolean value depends on the expression:

- If the expression is a guard expression, failure to evaluate or evaluating to a non-boolean value is equivalent to evaluating to `false`.
- If the expression is not a guard expression and evaluates to a non-Boolean value `Val`, an exception `{bad_filter, Val}` is triggered at runtime. If the evaluation of the expression raises an exception, it is not caught by the comprehension.

Examples (using a guard expression as filter):

    1> List = [1,2,a,b,c,3,4].
    [1,2,a,b,c,3,4]
    2> [E || E <- List, E rem 2].
    []
    3> [E || E <- List, E rem 2 =:= 0].
    [2,4]

Examples (using a non-guard expression as filter):

    1> List = [1,2,a,b,c,3,4].
    [1,2,a,b,c,3,4]
    2> FaultyIsEven = fun(E) -> E rem 2 end.
    #Fun<erl_eval.42.17316486>
    3> [E || E <- List, FaultyIsEven(E)].
    ** exception error: bad filter 1
    4> IsEven = fun(E) -> E rem 2 =:= 0 end.
    #Fun<erl_eval.42.17316486>
    5> [E || E <- List, IsEven(E)].
    ** exception error: an error occurred when evaluating an arithmetic expression
         in operator  rem/2
            called as a rem 2
    6> [E || E <- List, is_integer(E), IsEven(E)].
    [2,4]

## 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. (The remaining guards, if any, are not evaluated.)

`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`

## Guard Expressions

The set of valid guard expressions 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 following:

- Variables
- Constants (atoms, integer, floats, lists, tuples, records, binaries, and maps)
- Expressions that construct atoms, integer, floats, lists, tuples, records, binaries, and maps
- Expressions that update a map
- The record expressions `Expr#Name.Field` and `#Name.Field`
- Calls to the BIFs specified in tables Type Test BIFs and Other BIFs Allowed in Guard Expressions
- Term comparisons
- Arithmetic expressions
- Boolean expressions
- Short-circuit expressions (`andalso`/`orelse`)

BIF
`is_atom/1`
`is_binary/1`
`is_bitstring/1`
`is_boolean/1`
`is_float/1`
`is_function/1`
`is_function/2`
`is_integer/1`
`is_list/1`
`is_map/1`
`is_number/1`
`is_pid/1`
`is_port/1`
`is_record/2`
`is_record/3`
`is_reference/1`
`is_tuple/1`

Table: Type Test BIFs

Notice that most type test BIFs have older equivalents, without the `is_` prefix. These old BIFs are retained only for backwards compatibility and are not to be used in new code. They are also only allowed at top level. For example, they are not allowed in Boolean expressions in guards.

BIF
`abs(Number)`
`bit_size(Bitstring)`
`byte_size(Bitstring)`
`element(N, Tuple)`
`float(Term)`
`hd(List)`
`is_map_key(Key, Map)`
`length(List)`
`map_get(Key, Map)`
`map_size(Map)`
`max(A, B)`
`min(A, B)`
`node/0`
`node(Pid` | `Ref` | `Port)`
`round(Number)`
`self/0`
`size(Tuple` | `Bitstring)`
`tl(List)`
`trunc(Number)`
`tuple_size(Tuple)`

Table: Other BIFs Allowed in Guard Expressions

#### Change

The `min/2` and `max/2` BIFs are allowed to be used in guards from Erlang/OTP 26.

If an arithmetic expression, a Boolean expression, a short-circuit expression, or a call to a guard BIF fails (because of invalid arguments), the entire guard fails. If the guard was part of a guard sequence, the next guard in the sequence (that is, the guard following the next semicolon) is evaluated.

## Operator Precedence

Operator precedence in descending order:

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

Table: Operator Precedence

#### Change

Before Erlang/OTP 24, the `catch` operator had the lowest precedence.

#### Note

The `=` operator in the table is the match operator. The character `=` can also denote the compound pattern operator, which can only be used in patterns.

`?=` is restricted in that it can only be used at the top-level inside a `maybe` block.

When evaluating an expression, the operator with the highest precedence is evaluated first. Operators with the same precedence are evaluated according to their associativity. Non-associative operators cannot be combined with operators of the same precedence.

Examples:

The left-associative arithmetic 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

The non-associative operators cannot be combined:

    1> 1 < X < 10.
    * 1:7: syntax error before: '<'

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