8 Expressions

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

8.1 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 causes a badarg runtime error.

8.2 Terms

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

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

Examples:

X
Name1
PhoneNumber
Phone_number
_
_Height

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. They are however ignored by the compiler in the sense that they do not generate any warnings for unused variables.

Example:

The following code:

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

can be rewritten to be more readable:

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

This causes 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, []) ->
    [].

Notice that since variables starting with an underscore are not anonymous, this matches:

{_,_} = {1,2}

But this 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.

8.4 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 and receive expressions, and match expressions.

Match Operator = in Patterns

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.

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} -> ...

8.5 Match

The following matches Expr1, a pattern, against Expr2:

Expr1 = 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 occurs.

Examples:

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

8.6 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:keysearch(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.

-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.

8.7 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 (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

8.8 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 occurs.

Example:

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

8.9 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}. 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 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.

8.10 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. 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

ExprT is to 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. The return value of BodyT then 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 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.

There are two special cases for the timeout value ExprT:

infinity

The process is to wait indefinitely for a matching message; this is the same as not using a timeout. This can be useful for timeout values that are calculated at runtime.

0

If there is no matching message in the mailbox, the timeout occurs immediately.

8.11 Term Comparisons

Expr1 op Expr2

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

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

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.

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.

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> 1 > a.
false
4> #{c => 3} > #{a => 1, b => 2}.
false
4> #{a => 1, b => 2} == #{a => 1.0, b => 2.0}.
true

8.12 Arithmetic Expressions

op Expr
Expr1 op Expr2

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

8.13 Boolean Expressions

op Expr
Expr1 op Expr2

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

8.14 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),

From Erlang/OTP R13A, Expr2 is no longer required to evaluate to a Boolean value. As a consequence, andalso and orelse are now tail-recursive. For instance, the following function is tail-recursive in Erlang/OTP R13A and later:

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

8.15 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 that is a copy of the first argument. The procedure is a 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]

8.16 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 thrown.

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 badarg is triggered 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 thrown.

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 .. 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 an expression with bound variables or literals. 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.

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 .. Kn are any expressions with literals or bound variables. If 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, either with:

• A badmatch exception.

  This is if it is used in the context of the match operator as in the example.

• Or resulting in the next clause being tested in function heads and case expressions.

Matching in maps only allows for := as delimiters of 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

Matching an expression against an empty map literal, matches its type but no variables are bound:

#{} = Expr

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

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.

Two guard BIFs handle maps:

• is_map/1 in the erlang module
• map_size/1 in the erlang module

8.17 Bit Syntax Expressions

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

Each element Ei specifies a segment of the bit string. 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 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.

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>>.

Used in a bit string construction, Size is an expression that is to evaluate to an integer.

Used in a bit string 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 and bitstring it is the whole binary or bit string.

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

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

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.

Unit= unit:IntegerLiteral

The allowed range is 1..256. Defaults to 1 for integer, float, and bitstring, and to 8 for binary. No unit specifier must be given for the types utf8, utf16, and utf32.

The value of Size multiplied with the unit gives the number of bits. A segment of type binary must have a size that is evenly divisible by 8.

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..16#D7FF or 16#E000....16#10FFFF. Construction fails with a badarg exception if Value is outside the allowed ranges. The size of the resulting binary segment depends on the type or Value, or both:

• For utf8, Value is encoded in 1-4 bytes.
• For utf16, Value is encoded in 2 or 4 bytes.
• For utf32, Value is always be 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..16#D7FF or 16#E000..16#10FFFF. The match fails if the returned value falls outside those ranges.

A segment of type utf8 matches 1-4 bytes in the binary, if the binary 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 binary. The match fails if the binary 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 binary in the same way as an integer segment matches 32 bits. The match fails if the resulting integer is outside the legal ranges mentioned above.

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,H/bitstring>> = <<1,17,42:12>>.
<<1,17,1,10:4>>
12> H.
<<17,1,10:4>>
13> <<1024/utf8>>.
<<208,128>>

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.

8.18 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. Starting from Erlang/OTP R15, 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.

More examples are provided in Programming Examples.

8.19 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 Exit Reasons.

Examples:

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

Notice that catch has low precedence and catch subexpressions often needs to be enclosed in a block expression or in parentheses:

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 returns the value Any.

Example:

5> catch throw(hello).
hello

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

8.20 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.

An exception occurring during the evaluation of Body is 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

8.21 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

8.22 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.

8.23 List Comprehensions

List comprehensions is 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]

Here, 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 bit string generator is written as:
  BitstringPattern <= BitStringExpr.
  BitStringExpr must be an expression, which evaluates to a bitstring.
• 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 and bit string generator elements, for which all filters are true.

Example:

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

When there are no generators or bit string generators, a list comprehension returns either a list with one element (the result of evaluating Expr) if all filters are true or an empty list otherwise.

Example:

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

More examples are provided in Programming Examples.

8.24 Bit String Comprehensions

Bit string comprehensions are analogous to List Comprehensions. They are used to generate bit strings efficiently and succinctly.

Bit string comprehensions are written with the following syntax:

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

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

• A generator is written as:
  Pattern <- ListExpr.
  ListExpr must be an expression that evaluates to a list of terms.
• A bit string generator is written as:
  BitstringPattern <= BitStringExpr.
  BitStringExpr must be an expression that evaluates to a bitstring.
• A filter is an expression that evaluates to true or false.

The variables in the generator patterns, shadow variables in the function clause, surrounding the bit string comprehensions.

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

Example:

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

More examples are provided in Programming Examples.

8.25 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

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 following:

• The atom true
• Other constants (terms and bound variables), all regarded as false
• Calls to the BIFs specified in table Type Test BIFs
• Term comparisons
• Arithmetic expressions
• Boolean expressions
• Short-circuit expressions (andalso/orelse)

Notice that most type test BIFs have older equivalents, without the is_ prefix. These old BIFs are retained for backwards compatibility only 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.

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.

8.26 Operator Precedence

Operator precedence in falling priority:

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 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