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:
All subexpressions are evaluated before an expression itself is evaluated, unless explicitly stated otherwise. For example, consider the expression:
Expr1 + Expr2
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
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.
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
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,
Example:
The following code:
member(_, []) ->
[].
can be rewritten to be more readable:
member(Elem, []) ->
[].
This causes a warning for an unused variable,
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
For the
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,
If
Pattern1 = Pattern2
When matched against a term, both
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.
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]) -> ...
An arithmetic expression can be used within a pattern if it meets both of the following two conditions:
Example:
case {Value, Result} of
{?THRESHOLD+1, ok} -> ...
This feature was added in Erlang 5.0/OTP R7.
The following matches
Expr1 = Expr2
If the matching succeeds, any unbound variable in the pattern
becomes bound and the value of
If the matching fails, a
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]
ExprF(Expr1,...,ExprN) ExprM:ExprF(Expr1,...,ExprN)
In the first form of function calls,
Example:
lists:keysearch(Name, 1, List)
In the second form of function calls,
If
Examples:
handle(Msg, State)
spawn(m, init, [])
Examples where
Fun1 = fun(X) -> X+1 end
Fun1(3)
=> 4
fun lists:append/2([1,2], [3,4])
=> [1,2,3,4]
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
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,
Before 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 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
if
GuardSeq1 ->
Body1;
...;
GuardSeqN ->
BodyN
end
The branches of an
The return value of
If no guard sequence is evaluated as true,
an
Example:
is_greater_than(X, Y) ->
if
X>Y ->
true;
true -> % works as an 'else' branch
false
end
case Expr of
Pattern1 [when GuardSeq1] ->
Body1;
...;
PatternN [when GuardSeqN] ->
BodyN
end
The expression
The return value of
If there is no matching pattern with a true guard sequence,
a
Example:
is_valid_signal(Signal) ->
case Signal of
{signal, _What, _From, _To} ->
true;
{signal, _What, _To} ->
true;
_Else ->
false
end.
Expr1 ! Expr2
Sends the value of
receive
Pattern1 [when GuardSeq1] ->
Body1;
...;
PatternN [when GuardSeqN] ->
BodyN
end
Receives messages sent to the process using the send operator
(!). The patterns
The return value of
Example:
wait_for_onhook() ->
receive
onhook ->
disconnect(),
idle();
{connect, B} ->
B ! {busy, self()},
wait_for_onhook()
end.
The
receive
Pattern1 [when GuardSeq1] ->
Body1;
...;
PatternN [when GuardSeqN] ->
BodyN
after
ExprT ->
BodyT
end
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
ExprT ->
BodyT
end
This construction does not consume any messages, only suspends
execution in the process for
Example:
timer() ->
spawn(m, timer, [self()]).
timer(Pid) ->
receive
after
5000 ->
Pid ! timeout
end.
There are two special cases for the timeout value
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
Term comparison operators return the Boolean value of the
expression,
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
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
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
Expr1 orelse Expr2 Expr1 andalso Expr2
or
Returns either the value of
Example 1:
case A >= -1.0 andalso math:sqrt(A+1) > B of
This works even if
Example 2:
OnlyOne = is_atom(L) orelse
(is_list(L) andalso length(L) == 1),
From Erlang/OTP R13A,
all(Pred, [Hd|Tail]) ->
Pred(Hd) andalso all(Pred, Tail);
all(_, []) ->
true.
Expr1 ++ Expr2 Expr1 -- Expr2
The list concatenation operator
The list subtraction operator
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]
The complexity of
Constructing a new map is done by letting an expression
#{ 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
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,
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 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
If key
If key
If
To only update an existing value, the following syntax is used:
M#{ K := V }
Here
If key
If
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
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.
Matching of key-value associations from maps is done as follows:
#{ K := V } = M
Here
If the variable
Example:
1> M = #{"tuple" => {1,2}}.
#{"tuple" => {1,2}}
2> #{"tuple" := {1,B}} = M.
#{"tuple" => {1,2}}
3> B.
2.
This binds variable
Similarly, multiple values from the map can be matched:
#{ K1 := V1, .., Kn := Vn } = M
Here keys
If the matching conditions are not met, the match fails, either with:
A
This is if it is used in the context of the matching 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
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
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 are allowed in guards as long as all subexpressions are valid guard expressions.
Two guard BIFs handle maps:
>
<>]]>
Each element
Ei = Value |
Value:Size |
Value/TypeSpecifierList |
Value:Size/TypeSpecifierList
Used in a bit string construction,
Used in a bit string matching,
Notice that, for example, using a string literal as in
Used in a bit string construction,
Used in a bit string matching,
The value of
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
The value of
When constructing binaries, if the size
The types
When constructing a segment of a
When constructing, a literal string can be given followed
by one of the UTF types, for example:
A successful match of a segment of a
A segment of type
A segment of type
A segment of type
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 "
More examples are provided in
fun
[Name](Pattern11,...,Pattern1N) [when GuardSeq1] ->
Body1;
...;
[Name](PatternK1,...,PatternKN) [when GuardSeqK] ->
BodyK
end
A fun expression begins with the keyword
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
fun (Arg1,...,ArgN) -> Name(Arg1,...,ArgN) end
In
More examples are provided in
catch Expr
Returns the value of
For exceptions of class
For exceptions of class
For exceptions of class
Examples:
1> catch 1+2.
3
2> catch 1+a.
{'EXIT',{badarith,[...]}}
Notice that
3> A = catch 1+2. ** 1: syntax error before: 'catch' ** 4> A = (catch 1+2). 3
The BIF
Example:
5> catch throw(hello). hello
If
try Exprs
catch
[Class1:]ExceptionPattern1 [when ExceptionGuardSeq1] ->
ExceptionBody1;
[ClassN:]ExceptionPatternN [when ExceptionGuardSeqN] ->
ExceptionBodyN
end
This is an enhancement of
Notice that although the keyword
It returns the value of
If an exception occurs during evaluation of
If an exception occurs during evaluation of
The
try Exprs of
Pattern1 [when GuardSeq1] ->
Body1;
...;
PatternN [when GuardSeqN] ->
BodyN
catch
[Class1:]ExceptionPattern1 [when ExceptionGuardSeq1] ->
ExceptionBody1;
...;
[ClassN:]ExceptionPatternN [when ExceptionGuardSeqN] ->
ExceptionBodyN
end
If the evaluation of
An exception occurring during the evaluation of
The
try Exprs of
Pattern1 [when GuardSeq1] ->
Body1;
...;
PatternN [when GuardSeqN] ->
BodyN
catch
[Class1:]ExceptionPattern1 [when ExceptionGuardSeq1] ->
ExceptionBody1;
...;
[ClassN:]ExceptionPatternN [when ExceptionGuardSeqN] ->
ExceptionBodyN
after
AfterBody
end
Even if an exception occurs during evaluation of
If an exception occurs during evaluation of
The
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
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 Expr
catch
throw:Term -> Term;
exit:Reason -> {'EXIT',Reason}
error:Reason -> {'EXIT',{Reason,erlang:get_stacktrace()}}
end
(Expr)
Parenthesized expressions are useful to override
1> 1 + 2 * 3. 7 2> (1 + 2) * 3. 9
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
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
List comprehensions are written with the following syntax:
[Expr || Qualifier1,...,QualifierN]
Here,
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
Example:
1> [X*2 || X <- [1,2,3]]. [2,4,6]
More examples are provoded in
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:
<< BitString || Qualifier1,...,QualifierN >>
Here,
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
Example:
1> << << (X*2) >> || <<X>> <= << 1,2,3 >> >>. <<2,4,6>>
More examples are provided in
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.)
A guard is a sequence of guard expressions, separated
by comma (,). The guard is true if all guard expressions
evaluate to
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:
Notice that most type test BIFs have older equivalents, without
the
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 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