The Syntax of Funs

Funs are written with the syntax:

F = fun (Arg1, Arg2, ... ArgN) ->
        ...
    end

This creates an anonymous function of N arguments and binds it to the variable F.

If we have already written a function in the same module and wish to pass this function as an argument, we can use the following syntax:

F = fun FunctionName/Arity

With this form of function reference, the function which is referred to does not need to be exported from the module.

We can also refer to a function defined in a different module with the following syntax:

F = {Module, FunctionName}

In this case, the function must be exported from the module in question.

The follow program illustrates the different ways of creating funs:

-module(fun_test).
-export([t1/0, t2/0, t3/0, t4/0, double/1]).
-import(lists, [map/2]).

t1() -> map(fun(X) -> 2 * X end, [1,2,3,4,5]).

t2() -> map(fun double/1, [1,2,3,4,5]).

t3() -> map({?MODULE, double}, [1,2,3,4,5]).

double(X) -> X * 2.

We can evaluate the fun F with the syntax:

F(Arg1, Arg2, ..., Argn)

To check whether a term is a fun, use the test is_function/1 in a guard. Example:

f(F, Args) when is_function(F) ->
   apply(F, Args);
f(N, _) when is_integer(N) ->
   N.

Funs are a distinct type. The BIFs erlang:fun_info/1,2 can be used to retrieve information about a fun, and the BIF erlang:fun_to_list/1 returns a textual representation of a fun. The check_process_code/2 BIF returns true if the process contains funs that depend on the old version of a module.