I'm trying to write a recursive fun in an Erlang shell, but I keep getting an unbound variable exception:
1> Foo = fun(X) -> Foo(X) end. * 1: variable 'Foo' is unbound
This probably goes without saying, but I'm not trying to create an infinite loop! This is just a simple example of the error I'm getting.
You can do it with a little argument trick:
1> Foo = fun(F, X) -> F(F, X) end.
2> Foo(Foo, a).
The trick here is to send in the function as an argument to itself to allow recursion.
Alternative way to make it in one shoot:
1> Foo = fun(X) -> Fun = fun(F,Y) -> F(F,Y) end, Fun(Fun,X) end.
1> Foo = fun(Max) ->
1> Fun = fun(F, X) when X > Max -> ;
1> (F, X) -> [X | F(F, X+1)]
1> Fun(Fun, 0)
Alternatively, you can use the Y combinator. Y Combinator in Erlang explains.
You can also use the "Funs with names" variant:
Foo = fun F(X) -> F(X) end.
In this way it is easier to understand that
F is the function itself within the definition. Also,
F can be the same variable.
Obviously, Foo gets assigned only after the fun is defined, so it may not be accessed from within it.
I don't think that Erlang allows to call the anonymous function from itself. Just make it a named one.