How do you write a fun that's recursive in Erlang?

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.

-------------Problems Reply------------

You can do it with a little argument trick:

1> Foo = fun(F, X) -> F(F, X) end.
2> Foo(Foo, a).
<...infinite loop!>

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.
2> Foo(a).

For example:

1> Foo = fun(Max) ->
1> Fun = fun(F, X) when X > Max -> [];
1> (F, X) -> [X | F(F, X+1)]
1> end,
1> Fun(Fun, 0)
1> end.
2> Foo(10).

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, Foo and 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.

Category:recursion Views:0 Time:2009-05-15

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