Create a closure in Erlang: why are variables not bound?

Question

I have a piece of code that goes like this:

Fi_F = fun (F, I, Xs) -> 
           fun ( X ) -> 
                F( x_to_list(X, Xs, I) ) 
           end 
end,

I just need to turn a function of list to a function of one number. For example with Xs = [1,2,3] and I = 2, I expect this to grant me with function:

fun ( X ) -> F([ 1, X, 3]) end.

But somehow F, I and X are shadowed, not closured, so it fails in x_to_list with an empty list.

I'm still new to Erlang and think I'm missing something more conceptual, than a mere syntax problem.

UPD: Found a bug. I wrote x_to_list/3 this way:

x_to_list( X, L, I ) ->
    lists:sublist(L, I) ++ [ X ] ++ lists:nthtail(I+1, L).

So it counts list elements from 0, not 1. When I call it with I = 3, it fails. So this is not about closuring.

I still have shadowing warnings though, but it is completely another issue.

Answer 1

A somewhat quick and dirty implementation of x_to_list/3 (just to test) would be:

x_to_list(X, Xs, I) ->
   { Pre, Post } = lists:split(I-1, Xs),
   Pre ++ [X] ++ tl(Post).

Then, your code works without problems:

> Q = fun ( F, I, Xs ) -> fun (X) -> F( x_to_list(X, Xs, I)) end end.
> Y = Q( fun(L) -> io:format("~p, ~p, ~p~n", L) end, 2, [1,2,3] ).
> Y(4).
1, 4, 3
ok