Make list flat Prolog

Question

I am trying to do:

Flatten a nested list structure. Transform a list, possibly holding lists as elements into a 'flat' list by replacing each list with its elements (recursively).

Example: ?- my_flatten([a, [b, [c, d], e]], X). X = [a, b, c, d, e] And my attempt:

unwinder([], APP, APP).
unwinder([E|Es], T, APP) :- is_list(E), unwinder(E, T,APP), unwinder(Es, T,APP). 
unwinder([E|Es], T, APP) :- \+ is_list(E),  unwinder(Es, K, APP), append( [E], T, APP).

I suppose that the problem is append, especially I still have a problem with unification :)

Please explain in terms of unification.

Answer 1

One thing, your parameters K, T and APP have a messy in your predicate. In the second case:

unwinder([E|Es], T, APP) :- \+ is_list(E),  unwinder(Es, K, APP), append( [E], T, APP).

You go recursively to get K (i think) but never use it.

I re-write (a little) your code:

unwinder([], []).
unwinder([E|ES], APP) :-
    is_list(E), !,
    unwinder(E, K1),
    unwinder(ES, K2),
    append(K1, K2, APP). 
unwinder([E|ES], [E|K]) :-
    unwinder(ES, K).

First. Empty case.

Second. If the element is a list, go recursively through it and recursively through the rest of the list (ES). Then append the unified lists (K1 & K2).

Third. Don't check is list (you did it before), cons (|) the element (not a list) with unified list K.

Note: The ! symbol don't let go back the execution (backtracking) before that point. So, if the predicate fails in some point of second case, it can't go back to try the thrid.