I've only been working with Prolog for a couple days. I understand some things but this is really confusing me.
I'm suppose to write a function that takes a list and flattens it.
?- flatten([a,[b,c],[[d],[],[e]]],Xs).
Xs = [a,b,c,d,e]. % expected result
The function takes out the inner structures of the list.
This is what I have so far:
flatten2([],[]).
flatten2([Atom|ListTail],[Atom|RetList]) :-
atom(Atom), flatten2(ListTail,RetList).
flatten2([List|ListTail],RetList) :-
flatten2(List,RetList).
Now, this works when I call:
?- flatten2([a,[b,c],[[d],[],[e]]], R).
R = [a,b,c,d,e]. % works as expected!
But when I call to see if a list that I input is already flattened, is returns false instead of true:
?- flatten2([a,[b,c],[[d],[],[e]]], [a,b,c,d,e]).
false. % BAD result!
Why does it work on one hand, but not the other? I feel like I'm missing something very simple.
The definition of flatten2/2 you've given is busted; it actually behaves like this:
?- flatten2([a, [b,c], [[d],[],[e]]], R).
R = [a, b, c] ;
false.
So, given the case where you've already bound R to [a,b,c,d,e], the failure isn't surprising.
Your definition is throwing away the tail of lists (ListTail) in the 3rd clause - this needs to be tidied up and connected back into the list to return via RetList. Here is a suggestion:
flatten2([], []) :- !.
flatten2([L|Ls], FlatL) :-
!,
flatten2(L, NewL),
flatten2(Ls, NewLs),
append(NewL, NewLs, FlatL).
flatten2(L, [L]).
This one recursively reduces all lists of lists into either single item lists [x], or empty lists [] which it throws away. Then, it accumulates and appends them all into one list again out the output.
Note that, in most Prolog implementations, the empty list [] is an atom and a list, so the call to atom([]) and is_list([]) both evaluate to true; this won't help you throw away empty lists as opposed to character atoms.