Flatten a list in Prolog

Question

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

Answer 1

Here's an accumulator based version for completeness:

% flatten/2
flatten(List, Result) :- flatten(List, [], Result).

% auxiliary predicate flatten/3
flatten([], Result, Result).
flatten([Head| Tail], Part, Result) :- 
    is_list(Head),
    !, 
    flatten(Head, HR),
    append(Part, HR, PR),
    flatten(Tail, PR, Result).
flatten([Head| Tail], Part, Result) :- 
    append(Part, [Head], PR),
    flatten(Tail, PR, Result).
flatten(X, Part, Result) :-
    fail.