Flatten a list in Prolog

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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.

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