Question

I wanto to translate expressions like X = a(b(c(d))) in [a, b, c, d].

I guess i have to iterate/recurse the operator =.., but i don’t know how. I tried

flatten([], []).
flatten(Exp, X) :- Exp=..[H,T], flatten(T, Y), X is append([H], Y). 

but it does not seem to work.

Can someone help me?

Answer 1

flatten/2 is a built in usually, while is/2 introduces arithmetic evaluation. Thus your Prolog should warn you about these problems in your code.

If you are limiting the expression to unary terms, the code should be simplified (note I renamed the procedure):

flatterm(Exp, [H|R]) :-
    Exp =.. [H, T],
    !, flatterm(T, R).
flatterm(T, [T]).

test:

?- flatterm(a(b(c(d))),X).
X = [a, b, c, d].

You should try to understand why I swapped the base/recursive case, and the role of the ! (also known as cut).