Prolog - Convert a 'cons' term to a list

Question

I'm looking to convert a Prolog term similar to this:

cons(a, cons(b, cons(c, cons(d, nil ))))

into a list:

[a, b, c, d]

I have a function to verify that cons(a, cons(b, cons(c, cons(d, nil )))) is a proper list, as follows:

list(nil).

list(cons(_,X)):-
    list(X).

which yields,

?- list(cons(a, cons(b, cons(c, cons(d, nil ))))).
true.

Now, when I pass a term like, list(cons(a, cons(b, cons(c, cons(d, nil ))))). , and it's true , I want to then be able to convert it to a list. I can do all of the rest, I'm just stuck on the conversion from the term to a list.

Any assistance pointing me in the right direction would be greatly appreciated.

Answer 1

A simple solution would be:

list(nil, []).
list(cons(A,X), [A|L]):-
    list(X,L).

Example:

?- list( cons(a, cons(b, cons(c, cons(d, nil )))), L).
L = [a, b, c, d].