How do I find the longest list in a list of lists?

Question

I have a list of lists, and I need to find the longest one of them. If there are more than one with the same length it\'s the same which it returns. Thanks.

Answer 1

To have the length of longest list:

%sample: longest([[2,1,3],[7,5],[1,8,2,3,1],[2,7,1,4]],L,LEN).

longest([L], L, _) :-
   !.
longest([H|T], H, _) :-
   length(H, N),
   longest(T, X, N),
   length(X, M),
   N > M,
   !.
longest([_|T], X, LEN) :-
   length(X, LEN),
   longest(T, X, LEN),
   !.