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

We define longest/2 based on meta-predicate max_of_by/3 used in tandem with length/2:

longest(Xss,Ys) :-
   max_of_by(Ys,Xss,length).

Sample queries:

?- longest([[1],[2]],Xs).      % we expect   multiple solutions
  Xs = [1]                
; Xs = [2].                    % we    _get_ multiple solutions

?- longest([[2,1,3],[7,5],[1,8,2,3,1],[2,7,1,4]],Xs).
Xs = [1,8,2,3,1].              % succeeds deterministically