Question

Is it possible to reverse a list in Prolog with only two arguments? Such as:

reverse_list(List, Reversed).

This is not homework, I’m reading Seven Programming Languages in Seven Weeks and I got curious.

With three arguments you could use an accumulator (much like in functional programming):

reverseList([], Accumulator, Accumulator).
reverseList([Head|Tail], Accumulator, Solution) :-
  reverseList(Tail, [Head|Accumulator], Solution).
reverseList(List, Solution) :-
  reverseList(List, [], Solution).

Clarification: I saw a solution with append, I was wondering if you could do that without other prolog functions

Answer 1

sure:

reverseList([[], Accumulator, Accumulator]).
reverseList([[Head|Tail], Accumulator, Solution]) :-
  reverseList([Tail, [Head|Accumulator], Solution]).
reverseList([List, Solution]) :-
  reverseList([List, [], Solution]).

edit:actually that has only one :b

a non-cheating approach:

reverse([],[]).
reverse([H|T],L):-
    reverse(T,R),
    append(R,[H],L).

the problem is that the performance will be quite bad: you will recurse over the list and for each element you will do one append/3.
using time/1 and a random list of 1,000,000 elements:

accumulator:    % 2,000,003 inferences, 0.652 CPU in 0.652 seconds (100% CPU, 3066292 Lips)
arity-2         % 1,000,003 inferences, 0.178 CPU in 0.178 seconds (100% CPU, 5602426 Lips)