Recursive Prolog predicate for reverse / palindrome

Question

1. Can I get a recursive Prolog predicate having two arguments, called reverse, which returns the inverse of a list:

   Sample query and expected result:

Answer 1

There isn't an efficient way to define reverse/2 with a single recursive definition without using some auxiliary predicate. However, if this is nevertheless permitted, a simple solution which doesn't rely on any built-ins like append/3 (and should be applicable for most Prolog implementations) would be to use an accumulator list, as follows:

rev([],[]).
rev([X|Xs], R) :-
    rev_acc(Xs, [X], R).

rev_acc([], R, R).
rev_acc([X|Xs], Acc, R) :-
    rev_acc(Xs, [X|Acc], R).

rev/2 is the reversal predicate which simply 'delegates' to (or, wraps) the accumulator-based version called rev-acc/2, which recursively adds elements of the input list into an accumulator in reverse order.

Running this:

?- rev([1,3,2,x,4],L).
L = [4, x, 2, 3, 1].

And indeed as @false has already pointed out (+1),

palindrome(X) :- rev(X,X).