Prolog Beginner

Question

I'm trying to make predicates such that for lists X and Y, rmlast is true if X and Y are the same list except for the fact that Y has 1 element more. So [1] [1,2] or [3,2] [3,2,5].

rmlast([], [_]). :- true.
rmlast([X|xtail], [Y|ytail]) :- rmlast(xtail,ytail), X is Y.

This however, produces a false result for anything other than the base case.

Answer 1

Your code doesn't work because xtail and ytail are written with a lower case letter, and are hence not variables. This works:

rmlast([], [_]).
rmlast([X|T1], [X|T2]) :- rmlast(T1, T2).

Other than that:

1. As you see, there is no need to have variables X and Y in this case: just use X twice, in the appropriate places.

2. You write:

   rmlast([], [_]). :- true.
   

   That is the same as:

   rmlast([], [_]).
   :- true.
   

   As you can see, that second statement adds nothing and can hence safely be removed.