length/2

It is often useful to be able to calculate (or check) the length of a list. With arithmetic operators this is something that we can now do. The base is the empty list, which is obviously of length 0. This give us the Prolog clause:

length([ ], 0).

The recursion simply requires that we add 1 for each item on the list:

length([H|T], N):-
   length(T, N1),
   N is N1 + 1.