Why does these rules work? (finding last element in list in prolog)

Question

I am trying to understand the following set of rules, which supposed to find the last element in a list (for instance my_list(X,[1,2,3]) gives X=3).

my_last(X,[X]).
my_last(X, [_|L]) :- my_last(X, L).

I understand the first fact - X is the last element of a list if X is the only element in it, but how does the second rule work? this looks a bit strange to me as a prolog noob, I'd love to know if there's an intutive way to interprate it. T

Answer 1

Read the Prolog predicate as logical rules. If there is more than one predicate clause for the predicate, you can think of it as "OR":

my_last(X,[X]).

X is the last element of the list, [X].

my_last(X, [_|L]) :- my_last(X, L).

X is the last element of the list [_|L] if X is the last element of the list L.

The second clause is a recursive definition. So it does, ultimately, need to terminate with the recursive call not matching the head of the recursive clause that is calling it. In this case, L eventually becomes [] and will no longer match [_|L]. This is important because, otherwise, you will get an infinite recursion.