Prolog Mutual recursion

Question

Ok I'm writing some code to go through a check each value by mutual recursion in Prolog. This is my code so far:

semestersok(SP) :-
    [Prior|Tail] = SP,
    sem1ok(SP).



%% sem1ok(SP) :- checks semester 1 of SP is ok
sem1ok(SP) :-
    [Sem1|Tail] = SP,
    sem2ok(Tail).

%% sem2ok(SP) :-
sem2ok(SP) :-
    [Sem2|Tail] = SP,
    sem1ok(Tail).

I haven't put any code in yet to do with checking (There's two relations as it has to check alternating values), I'm having a problem with the code cycling through until it has an empty list, then it fails and comes back with false (no). Since This code isn't manipulating any code I believe it should come back true as it stands right now. Why isn't it?

Answer 1

You need some rules for the empty list. Add this:

sem1ok([]).
sem2ok([]).

Also, the code might be more intuitive if you write it like this (as the distinction between the rule matching the empty list and the one matching the non-empty list is more clear):

% Rules for sem1ok/1
sem1ok([]).
sem1ok([Sem1|Tail]):-
    ok(Sem1), % Something involving Sem1
    sem2ok(Tail).

% Rules for sem2ok/1
sem2ok([]).
sem2ok([Sem2|Tail]):-
    ok(Sem2), % Something involving Sem2
    sem1ok(Tail).