Prolog：将递归谓词放入Variable

Question

I'm beginner in prolog and I need to resolve the following exercise:

Write a 3-predicate prereqchain(P,Q,L) that, given two courses P and Q, returns in L a representation of all the steps on a prerequisite path from P to Q. For example prereqchain(comp085,itec450,L) should give the answer

L=prerequisite(comp085, comp101, 
prerequisite(comp101, comp281, 
prerequisite(comp281, itec450))).

Basically, the knowledge data base stand for the prerequisite relationship between the degree's courses. My main problem is, How can I capture the recursive calls trace and put it into variable ? On the other hand, is not really all the trace, because I just need the calls where the variables was replaced with the values.

Thanks a lot!

Answer 1

What's your base case? It's going to look like this:

prereqchain(Start, End, prerequisite(Start, End)) :-
    prerequisite(Start, End).

This gives you the following situation:

?- prereqchain(comp281, itec450, L).
L = prerequisite(comp281, itec450).

So this is the first thing you need, now what happens when you ask for prereqchain(comp101, itec450) ? You need an inductive case that follows the rest of the chain:

prereqchain(Start, End, prerequisite(Start, Middle, Tail)) :-
    prerequisite(Start, Middle),
    prereqchain(Middle, End, Tail).