I'm just learning Prolog, and I'm reviewing lecture notes and all the notes say is that:
given the following definition for directed graphs:
path(X, Y) :- edge(X, Y).
path(X, Y) :- edge(X, Z), path(Z, Y).
If we wanted to make this an undirected graph, defining
edge(X, Y) :- edge(Y, X). alone doesn't work, and I can't figure out why.
X to Y has an edge if Y to X has an edge. Seems to make sense to me.
The notes don't really clarify why not, but it does define that the proper solution would be:
edge1(X, Y) :- edge(X, Y).
edge1(X, Y) :- edge(Y, X).
to what we already had.
Can anyone explain this to me, please and thanks? <3
Because rules don't work the same as facts, and you are spiraling off into an infinite loop if you use the same predicate.
Let's take the example ?-edge(5,2). We will end up calling --
edge(5,2) :- edge(2,5).
Ok, what happens when we call edge(2,5)?
edge(2,5) :- edge(5,2).
... Uh oh. Logic circle.
When using edge1, you are simply creating a wrapper for your predicate to escape the recursive definition.