🚀 go-pugleaf

From: swk@seymour.mlb.semi.harris.com (Song W. Koh)
>prolog beginner.  I am having trouble describing transitive and
>symetric relations without stack overflows.  Things like:
>
>distance(X,Y,D) :- distance(Y,X,D).
>
>and
>
>distance(X,Z,D) :- distance(X,Y,D1), distance(Y,Z,D2), D is D1 + D2.

No need for complex solutions involving meta-interpreters. I
beleive the reasons why novices commonly make mistakes like
this is that Prolog is taught as a mysterious theorem-proving
system, rather than as just another programming language. If it
were taught in the latter way, even novices would soon see that
the first clause above will lead straight to a loop.

You need to represent your known distances as facts with a separate
predicate:

dist(a,b,4).
dist(b,c,5). etc.

Now have:

distance(X,Y,D) :- dist(X,Y,D).
distance(X,Y,D) :- dist(Y,X,D).
distance(X,Y,D) :- dist(X,Z,D1),distance(Z,Y,D2),D is D1+D2.

OK, but this can still loop, consider:

dist(a,b,4).
dist(b,c,5).
dist(a,c,3).
dist(a,d,2).
dist(d,e,1).

with query:

distance(a,e,D).

The effect will be to cycle round teh loop in the graph.
You need to keep a record of points 'visited' to prevent
cycles like this. Think about it a bit more.

Matthew Huntbach