🚀 go-pugleaf

fdb@syllogic.nl (Frank de Bruijn) writes:

>swk@seymour.mlb.semi.harris.com (Song W. Koh) writes:

>>[I'm a] prolog beginner.  I am having trouble describing transitive and
>>symetric relations without stack overflows.  Things like:

>>distance(X,Y,D) :- distance(Y,X,D).

>I see, are you blind, or just too lazy to read a decent textbook on
>PROLOG? Even the "not so good books" describe this.

>The standard solution is to use a different predicate name for the distance
>facts. (It has always struck me as a bit of a diappointment though)

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

This will not work: it will not find distances of points that require
paths more than two long. A correct formulation of the transitive
clause is

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

So be sure to get your answers right before you flame someone for
asking the obvious.

	Torben Mogensen (torbenm@diku.dk)