🚀 go-pugleaf

In article <SWK.92Aug10220513@seymour.mlb.semi.harris.com>
swk@seymour.mlb.semi.harris.com (Song W.  Koh) writes:

# I am having trouble describing transitive and
# symmetric relations without stack overflows.  Things like:
# 
# distance(X,Y,D) :- distance(Y,X,D).


The key is to use a different functor for the DATA and the PROGRAM.  If
you use distance/3 to specify data and distance_between/3 to describe
the symmetric relation, then the problem will go away. 

distance_via/4 I've included for interest; it finds the distance from A
to B via exactly one intermediate point.  If you don't know how many
intermediate points there will be on the journey, you cannot do it this
way; you will want a solution that uses lists.  (If you can't do it
yourself, ask me and I will post you a version that does it that way.)

This code was tested on Edinburgh Prolog version 1.5.04, version 12
September 1988. 


% Ken Johnson  11 August 1992
%
% You may use and reproduce this code freely but if you sell copies at a
% profit I want a share. 

distance_between(A,B,N) :-		% Distance on a one-step journey
	distance(A,B,N).

distance_between(A,B,N) :-
	distance(B,A,N).

distance_via(A,Via,B,D) :-		% Distance on a 2-step journey
	distance_between(A,Via,D1),
	distance_between(Via,B,D2),
	D is D1 + D2.

% The following distances are not meant to be accurate!

distance(edinburgh,glasgow,40).
distance(edinburgh,london,400).
distance(edinburgh,newcastle,100).
distance(edinburgh,inverness,80).
distance(inverness,wick,80).
distance(london,dover,50).
distance(london,penzance,250).
distance(dover,calais,20).



-- 
////  Advice to dieters:          ////  Ken Johnson, A I Applications Institute
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