🚀 go-pugleaf

In article <7199@skye.ed.ac.uk>, ken@aiai.ed.ac.uk (Ken Johnson) writes:
|> In article <1992Aug13.111745.17980@odin.diku.dk> torbenm@diku.dk (Torben
|> AEgidius Mogensen) writes:
|> #  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.
|> This may loop indefinitely.

Agreed.


|> For instance, suppose you invoked it with
|>          ?- distance(london,paris,X).
|> when `paris' was not mentioned in any distance fact.

Hardly, as seen below.  (Note that since there is no distance_fact
in the program, your precondition is fulfilled.  Also, the dynamic
declaration may not be necessary in your flavor of Prolog.)

----- Begin Included Transscript -----
  mint[254]: prolog
  SICStus 2.1 #5: Tue May 12 12:04:21 MET DST 1992
  | ?- [user].
  | :- dynamic distance_fact/3.
  | distance(X,Z,D) :-
       distance_fact(X,Y,D1),
       distance(Y,Z,D2),
       D is D1 + D2.
  | {user consulted, 40 msec 672 bytes}

  yes
  | ?- distance(london,paris,X).

  no
  | ?- ^D
------ End Included Transscript ------

However, the goal  ?- distance(london,paris,X)  leads to an infinite
loop in Prolog if you can deduce both the facts
  distance_fact(london,paris,_)
and
  distance_fact(paris,london,_)
from your program.

-- 
Robert Marti                    |  Phone:    +41 1 254 72 60
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