🚀 go-pugleaf

Jussi.Rintanen@hut.fi
(Message-ID: <1992Dec13.173016.8849@nntp.hut.fi>) asks:

> Question 1: What implementations of Prolog implement the occurs check?
> (Our university has both Sicstus and NU-Prolog.)
 
The current draft International Standard for Prolog provides built-in
predicates 

1)  unify_with_occurs_check/2

   For any arguments unify_with_occurs_check(X, Y) always 
   succeeds or fails, and never generates an error or 
   gives an undefined result.  

2)   =/2 (unifies) 

   When unification involves infinite trees either as arguments 
   or as a partial result of some top-down unification algorithm,
   the behaviour is undefined.  A standard-conforming processor 
   might consistently succeed or fail for a unification that is 
   formally undefined.  This predicate can be implemented 
   much more efficiently than unify_with_occurs_check/2.

For more information, see 
    R S Scowen, "An overview of Prolog standardization 
    --- progress, problems and solutions", pp922-936 
    of "Logic programming --- proceedings of the eighth 
    international conference" (K Furukawa: Editor,  MIT Press, 
    Cambridge, Massachusetts, 1991. ISBN 0-262-56058-5.)

Roger Scowen (ISO/IEC JTC1 SC22 WG17 Prolog convener),
DITC/93, National Physical Laboratory,
Teddington, Middlesex, Great Britain   TW11 0LW
Telephone: +44 81 943 6956
E-Mail: rss@seg.npl.co.uk         Fax: +44 81 977 7091
18 December 1992