🚀 go-pugleaf

 Saumya Debray writes:
 > 
 > Fernando Pereira writes:
 > > Nonetheless, the efficiency reasons that led to Prolog's
 > > unification not having the check are pretty compelling, so I do not advocate
 > > adding it to Prolog in all cases. 
 > 
 > While I believe this statement, it would be interesting to actually see some
 > numbers (preferably for programs other than nrev) showing how much more
 > expensive unification becomes with the occurs check.  Do the implementors
 > of Sepia, NU-Prolog, and other systems that provide both kinds of
 > unification have any numbers they'd care to post?

 Some benchmarks were published in a paper presenting an architecture design of
 a RISC processor for Prolog, in which the occur-check is always performed if
 necessary. They use the WAM, and take advantage of the fact that only
 get_value instructions really need to call the occur-check routine. According
 to their results, 5 % of the time is spent performing the occur-check... It's
 not that much !! Their benchmark programs include quite large programs, up to
 5600 lines. 

 With some analysis (even a simple groundness analysis can be useful, but there
 are also some more sophisticated analysis schemes), I'm sure that on most
 programs, the overhead of the occur-check on a compiled Prolog can be negligible.


 Reference:

 @inproceedings{ DRSW-89,
       author = "Peter Dreussen and Wolfgang Rosentiel and Klaus Erik Schauser and
                 J{\"{o}}rg Wedeck",
       Title  = "{Architecture Design of a RISC Processor for Prolog}", 
       Booktitle = "Euromicro 89",
       Year      =  1989 }

 
 B. Dumant