🚀 go-pugleaf

In article <1992Dec18.093045.21069@dcs.qmw.ac.uk> mmh@dcs.qmw.ac.uk (Matthew Huntbach) writes:
>In article <24435@alice.att.com> pereira@alice.UUCP () writes:
>>If you think of Prolog programs as *logic* programs where the logic is
>>(a fragment of) first-order logic, what is wrong it that the answers
>>derived without the occurs-check are in general unsound. 
>Well, yes, I'm aware of this, but Prolog is such a compromised
>form of logic that I wonder if it matters.
Right. Since we've had a hurricane, why not go for an earthquake as well?
More seriously, except for the lack of an occurs check, the original
*pure* Prolog is sound, and *many* interesting deductive tasks can
be coded in pure Prolog. Furthermore, extralogical features like assert
can be used in constrained *logical* ways, eg. for lemma storage,
and the resulting programs would be sound with respect to well-defined
logical systems, *if they were executed with the occurs check.* The
problem with the lack of an occurs check is that it can hit you
in unforeseen and mysterious ways, while the nonlogical parts
of Prolog hit you *only* if you use them unwisely.

Fernando Pereira
2D-447, AT&T Bell Laboratories
600 Mountain Ave, PO Box 636
Murray Hill, NJ 07974-0636
pereira@research.att.com