Path: utzoo!mnetor!uunet!mcvax!ukc!its63b!aiva!richard
From: richard@aiva.ed.ac.uk (Richard Tobin)
Newsgroups: comp.lang.prolog
Subject: Re: compatibility/elegance & *theory*
Message-ID: <383@aiva.ed.ac.uk>
Date: 28 Apr 88 01:29:26 GMT
References: <136@vor.esosun.UUCP> <841@cresswell.quintus.UUCP> <365@aiva.ed.ac.uk> <903@cresswell.quintus.UUCP>
Reply-To: richard@uk.ac.ed.aiva (Richard Tobin)
Organization: Bannerman's Bar, Cowgate, Edinburgh
Lines: 35

In article <903@cresswell.quintus.UUCP> ok@quintus.UUCP (Richard A. O'Keefe) writes:
>Actually, call/1 in Prolog is a pretty straightforward APPLY.  (There is
>a family of call/N things each adding N-1 extra arguments.)  The problem
>is that ','/2 and so on are FEXPRs.  (Is everyone nicely confused now?)

Yup.

>The point is that call(','(write(a)), write(b)) ends up calling write(a)
>and write(b), not because call/2 does anything special, but because (A,B)
>calls A and B.  It is as if you did
>	(APPLY #'(LAMBDA (X Y) (AND (EVAL X) (EVAL Y)))
>	    '(PRINT 'a) '(PRINT 'b) '())
>in Lisp.

Is this really true?  Isn't it the case that in all reasonable prologs
call/1 *is* a simple interpreter, with the result that (in particular)
cut works reasonably?

For example,
  X = (repeat, write(x), !, fail), call(X).
prints just one x, but if I define the operator 'and' (with precedence like
that of comma) to be
  A and B :- call(A), call(B).
then
  X = (repeat and write(x) and ! and fail), call(X).
prints rather a lot of x's.

Are you being disingenuous here, or have I just eaten too much microwave 
popcorn?

-- Richard
-- 
Richard Tobin,                         JANET: R.Tobin@uk.ac.ed             
AI Applications Institute,             ARPA:  R.Tobin%uk.ac.ed@nss.cs.ucl.ac.uk
Edinburgh University.                  UUCP:  ...!ukc!ed.ac.uk!R.Tobin