Path: utzoo!mnetor!uunet!seismo!sundc!pitstop!sun!quintus!ok
From: ok@quintus.UUCP (Richard A. O'Keefe)
Newsgroups: comp.lang.prolog
Subject: Re: compatibility/elegance & *theory*
Message-ID: <919@cresswell.quintus.UUCP>
Date: 30 Apr 88 02:37:41 GMT
References: <136@vor.esosun.UUCP> <841@cresswell.quintus.UUCP> <365@aiva.ed.ac.uk> <383@aiva.ed.ac.uk>
Organization: Quintus Computer Systems, Mountain View, CA
Lines: 43

In article <383@aiva.ed.ac.uk>, richard@aiva.ed.ac.uk (Richard Tobin) writes:
> 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?

Yes it is true, and no it is not the case that call/1 *is* an interpreter.
Be careful to distinguish what something _IS_ from how it is _implemented_.
Let me sketch a different implementation:

	call(Goal) :-
		callable(Goal),
		locate_procedure_record(Goal, Proc),
		!,
		unpack_goal_and_jump(Goal, Proc).
	call(Goal) :-
		callable(Goal),
		!,
		<report a call to an undefined predicate>.
	call(Goal) :-
		<report a call to a variable, number, &c>.

So, for example, call(append(A,B,C)) is basically an indirect jump to the
(presumably compiled) code for append/3; this jump having occurred nothing
further takes place that could be called "interpretation".

Accompanying this would be definitions like

	','(A,B) :-
		label(ChoicePoint),
		interpret((A,B), ChoicePoint).

	';'(A,B) :-
		label(ChoicePoint),
		interpret((A;B), ChoicePoint).

This is what I meant by saying that __call__ is like EVAL, but that
__,__ and __;__ and so on are FEXPRs.  It is not that 'call' interprets
anything, but that if you call ','/2 IT will do certain things.  (Think
of the compiler as doing partial execution.)

> Are you being disingenuous here, or have I just eaten too much microwave 
> popcorn?
I'm afraid it must be the latter.  Try cooking it first (:-).