Path: utzoo!utgpu!watserv1!watmath!att!pacbell!pacbell.com!mips!swrinde!zaphod.mps.ohio-state.edu!samsung!cs.utexas.edu!bradley
From: bradley@cs.utexas.edu (Bradley L. Richards)
Newsgroups: comp.lang.prolog
Subject: Re: Meta-programming question
Message-ID: <12064@cs.utexas.edu>
Date: 31 Aug 90 16:24:40 GMT
References: <199@qt.cs.utexas.edu> <3646@goanna.cs.rmit.oz.au>
Organization: U. Texas CS Dept., Austin, Texas
Lines: 48

First, many thanks to all the folks who responded to my question!

>ok@goanna.cs.rmit.oz.au (Richard A. O'Keefe) writes:

>Sigh.  This is why we wanted a Prolog standard...
>
>It isn't perfectly clear what you don't want bound.  If you are probing
>for a clause with a pattern, and you are happy for that pattern to get
>instantiated, but you want (a copy of) the original clause, do
>
>        head_clause_nobind(Head, Clause) :-
>            clause(Head, _, Ref),
>            instance(Ref, Clause).

Indeed this is exactly what I want to do.  And I'm in strong agreement with
your first comment, because Arity *does not* support the clause/3 predicate.
I attempted to write one using recorded/3 when I found that it can in fact
access clauses--but, sadly, it doesn't do so in a consistent manner.

So, it looks like I'm stuck with the effective but slow version

>        head_nobind_clause_nobind(Head, Clause) :-
>            functor(Head, F, N),
>            functor(Skel, F, N),
>            clause(Skel, Body),
>            \+ \+ Skel = Head,
>            make_clause(Skel, Body, Clause).

>May I respectfully suggest that trying to use the "raw" clause store
>is almost always a bad idea?  Suppose you are doing your meta-programming
>and start tinkering with the append/3 predicate.  What if your meta-program
>uses append/3?  You want to keep the levels separate.

Indeed you are right.  I've been away from Prolog for a while so I'm not sure
if this is "standard," but Arity provides the option of creating different
"code worlds," and I keep the meta-programming stuff isolated this way.  This
has the advantage that when I want to execute a predicate in the object
program, I can merely change worlds and call the appropriate predicate.
Presuming it doesn't go off in never-never land, and that I don't want any
intermediate results, this is faster than running a metainterpreter.

Thanks again!
Bradley

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 Bradley L. Richards       uucp:  cs.utexas.edu!bradley
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