Re: Newbie, *really* basic question

From: Scott Bourbon (SCottB_at_hotmail.com)
Date: 07/05/04 

Date: Mon, 05 Jul 2004 07:11:03 +0200

Hi there,

I haven't tried with Ciao yet: after years of absence I'm back to
Prolog, finding a lot of interesting novelties.

Now: I suppose this is implementation dependent, but at least with
gprolog 1.2.16 you can get what you expected with

set_prolog_flag(unknown,fail).

| ?- likes(mary,john).
likes(mary,john).
uncaught exception: error(existence_error(procedure,likes/2),top_level/0)
| ?- set_prolog_flag(unknown,fail).
set_prolog_flag(unknown,fail).

yes
| ?- likes(mary,john).
likes(mary,john).

no
| ?-

It is to say you can get some control over the kind of Prolog reaction
to a query containing an unknown predicate.

Actually, the set_prolog_flag can act on various other aspects of the
interpreter, in gprolog;

Cheers,

        Andrea Fedeli.

PS: I've just checked with SWI-Prolog: set_prolog_flag(unknown,fail). is
accepted, but it does not produce the same effect that in gnu prolog. 

--
Bryan Glennon wrote:
> So, with time on my hands I decided to learn Prolog. Downloaded Caio 
> Prolog, and got a copy of Programming in Prolog by Clocksin & Mellish. I 
> am stuck on the first example. Here's the session output:
> 
> Ciao-Prolog 1.9 #342: Sat Apr 24 20:46:05 2004
> ?- likes(john, mary).
> {ERROR: user:likes/2 - undefined predicate}
> 
> no
> ?-
> 
> I realize it's telling me that there is no predicate "likes" that takes 
> 2 arguments, but nothing I have read indicated that I need to define 
> that predicate somewhere.
> 
> What am I missing?
> 
> Thanks,
> Bryan

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