Newsgroups: comp.lang.prolog
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From: marion@transport.ucl.ac.uk (Marion Edwards)
Subject: Re: Predicate Not Defined (Porting ALS to LPA)
Sender: news@ucl.ac.uk (Usenet News System)
Message-ID: <1994Dec5.111410.30307@ucl.ac.uk>
Date: Mon, 5 Dec 1994 11:14:10 GMT
Reply-To: marion@transport.ucl.ac.uk (Marion Edwards)
References:  <3bjpld$nu5@giga.bga.com>
Organization: Centre for Transport Studies, UCL
Lines: 22

In article <3bjpld$nu5@giga.bga.com>, jem@mba.com (Jay Margolis) writes:
|> 
|> I am porting a prolog program from ALS prolog to LPA Prolog.  In ALS,
|> when a term is found on the right side of the horn clause that is
|> not defined, it assumes failure, and backtracks.  But in LPA Prolog,
|> it gets a run-time error: predicate not defined.
|> 
|> Accoding to my Prolog book Moss[94], it says that some Prologs require
|> that you use the unknown(_,fail) to cause a normal failure when finding
|> an undefined predicate.  I could not get this to work with LPA.


In LPA 386-Prolog (version 1.1) for DOS you need:

    unknown_predicate_handler( _, fail)

Marion Edwards

Centre for Transport Studies               marion@transport.ucl.ac.uk
University College London
Gower Street
LONDON              WC1E 6BT
