From: Michael Kifer <kifer@cs.sunysb.edu>

Date: Tue, 11 Mar 2008 02:31:13 -0400

To: axel@polleres.net

Cc: "Public-Rif-Wg (E-mail)" <public-rif-wg@w3.org>

Message-ID: <9478.1205217073@cs.sunysb.edu>

Date: Tue, 11 Mar 2008 02:31:13 -0400

To: axel@polleres.net

Cc: "Public-Rif-Wg (E-mail)" <public-rif-wg@w3.org>

Message-ID: <9478.1205217073@cs.sunysb.edu>

Axel wrote: > > I was talking to Harold about the DTB document again and we agreed both > that we do not really like the "negative guards", because they seem to > introduce some special form of negation, which is a bit weird, as > otherwise negation is not part of bld... > > Now let's take the use case, which I suppose brought up the whole idea > about negative guards: > > > I assume the idea was that guards where meant to do some type checking > to prevent other builtins to fail with an error... so we need these for > instance to right something like: > > q(X+1) :- p(X) if X is numeric. > q(0) :- p(X) if X is not Numeric. > > i.e. to make a case distinction, concerning whether the arbuments are > "allowed" for the built-in which should e "guarded". > > Now with the current means we would need to write this something like > as follows: > > q(numeric-add(X,1)) :- p(X), isInteger(X) . > q(numeric-add(X,1)) :- p(X), isDecimal(X) . > q(numeric-add(X,1)) :- p(X), isLong(X) . > > q(0) :- p(X), isNotInteger(X), isNotLong(X), isNotDecimal(X). Decimal a supertype of Integer and long, so you do not need all that complexity: q(numeric-add(X,1)) :- p(X), isDecimal(X). q(0) :- p(X), isNotDecimal(X). > whereas we could write this simpler and without negative guards if we > just add one more guard except the current datatypeguards: > > q(numeric-add(X,1)) :- p(X), isInteger(X) . > q(numeric-add(X,1)) :- p(X), isDecimal(X) . > q(numeric-add(X,1)) :- p(X), isLong(X) . > q(0) :- p(X), isError(X+1). > > I don't know whether this solves all use cases for negative guards, but, > before buying into sneaking in some form of negation which we can later > on have in dialects with negation for free... maybe adding the isError() > guard for the moment and leaving out the negative guards would do for > the moment? How do you define the semantics of isError? mReceived on Tuesday, 11 March 2008 06:31:56 GMT

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