From: Jos de Bruijn <debruijn@inf.unibz.it>

Date: Tue, 11 Mar 2008 12:03:04 +0100

Message-ID: <47D666E8.807@inf.unibz.it>

To: axel@polleres.net

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

Date: Tue, 11 Mar 2008 12:03:04 +0100

Message-ID: <47D666E8.807@inf.unibz.it>

To: axel@polleres.net

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

Thinking a bit about implementation, implementing built-ins such as isNotString might not be as straightforward as I had originally thought. Namely, isString and isNotString will give rise to multiple minimal models: for every IRI a there will be one minimal model in which isString(a) is true and one minimal model in which isNotString(a) is true. The lack of a single minimal model prevents the use of the usual fixpoint operator for computation. Best, Jos Jos de Bruijn 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. > > the most natural way to write these rules is: > > q(X+1) :- p(X), isNumeric(X). > q(0) :- p(X), isNotNumeric(X). > > In fact, I would be surprised if we would not have the isNumeric and > isNotNumeric built-in predicates. (we already have built-ins such as > numeric-add and numeric-subtract, so it will be rather silly not to have > the isNumeric and isNotNumeric predicates) > > Now, your first example is an approximation of this, which can be > simplified, as Michael pointed out. > > Your second example is an encoding that one can only understand if one > would know about the intricacies of the way RIF deals with errors. > Guards are a way to prevent errors occurring in the first place, thereby > not requiring users to know about the intricacies of the semantics of > errors. > > > Then, I would argue that having "negation" in built-in predicates does > not introduce negation in the language. > In fact, we already have negation in our built-in predicates. For > example, greater-than is the negation of less-or-equal. > Perhaps negation is also not the proper term in this context. Perhaps > we should rather talk about "complement". Greater-than is the > complement of less-or-equal; numeric-equal is the complement of > numeric-unequal; isNumeric is the complement of isNotNumeric. > Implementing the complement of a built-in predicate is as complex as > implementing the built-in predicate itself. > > Finally, complement of built-in predicates is quite different from the > (default) negation usual in logic programs. > Consider isNotString(ex:myiri). This statement may be true in some > models, and false in others. > Consider now not isString(ex:myiri). This statement is true in the > minimal model, because it is not known that ex:myiri is a string. > > > Best, Jos > > >> >> 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). >> >> 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? >> >> Axel >> >> > -- debruijn@inf.unibz.it Jos de Bruijn, http://www.debruijn.net/ ---------------------------------------------- One man that has a mind and knows it can always beat ten men who haven't and don't. -- George Bernard ShawReceived on Tuesday, 11 March 2008 11:03:20 UTC

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