From: Axel Polleres <axel.polleres@deri.org>

Date: Sat, 05 Jan 2008 13:34:41 +0100

Message-ID: <477F7961.1030806@deri.org>

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

CC: RIF WG <public-rif-wg@w3.org>

Date: Sat, 05 Jan 2008 13:34:41 +0100

Message-ID: <477F7961.1030806@deri.org>

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

CC: RIF WG <public-rif-wg@w3.org>

Michael Kifer wrote: > Last Tuesday we were unable to resolve the issue of builtin functions > because we could not decide on their semantics. The problem is that builtin > functions are partial and are supposed to return an error if given wrong > arguments. > > One approach we considered was to simulate them using predicates by > translating functional expressions into formulas that involve builtin > predicates. Unfortunately, this has a problem that occurrences of such > functions in the conclusions of rules must have a different translation > than the occurrences in rule premises. This was perceived as a problem if > we were to consider BLD as a subset of the (future) first-order dialect, > since the first-order dialect will not have such a distinction. > > I thought a bit about a direct model-theoretic semantics, and there is at > least one solution that does not seem too complicated. Comments are welcome. > > 1. We need a special constant (not sure if it should be in rif:IRI or > in its own symbol space) of the form rif:error. > > 2. The domain of interpretation of any semantic structure will also have a > special distinguished element called _|_. > > 3. All semantic structures will be required to interpret rif:error as _|_ > (i.e., I_const(rif:error) = _|_) > > 4. The builtin functions will be defined so that they will return rif:error > whenever their arguments are of the wrong type. > > 5. For predicates, we have two options. > a. The simplest option is to say that a predicate, p(a,b,c,...), is false if > any of its arguments evaluates to _|_ in the interpretation. > > b. This option introduces a new truth value, called E (error) such that > ~E = E, E/\F=F, E/\T=E, E\/F=E, E\/T=T. Then we can say that > p(a,b,c,...) has truth value E if at least of of the args is _|_. > > The advantage of option (b) over option (a) is that we have an explicit > representation for errors. The handing of truth value E in option b) seems to be very exactly the way SPARQL handles errors in built-ins in FILTER expressions. > The disadvantage is that it is much more complicated. Many results need > to be ported to account for this new truth value, and I did not check > this carefully. Quite possible that this idea breaks somewhere. > > I think option (b) is too much work for very little gain. > > Comments? (esp. if anyone can see whether option (a) breaks somewhere) Does that mean you suggest only builtin-functions and no bult-in predicates? -- Dr. Axel Polleres email: axel@polleres.net url: http://www.polleres.net/ rdf:Resource owl:differentFrom xsd:anyURI .Received on Saturday, 5 January 2008 12:34:59 UTC

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