From: Sandro Hawke <sandro@w3.org>

Date: Fri, 16 Mar 2001 07:30:00 -0500

Message-Id: <200103161230.HAA02439@hawke.org>

To: bgrosof@MIT.EDU

cc: www-rdf-logic@w3.org

Date: Fri, 16 Mar 2001 07:30:00 -0500

Message-Id: <200103161230.HAA02439@hawke.org>

To: bgrosof@MIT.EDU

cc: www-rdf-logic@w3.org

[ rdf-logic: This e-mail is continuing a face-to-face conversation Benjamin Grosof and I were having yesterday. I'm trying to understand how KR language expressiveness theory applies to RDF. Others are welcome to join, of course. ] Okay, this morning my brain is properly in gear. Let's say we want to capture the grandparent relation. Intuitively we know grandparent(a,c) means something like parent(a,b) and parent(b,c). But how do we write this? One direction is easy: grandparent(?a, ?c) <--- parent(?a, ?b) and parent(?b, ?c) parent(gregorian, sandro) parent(sandro, bret) we can now infer: grandparent(gregorian, bret). But what if we're given grandparent(gregorian, bret) isLoved(?x) <---- parent(?x, ?y) and we'd like to prove isLoved(gregorian)? In FOL you can say forall a forall c grandparent(a,c) implies exists b suchthat parent(a, b) and parent(b, c) In NILE you say rule "" ($a grandparent $c ==> $a parent !b; !b parent $c) In Prolog (but not Datalog) you can say: parent(X, parent_of(X)) :- grandparent(X, Y). In Prolog or Datalog you can rephrase in terms of a 3-ary relation: gen3(X,Y,Z) :- parent(X,Y), parent(Y,Z). parent(X,Y) :- gen3(X,Y,Z). parent(Y,Z) :- gen3(X,Y,Z). which lets us rephrase our test case: gen3(gregorian, whatever, bret). isLoved(X) :- parent(X, Y). and then infer isLoved(gregorian) as we want. (In practice, this set of rules puts prolog into a loop, although XSB should handle it with tabling.) But how else? And thus my hypothesis: datalog with only binary predicates is too weak to talk about transitive relations. -- sandroReceived on Friday, 16 March 2001 07:30:39 UTC

*
This archive was generated by hypermail 2.3.1
: Wednesday, 2 March 2016 11:10:34 UTC
*