LISA, Multislots and thoughts about Pattern matching

LISA has no multislots. That's just the way it is. You can add a list to a slot, but you have to match it as-is unless you provide a custom test-function.

About a week ago, Pedro Kröger announced on the Lisa Users mailing list that he was converting the Zebra puzzle from CLIPS to LISA. The rule syntax is so similar between the engines that it ought to have been a quite straightforward and simple exercise. But, since the CLIPS solution uses implied deftemplates (which are implemented as multislots) the LISA program wouldn't work.

This, and many other situations like it, has made me wonder about why nested fact structures aren't supported. Matching, on any level in a nested list, is actually described in Forgy's thesis (from 1979) but hasn't, for some reason, been implemented in most rule engines. Probably because it's faster to only allow flat structures.

The Rete implementation described in the thesis constructs an awful lot of nodes but I'm not sure you need all of them. A rule compiler ought to be able to figure out which part of a structure to test without doing too many feature-tests ("is the second subelement a list of 3 subelements" etc).