[Fwd: Re: [cells-devel] Logic Programming in Cells]

[Thomas F. Burdick]

Kenny Tilton writes:

 > The bigger problem is that half-way thru I remembered that I kind of 
 > like the idea of setting a model in motion and then just having it run 
 > without further input because one change always leads to another. We'd 
 > have to have a hook somewhere to stop and check the event queue so the 
 > poor user can get in on the fun.
 > 
 > I cannot say for sure I will ever do that, but it Sounds Right. So I 
 > will look instead at maintaining a simulated propagation stack just  to 
 > help debugging (a known need) with an eye towards someday possibly 
 > implementing "rule calculation backout". Am I right that the condition 
 > system would be too slow for logic programming? You mentioned the rule 
 > would "call FAIL". What would FAIL do?

FAIL would do some sort of nonlocal exit :-)

Uhm, I'm not sure.  Currently, the FAIL I have signals a
NO-VALID-STATE condition, the code that runs the generators
establishes a BACKTRACK restart, and wrapped around the very outside
is a handler for NO-VALID-STATE that invokes the BACKTRACK restart if
it can find it.  I did this with the idea that Cells is probably not
up to being a full-featured LP engine, so it's probably better to err
on the side of letting the user hook in.

In the no-stack situation, FAIL could cause a RETURN-FROM back into
the propagation machinery, which would certainly be faster.  Whether
that would make enough of a difference to enable a backtracking Cells
to be used for more general LP stuff, I don't know.  Only one way to
find out, I guess.