[cells-devel] Thought For Someone Who Would Use Cells to Do Prolog

[Thomas F. Burdick]

Marco Antoniotti writes:
 > As an aside, any Prolog implementation needs a good unification routine.
 > 
 > Now, you also want a good unification routine that does CL objects 
 > properly.
 > 
 > So you cannot help but using the CL-UNIFICATION package from 
 > http://common-lisp.net/project/cl-unification
 > 
 > I know it is a shameless plug, but you simply cannot avoid it :)

Shameless but well timed, I hadn't realized that your cl-unification
goes through slot accessors.  Very nice.

Just to dispell any illusions, I'm not trying to turn Cells into a
Prolog interpreter.  I kind of want a Real Constraints system, but
know the problems associated with that, so I'm just playing with
trying to mix a little LP into Cells to see if I can get something
where my code is a little clearer than what I could get otherwise.

The other motivation is that, in the past, I've had situations where I
got data out of a database, and fed it line-by-line into a c-input
until I got a consistent model, like:

  (do-results (x y z) (db (generate-query a b c))
    (setf (values (x obj) (y obj) (z obj))
          (values x y z))
    (when (model-is-good obj) (return obj)))

I'd prefer to be able to integrate this into the model, so I could
have something more like:

  (defmodel my-model ()
    ((a :initform (c-input () ...) :accessor a)
     (b :initform (c-input () ...) :accessor b)
     (c :initform (c-input () ...) :accessor c)
     (xyz :initform (c-generator ((it (db-query-iterator *db*
                                        (generate-query (^a) (^b) (^c)))))
                      (if (more-tuples-p it)
                          (next it)
                          (fail)))
          :accessor xyz)
     ... x y and z here depend on xyz, and if something is inconsistent in
         the model, their formulas call fail instead of causing (^model-is-good)
         to be nil ... ))

 > On May 26, 2005, at 12:20 AM, Kenny Tilton wrote:
 > 
 > > If your Prolog-in-CL uses  slot readers/accessors, dependencies will 
 > > arise. So the position (or whatever GUI layout attribute you have in 
 > > mind) can be calculated by a cl-prolog operating over CLOS instances 
 > > via CLOS slot accessor GFs, and the necessary dataflow will arise 
 > > naturally.

Yeah, it could be that some Allegro Prolog enhancements would handle
all of this as well.  But in the cases where I just want a *little*
Logic, making Cells handle it might not be a bad idea.  Certainly what
I have is entertaining.

Now to look at your recent changes, and to actually restore the old
dependencies on backtracking.