Dear Christian,
I understood that Alice integrates Gecode and that Gecode supports
batch recomputation -- which can significantly contribute to a more
efficient search. I also understood that Mozart does not support batch
recomputation yet.
Are there perhaps any plans to integrate Gecode in Mozart as well?
Best,
Torsten
On 26.10.2005, at 20:06, Christian Schulte wrote:
Dear all,
I have been following this discussion with quite some interest,
however I
was unably to throw in something. Here it comes.
The first point I would like to re-iterate on (Grégoire and Raphaël
already
went there) is the tremedous difference between the essence of
constraint
programming and generate and test. The very idea of constraint
programming
is to open up variables so that they can specify what are values which
still
can participate in a solution. By this, one describes (partially) a
set of
solutions to the problem. With generate and test, one only describes a
single partial assignment. Why this matters: the inference you can do
in
principle for generate and test is to find out that the current partial
assignment can not be extended to a solution. And if one finds out the
only
thing is to backtrack! Now for constraint programming, inference
(constraint
propagation, that is) can remove some assignments (as described by the
projections onto the involved variables) without being forced to
backtrack.
The difference is typically at least exponential.
The other point I'd like to make is to discourage Russ to implement
propagators in Oz. Yes, one can do it. And yes, one can dig a hole
with a
spoon. However, pick your tool well! Evidence has it that you are way
better
of to implement a constraint as a propagator in C++ than in Oz. This is
partially due to efficiency concerns (these guys need to be really
really
fast). But alos, as it comes to numerical and graph algorithms, C++ is
just
the better pick. Of course, I am somehow biased as I abandoned Oz in
favour
of my new pet: Gecode, a constraint programming environment entirely
developed in C++ (check www.gecode.org, we are going to release on Nov
24th).
Christian
-----Original Message-----
From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED]
On
Behalf Of Russ Abbott
Sent: Wednesday, October 26, 2005 7:43 PM
To: Raphael Collet
Cc: [EMAIL PROTECTED]
Subject: Re: Implementing constraints
On 10/26/05, Raphael Collet <[EMAIL PROTECTED]> wrote:
Russ Abbott wrote:
The question is: how can one do
that in Oz? How can one express range constraints and modifications
to
those range constraints at the Oz level? I imagine it can be done.
But
it will take a bit of work.
Here is how to code that constraint in Oz if Xs is a list of integers:
No dispute. If you can code stuff as integers and use FD, you get more
leverage.
A question that comes to my mind (and that I asked a few messages ago
but not as fully) is why aren't these features already available at
the
Oz level. Oz must have much of this mechanism already built. Why must
one write C++ code to get at it?
There are two answers to that question. First, quite a few
combinatorial problems can be easily "coded" with integers or integer
sets. Second, noone has had the time to implement it yet :-(
That was my question. Thanks for the answer.
I would think that since the underlying mechanisms already exist,
making
them visible at the Oz level wouldn't take that much work--not that I'm
volunteering to do it.
Since Oz prides itself as being a platform for exploring issues in
programming language design (and not just a language for writing FD
constraint programs), exposing the mechanisms that make it possible to
reason about range constraints in general would further that vision. I
would
urge that this be elevated to a higher priority.
-- Russ
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Torsten Anders
Sonic Arts Research Centre
Queen's University Belfast (UK)
www.torsten-anders.de
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