Re: [eclipse-clp-users] Assignment propagation queue

From: Kish Shen <kisshen_at_...5...>
Date: Tue, 03 Feb 2009 14:56:56 +0000
Lars Kotthoff wrote:
>>  > or does it use a heuristic like most constrained variable first?
>>
>> You would have to explain more precisely what you mean by that in the
>> context of a propagator-queuing implementation.  Propagators typically
>> involve many variables.  You could write a propagator that increases
>> its own priority when it involves highly constrained variables.
> 
> I was thinking along the lines of taking the highest number of constraints any
> of the variables covered by the propagator is involved in and using that as the
> priority.
> 
> Do all propagators that eclipse defines have the same priority then?
> 
Hi Lars,

No, ECLiPSe does not "define" the priority of propagators.

The priority for a propagator (implemented as one or more goals that are 
suspended) is specifed when you suspend (or resuspend) the propogator -- 
see the documentation for suspend/3,4 (you may also want to read the 
`Advance Control Features' chapter of the User manual, 
http://www.eclipse-clp.org/doc/userman/umsroot104.html).

I assume you are questioning how the priority can be adjusted when you 
resuspend -- to do so, the propagator goal must remember the old 
priority. In fact, since propagators are simply normal ECLiPSe goals, 
you can resuspend a different goal (effectively a different propagator), 
because you no longer need the original propagator.

I guess I should explain more clearly what I meant when I said ECLiPSe 
do not directly implement propagators in my last message. I was thinking 
  in terms of what is available in some other (Constraint Logic) 
Programming languages, which provide special syntax for directly 
encoding propagators -- examples are indexicals (used in SICStus, 
probably others), action rules (used in B-Prolog). ECLiPSe does not have 
such specialised syntax, and instead you write the code for propagation 
using normal ECLiPSe syntax.

Cheers,

Kish



(in fact you may suspend a different goal from the original, e.g. 
because the further propagation needed have become simpilier)
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Received on Tue Feb 03 2009 - 14:56:59 CET

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