mark richardson wrote:
Hi,
I'm slowly working through tutorials on FD's and constraint based
programming, working up to a final year degree project next year and
there is something I'm not sure I understand fully.
I was trying to find details about user defined propagators. I know
that you can write your own in C++ but why can't you create one using
standard Oz? or can you?
For example, say you have a function {Between X} that returns true if
X is between 7 and 10.
Why can't this be used as a constraint? or if you can, how?
If some patient person could explain this a little I would be most
grateful (or even just point me to some reference I haven't found yet).
Regards
Mark Richardson
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I'm getting this confused I think.
I want to use an Oz function to 'constrain' not propagate.
Sorry about that, but I'm still confused about whether I can use a
standard Oz function as a constraint.
Hi,
yes :-) In fact you could probably use a normal function as a
constraint, but given that the function does no propagation this would
just serve as an "assert" or something like that. The search itself
would be just generate-and-test, which is probably not what you want.
If say your function was what you said,
fun {Constr X}
7 =< X andthen X =< 10
end
(I hope my Oz syntax is still OK :-)) then saying "thread {Constr
X}=true end" (threaded so that it looks like a propagator) should do it.
You have to understand what it actually does - "7 =< X" waits for X to
get bound within the space where the thread runs. When that happens, the
function compares the actual value with the bounds and, since the result
is already constrained (bound) to "true", an unification exception is
thrown when the condition does not hold. An exception in a space might
turn into a failure in the case of unification error (I'm not sure...),
however in case that it doesn't you can still wrap your function call like
thread
try
{Constr X}=true
catch _ then
fail
end
end
This works for other errors as well, BUT you should try to have a
specific filter in your "catch" clause (not just "_") so that you don't
treat your programming errors as logical failures in your space!
However, writing Oz constraints is not impossible, you can just have an
Oz thread hanging around waiting for some events on FD (or other)
variables. For example, you can have a loop in your "propagator thread",
use FD.watch.min to wait for a domain change (multiple waits can be
combined e.g. using Value.waitOr), then use some reflection like
FD.reflect.min to get actual domain info and then, if needed, post some
new constraint or just perform a domain reduction as a reaction.
Hope this helps,
Filip
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