[mercury-users] Constraints and conditionals in Mercury

[Julien Fischer]

On Mon, 14 Apr 2008, Mark Brown wrote:

> On 13-Apr-2008, Peter Schachte <schachte at csse.unimelb.edu.au> wrote:
>> Ralph Becket wrote:
>>> Richard A. O'Keefe, Friday, 11 April 2008:
>>>> knights_tour/1 has one argument, an integer.  It is known to be ground.
>>>> There is a conditional
>>>>
>>>> 	if knights_tour(N) then ... else ...
>>>>
>>>> The implementation of knights_tour/1 uses constraints *internally* to
>>>> determine whether there exists at least one knight's tour on an NxN
>>>> board or not.  Suppose that knights_tour(N) leaves some constraints
>>>> pending when it exits, and that for some value of N those constraints
>>>> are not satisfiable.
>>>
>>> Right now: we don't catch this situation.  We have discussed the
>>> problem, however.  One suggestion was to look out for cases where
>>> a solver variable goes out of scope without having become ground
>>> and require some kind of promise of good behaviour from the programmer.
>>
>> The problem with that is that the promise could be a lie, and there may be no
>> way for the caller to make it true.  I suggested there should be a way for
>> Mercury to force a constraint solver to either satisfy all pending
>> constraints if it ca, fail if it can determine they're unsatisfiable, or
>> throw an exception if it can't do either.  The Mercury compiler would then
>> force a decision in negated contexts and anywhere else it has to commit.
>
> For commits, there is the ability to do this using the function trailing
> interface.  The callback, when called, is passed a "reason" for the trail
> being unwound.  The functions can be called not just on failure, but also
> on exceptions, commits, etc.  These functions could be made to check for
> floundering.
>
> (Sorry, due to network problems I can't look up the URL for this.)

It is:

<http://www.mercury.csse.unimelb.edu.au/information/doc-latest/mercury_ref/Delayed-goals-and-floundering.html#Delayed-goals-and-floundering>

Julien.
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