[m-dev.] for review: constraint propagation

[Fergus Henderson]

On 29-Feb-2000, Simon Taylor <stayl at cs.mu.OZ.AU> wrote:
> 
> Fergus wrote:
> > A few general comments:
> > 
> >  -	Constraint propagation might not preserve unique-mode-correctness.
> > 	Do you check to ensure that unique-mode-correctness is preserved?
> 
> Could you give an example of a case where constraint propagation
> might not preserve unique-mode-correctness?

	:- mode p(mdi, out) is semidet.
	p(X, Y) :-
		q(X, Y),
		test(X).

	:- mode q(mui, out) is det.
	:- mode test(mdi) is semidet.

Here if you treat `test' as a constraint, and move it before
the call to q/2, then it will violate unique-mode-correctness.
I think the same example will also apply if you replace the mostly unique
modes with unique modes (s/mdi/di/g, s/mui/ui/g).

(But perhaps I should be asking you for a proof, rather than you asking me
for a counter-example ;-)

-- 
Fergus Henderson <fjh at cs.mu.oz.au>  |  "I have always known that the pursuit
WWW: <http://www.cs.mu.oz.au/~fjh>  |  of excellence is a lethal habit"
PGP: finger fjh at 128.250.37.3        |     -- the last words of T. S. Garp.
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