[mercury-users] Newbie problem. :)

[Peter Schachte]

> Given the clause
>     :- p(X), q(X).
> "p(X) and q(X) cannot both be true", then
> a disjunction
>     ( ... p(T) ... ; ... q(T) ... )
> in a mode where T is ground can be compiled as
> an if-then-else.

But for determinism, you also need to know that

	p(X) ; q(X).

I think it would be better to allow declarations something like

	:- assertion p(X) <=> \+ q(X).

Since exclusive or would be a lot more natural for this, maybe the
syntax should be

	:- assertion p(X) ! q(X).

or maybe even something more specialized:

	:- exclusive p(X), q(X).

This generalizes nicely to n goals:

	:- exlusive p1(X), ... pn(X).

This means
	p1(X) => \+ p2(X), ..., \+ pn(X) and
	...
	pn(X) => \+ p1(X), ..., \+pn-1(X) and
	p1(X) ; ... ; pn(X).


-- 
Peter Schachte                     Whatever you can do or dream you can,
mailto:schachte at cs.mu.OZ.AU        begin it; Boldness has genius, power and
http://www.cs.mu.oz.au/~schachte/  magic in it.
PGP: finger schachte at 128.250.37.3      -- Goethe 
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