[mercury-users] Mutual Exclusivity & Exhaustiveness

[Ralph Becket]

Simon Taylor, Thursday,  3 January 2002:
> On 03-Jan-2002, Ralph Becket <rafe at cs.mu.OZ.AU> wrote:
> > Simon Taylor, Thursday,  3 January 2002:
> > > On 03-Jan-2002, Ralph Becket <rafe at cs.mu.OZ.AU> wrote:
> > > 
> > > We definitely want to disallow examples like:
> > > 
> > > :- promise_exclusive(X::ground, Y::ground) is det :-
> > > 		( foo(X)
> > > 		; bar(X, Y)
> > > 		; baz(Y)
> > > 		).
> > 
> > Hmm, from the above we can infer
> > 
> > :- promise_exclusive(X::ground) is det :-
> > 	( foo(X)
> > 	; baz(X)
> > 	).
> 
> How? The arguments of foo and baz need not even have the same type.

Brain failure on my part.

> > By the way, I don't think the connective should be ":-". 
> 
> Actually, my suggested syntax doesn't parse. Oops.
> 
> > The usual reading of "P :- Q" is "P if Q".  What we're trying to say here is
> > that the LHS implies the RHS!  So something like `for` would be a better
> > choice.
> 
> I'd rather not invent another connective just for this.
> 
> Maybe this would be better:
> 
> :- promise_exclusive(X::ground) is det where
> 	( foo(X)
> 	; baz(X)
> 	).
> 
> > Going further, the use of disjunction is wrong, too.  Maybe a plain list
> > would be better:
> >
> > :- promise_exclusive(X::ground) `for` [
> > 	foo(X)
> > 	bar(X, _)
> > 	baz(X)
> > ].
> 
> The disjunction mimics how code using the promise is written.
> It also has the correct variable scoping rules.
> I don't see how a list is any more correct. The elements of
> the "list" don't have the same type.

Okay, if we say that ";" should be read as exclusive-or in a
promise_exclusive declaration.

- Ralph
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