[mercury-users] Another Mercury Modes Problem

[David Overton]

On Tue, May 28, 2002 at 04:45:12PM +1000, Ralph Becket wrote:
> David Overton, Tuesday, 28 May 2002:
> > On Tue, May 28, 2002 at 03:01:07PM +1000, Ralph Becket wrote:
> > > 
> > >     % These should be built-in, but aren't...
> > >     %
> > > :- mode di(I) == unique(I) >> dead.
> > > :- mode uo(I) ==      free >> unique(I).
> > 
> > The inst `unique(...)' takes a list of functors separated by `;' (the same
> > as `bound(...)'), not an inst.
> 
> I was unsure of that syntax.  How does one apply the unique attribute to
> a named inst?

You can't at the moment, although it might be a useful feature to add.

> 
> > `di/1' is defined in builtin.m as
> > 
> > 	:- mode di(I) == (I >> dead).
> > 
> > `uo/1' is not defined, but it would be the same as `out/1' if it were.
> 
> This would appear to require that every inst come in both shared and
> unique flavours, rather than being able to apply the unique attribute to
> the top-level functor at will.

Yes, if you require both ``flavours'' you need to declare two separate
insts.

> Which reminds me of a discussion I was having with Fergus: Fergus tells
> me that uniqueness goes "all the way down" and that this is what one
> typically wants.  I can't see it, myself: I agree that one often wants a
> unique "spine", but the other arguments to a functor needn't (won't) be
> unique in the common case, as far as the examples I've been able to come
> up with are concerned.  Would it cause major problems if uniqueness only
> applied to the functor, rather than recursively to all its arguments as
> well?

The uniqueness of `unique' goes all the way down.  If you use
`unique(...)' you can have non-unique insts in the arguments of the
functors, e.g.

	unique(no ; yes(ground))

is a perfectly valid inst although note that

	bound(no ; yes(unique))

is _not_ allowed.



David
-- 
David Overton      Computer Science and Software Engineering
PhD Student        The University of Melbourne   +61 3 8344 9159
Research Fellow    Monash University (Clayton)   +61 3 9905 5779
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