[mercury-users] some [T] ...

[Mark Brown]

On 18-Nov-2002, Michael Day <mikeday at yeslogic.com> wrote:
> 
> > 	:- type abc ---> a ; b ; c.
> > 
> > 	:- subtype ab < abc ---> a ; b.
> 
> By "including those inside typeclass declarations" did you mean that I
> could declare the following type class:
> 
> :- typeclass foo(T) where [ func foo(T) = string ].
> 
> and make ab an instance of it?
> 
> :- instance foo(ab).

No, that would be disallowed.  What I mean is that you would be allowed to
refer to subtypes inside the pred/func declarations in the 'where [ ... ]'
part of a typeclass definition.  So you could write:

	:- typeclass bar(T) where [ func bar(T, ab) = string ].

While you would not be allowed to write something like

	:- instance foo(ab),

you could still call a method of this typeclass on the subtype ab, provided
that abc is declared to be a instance of foo, and the method is
polymorphically moded.  In this respect method calls are no different
to ordinary calls.

> > 	:- pred q(z(ab), io, io).
> > 	:- mode q(in, di, uo) is det.
> > 
> > becomes
> > 
> > 	:- pred q(z_rep(abc), io, io) is det.
> > 	:- mode q(in(bound(z(Z))), di, uo) is det.
> 
> Wow, so does that mean I could have lists and maps and god knows what else
> all containing predicates? Is it possible to say:
> 
> :- pred q(list(z(ab)), io, io).
> :- mode q(in, di, uo) is det.
> 
> and everything just works?

Yes, provided the mode that results from propagating the subtype
information into the existing mode is a valid one.  As Fergus mentioned,
though, you wouldn't easily be able to use lists/maps from the current
version of the standard library.  See the recent discussion on the
mercury-developers mailing list for one proposal of how we may deal with
this.

Cheers,
Mark.

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