[m-rev.] for review: type_spec and typeclass constraints

[Fergus Henderson]

On 20-Aug-2002, Simon Taylor <stayl at cs.mu.OZ.AU> wrote:
> From Fergus's review of type specialization on mercury-developers 
> in September 1998:
> > > +
> > > +             % A constraint says nothing if the types are all ground.
> > > +             list__member(Type, Types),
> > > +             \+ term__is_ground(Type)
> >
> > This is not always true.  Constraints on ground types can make sense
> > if the instance declaration is visible in the caller but not in the
> > callee.
> >
> > For example, if you have an abstract type `adt' declared in the
> > interface of one module
> > 
> >         :- module foo.
> >         :- interface
> >         :- class c where [...].
> >         :- type adt.
> > 
> >         :- implementation.
> >         :- instance c(adt) where ...
> > 
> >         foo:p :-
> >                 bar:q.
> > 
> > then in a different module you could have
> > 
> >         :- module bar.
> >         :- interface.
> >         :- import_module foo.
> >         :- pred q <= c(int).
> >         q :- ...
> > 
> > and here the ground constraint `c(int)' makes sense, since the instance
> > declaration isn't visible inside `bar', it's only visible from the caller
> > in the implementation of module `foo'.
> 
> But I see that David Overton has recently committed the following change,
> which disallows cases like this:

Ouch!
I guess we should add an example like that to the test suite.

David, was the part of the change which disallowed examples
like that intentional?  If so, what's the rationale?

> > date: 2002/07/26 06:33:11;  author: dmo;  state: Exp;  lines: +18 -9
> > 
> > Allow constraints on pred/func, instance and typeclass declarations to
> > constraint arbitrary types rather than just type variables.  The only
> > restriction is that each constraint must contain at least one type
> > variable and that all type variables in the constraint must also occur
> > somewhere else in the declaration.
> > 
> > compiler/prog_io_typeclass.m:
> >         When parsing class constraints (on pred/func, instance and
> >         typeclass declarations) remove the restriction that the
> >         arguments must all be variables.  Instead, ensure that at least
> >         one argument contains at least one variable.

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