[m-rev.] for review: bug fix in existential constraints

[Julien Fischer]

On Fri, 15 Apr 2005, Julien Fischer wrote:

>
> On Thu, 14 Apr 2005, Mark Brown wrote:
>
> > Estimated hours taken: 2
> > Branches: main, release-0.12
> >
> > Only allow existential constraints to constrain existentially quantified
> > type variables.  Likewise only allow universal constraints to constrain
> > universally quantified type variables.
> >
> > We previously didn't check these conditions, and in fact the reference
> > manual allowed otherwise.  However, the implementation did not support
> > existential constraints on universally quantified type variables, and
> > will not do so for the foreseeable future.  The implementation does support
> > universal constraints on existentially quantified variables, however these
> > are not useful since no caller will ever be able to satisfy the constraints.
> >
> > compiler/check_typeclass.m:
> > 	Check these conditions as part of typeclass checking.
> >
> > 	Also return a bool from the recently added check for concrete
> > 	instances, indicating whether errors were found or not.
> >
> The check for concrete instances hasn't yet been added to the release
> branch.
>
It has now.

Julien.
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