Constrained Type Families?

[Edward Kmett]

If and when that feature lands would it be possible to use it to bypass a
current limitation in class associated types?

Notably if a class associated type has a more general kind, we currently
can't give a default definition for it that has a tighter kind.

e.g. I have some classes which are technically polykinded but where 90% of
the instances instantiate that kind as *. The status quo prevents me from
putting in a type default that would only be valid when the kind argument
is *.

-Edward

On Tue, Mar 8, 2016 at 8:21 PM, Richard Eisenberg <eir at cis.upenn.edu> wrote:

>
> On Mar 8, 2016, at 7:17 PM, Evan Austin <e.c.austin at gmail.com> wrote:
>
> The wiki page for Phase I of Dependent Haskell describes an approach to
> constrained type families:
>
> https://ghc.haskell.org/trac/ghc/wiki/DependentHaskell/Phase1#Typefamilyequationscanbeconstrained
>
> Did that land in GHC 8.0 and, if so, is the updated syntax documented
> somewhere?
>
>
> No, it didn't make it. The motivating test case seemed contrived and so we
> punted on this one.
>
> Do you have a use case that really needs this feature? That would help to
> motivate it for 8.2 or beyond.
>
> Thanks!
> Richard
>
>
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