Constrained Type Families?

[Edward Kmett]

It has only been a low level irritant. Until Evan mentioned the link above,
I'd mostly just pushed it aside and learned to live with it.

This is probably the first time I've mentioned it outside of IRC.

-Edward

On Tue, Mar 8, 2016 at 8:31 PM, Richard Eisenberg <eir at cis.upenn.edu> wrote:

> I see no good reason for this restriction -- I think that we should just
> remove the restriction instead of cooking up a workaround. Have you brought
> this up before? Perhaps make a ticket.
>
> Richard
>
> On Mar 8, 2016, at 8:24 PM, Edward Kmett <ekmett at gmail.com> wrote:
>
> 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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>>
>>
>
>
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