Constrained Type Families?
eir at cis.upenn.edu
Wed Mar 9 01:31:31 UTC 2016
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.
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 *.
> 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:
>> 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.
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