Constrained Type Families?

[Richard Eisenberg]

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
> 
> 
> _______________________________________________
> ghc-devs mailing list
> ghc-devs at haskell.org
> http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devs
> 
> 

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mail.haskell.org/pipermail/ghc-devs/attachments/20160308/744edaf3/attachment-0001.html>