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 <[email protected]> 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 <[email protected]> wrote: > > On Mar 8, 2016, at 7:17 PM, Evan Austin <[email protected]> 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 > [email protected] > http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devs > >
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