Constrained Type Families?
ekmett at gmail.com
Wed Mar 9 01:24:15 UTC 2016
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
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
> 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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> ghc-devs at haskell.org
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