eir at cis.upenn.edu
Fri Jul 31 15:10:19 UTC 2015
Let's rewrite with explicit kind variables, noting that b is also poly-kinded:
class Dep k k2 (a :: k) (b :: k2) | a -> b k2
-- if a determines b, it surely determines k2
instance Dep k * x y => Dep (Maybe k) * (Just x) (Maybe y)
Actually, even with the kinds explicit, it still looks valid to me. Post a bug report?
On Jul 31, 2015, at 9:54 AM, Gabor Greif <ggreif at gmail.com> wrote:
> Hi all,
> say I want to instantiate
> class Dep (a :: k) b | a -> b
> instance Dep x y => Dep (Just x) (Maybe y)
> Is this supposed to work? I get "The coverage condition fails" errors.
> For simple cases like
> instance Dep True Bool
> etc. it seems to work fine.
> Thanks and cheers,
> ghc-devs mailing list
> ghc-devs at haskell.org
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