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?
Richard
On Jul 31, 2015, at 9:54 AM, Gabor Greif
Hi all,
say I want to instantiate
class Dep (a :: k) b | a -> b
as
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,
Gabor _______________________________________________ ghc-devs mailing list ghc-devs@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devs