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 <[email protected]> wrote:
> 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
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