Can GHC propose kind restrictions?
This is UX. As my code gets more polykinded I find myself in situations such as these all the more often
instance (Comonad f, Representable f, Comonad g, Representable g) => Comonad (Compose f g) where
extract = undefined
duplicate = undefined
instance ComonadApply (Compose f g) where
(<@>) = undefined
/tmp/a.hs:20:1035: error:
• No instance for (Comonad (Compose f g))
arising from the superclasses of an instance declaration
• In the instance declaration for ‘ComonadApply (Compose f g)’

20  instance ComonadApply (Compose f g) where
 ^^^^^^^^^^^^^^^^^^^^^^^^^^
Failed, modules loaded: none.
It would help me if GHC looked up a Comonad (Compose _ _)
instance, compared the kinds
Comonad (Compose (f :: Type > Type) (g :: Type > Type) :: Type > Type)
ComonadApply (Compose (f :: k > Type) (g :: Type > k) :: Type > Type)
a simple suggestion like this would be helpful
• No instance for (Comonad (Compose f g))
arising from the superclasses of an instance declaration
Try adding a kind signature (ComonadApply (Compose (f :: Type > Type) g)).
Even more amazing would be
• No instance for (Comonad (Compose f g))
arising from the superclasses of an instance declaration
Try adding a context
(Comonad f, Representable f, Comonad g, Representable g)
=> ComonadApply (Compose f g)
Which the compiler could in theory guess, since following GHC's suggestion iteratively you end up with
ComonadApply (Compose (f :: Type > Type) g)
 ==> add (Comonad f) to the context of the instance declaration
Comonad f => ComonadApply (Compose f g)
 ==> add (Comonad g) to the context of the instance declaration
(Comonad f, Comonad g) => ComonadApply (Compose f g)
 ==> add (Representable g) to the context of the instance declaration
(Comonad f, Comonad g, Representable g) => ComonadApply (Compose f g)
 ==> add (Representable f) to the context of the instance declaration
(Comonad f, Comonad g, Representable g, Representable f) => ComonadApply (Compose f g)