show kind signatures for (gadt) type constructors

[rwbarton@…]

I like to write GADT declarations in the following style:

{-# LANGUAGE GADTs, KindSignatures #-}
data Foo :: (* -> *) -> * -> * where
  Bar :: f x -> Foo f (f x)

so I don't give (necessarily unused) names to the type parameters of the type constructor Foo. If I don't export the data constructors of Foo but only Foo itself then the documentation generated by Haddock consists solely of

data Foo

This is uninformative and potentially misleading (it seems to suggest that Foo has kind *). It would be better to produce

data Foo :: (* -> *) -> * -> *

This is a particularly bad set of circumstances (no constructors for the reader to infer the kind from) but in general I think it would be nice to preserve any kind signatures written in the program source.