Data families should not instantiate to non-Type kinds (#14420) · Issues · Glasgow Haskell Compiler / GHC

Data families should not instantiate to non-Type kinds

data family Any :: k  -- allowable now due to fix for #12369

type family F (a :: Bool) :: Nat where
  F True  = 0
  F False = 1
  F Any   = 2

ghci> :kind! F True
F True :: Nat
= 0
ghci> :kind! F False
F False :: Nat
= 1
ghci> :kind! F Any
F Any :: Nat
= 2

Oh dear.

We should require that any instantiation of a data family be to a kind that ends in Type.

Trac metadata