Reification drops necessary kind annotations
Consider the following:
{-# LANGUAGE DataKinds, PolyKinds, TypeFamilies, TemplateHaskell #-}
import Language.Haskell.TH
type family Poly (a :: k) :: *
type instance Poly (x :: Bool) = Int
type instance Poly (x :: Maybe k) = Double
$( do info <- reify ''Poly
runIO $ putStrLn $ pprint info
return [] )
Compiling with HEAD yields this output:
type family Main.Poly (a_0 :: k_1) :: *
type instance Main.Poly x_2 = GHC.Types.Double
type instance Main.Poly x_3 = GHC.Types.Int
The problem is that the type patterns in the reified instances are just plain variables, without their kind annotations. This omission makes the instance declarations unfaithful to the original meaning.