{-# LANGUAGE DataKinds, TypeFamilies, FunctionalDependencies #-}
module Main where

import Data.Kind (Type)

data Tag = A | B | C

data STag tag where
  SA :: STag A
  SB :: STag B
  SC :: STag C

-- class Fam (tag :: Tag) (t :: Type) | tag -> t where
-- instance {-# OVERLAPPABLE #-} Fam a Bool where
-- instance {-# INCOHERENT #-} Fam A Int where

type family Fam tag where
  Fam A = Int
  Fam _ = Bool

class Foo (tag :: Tag) where
  foo :: STag tag -> Fam tag

instance {-# OVERLAPPABLE #-} Fam tag ~ Bool => Foo tag where
  foo _ = True
instance Foo A where
  foo _ = 42

main = do
  print $ foo SA
  print $ foo SB
  print $ foo SC

-- foo :: forall tag a. STag tag -> Fam tag
-- foo SA = 42 :: Int
-- foo _ = True :: Bool