{-# LANGUAGE GADTs #-}

{-# LANGUAGE DataKinds #-}

{-# LANGUAGE KindSignatures #-}

{-# LANGUAGE RankNTypes #-}

{-# LANGUAGE PolyKinds #-}

{-# LANGUAGE StandaloneDeriving #-}

{-# LANGUAGE UndecidableInstances #-}

data Param = Param1 | Param2

data T (p :: Param) where

  TInt :: Int -> T Param1

  TInteger :: Integer -> T Param1

  TBool :: Bool -> T Param2

data U (p :: Param) where

  UDouble :: Double -> U Param1

  UString :: String -> U Param2

  

data F (t :: Param -> *) where

  F :: t Param1 -> t Param2 -> F t

  

f :: T a -> U a

f (TInt x) = UDouble (fromIntegral x)

f (TInteger x) = UDouble (fromIntegral x)

f (TBool x) = UString (show x)

class MyFunctor f where

  myFmap :: (forall a. t a -> u a) -> f t -> f u

  

instance MyFunctor F where

  myFmap f (F x1 x2) = F (f x1) (f x2)

deriving instance Show (U a)

deriving instance (Show (t Param1), Show (t Param2)) => Show (F t)

main = print $ myFmap f (F (TInt 42) (TBool False))