{-# LANGUAGE KindSignatures #-}
{-# LANGUAGE GADTs #-}
{-# LANGUAGE DataKinds #-}


data OutputTag = A | B | C

data Input (o :: OutputTag) where 
  I1 :: Int    -> Input A
  I2 :: String -> Input B
  
data Output (o :: OutputTag) where 
  OA :: String -> Output A
  OB :: Bool -> Output B
  OC :: Output C

inputToOutput :: (tag ~ tag') => Input tag -> Output tag'
inputToOutput (I1 0) = OC
inputToOutput (I1 i) = OA (show i)
inputToOutput (I2 s) = OB (s == "x")



main :: IO ()
main = let OA s = inputToOutput (I1 4) in print s