Playing with GADTs and destructuring them.

gadts.hs

{-# LANGUAGE StandaloneDeriving #-}
{-# LANGUAGE GADTs #-}

data D a where
  M :: D Bool
  F :: Int -> D Int

deriving instance Show (D a)

main :: IO ()
main =
  let a = M
      f = F 3
   in case a of
        M -> putStrLn "Moriarty"
        (F 3) -> putStrLn "F x 3"