Showing GADTs

show-gadts.hs

data Parser a where
  Val :: Show a => a -> Parser a
  Cat :: Parser a -> Parser b -> Parser (a, b)

instance Show (Parser a) where
  showsPrec p x =
    showParen (p > 10) $
      case x of
        Val n ->
          showString "Val " .
          showsPrec 11 n
        Cat a b ->
          showString "Cat " .
          showsPrec 11 a .
          showChar ' ' .
          showsPrec 11 b


data Parser2 a where
  Val2 :: Int -> Parser2 a
  Cat2 :: Parser2 a -> Parser2 b -> Parser2 (a, b)

instance Show (Parser2 a) where
  showsPrec p x =
    showParen (p > 10) $
      case x of
        Val2 n ->
          showString "Val2 " .
          showsPrec 11 n
        Cat2 a b ->
          showString "Cat2 " .
          showsPrec 11 a .
          showChar ' ' .
          showsPrec 11 b