oh dear

polytypes.hs

{-# LANGUAGE DataKinds #-}

data ErrorCatA = WTFError String | OMGError String
data ErrorCatB = BadError Int | TerribleError Float

data GenericError = CatA ErrorCatA | CatB ErrorCatB
class GError a where
    wrap :: a -> GenericError

instance GError ErrorCatA where
    wrap = CatA

instance GError ErrorCatB where
    wrap = CatB

handleError :: GenericError -> IO ()
handleError err =
    putStrLn $ case err of
        CatA (WTFError s) -> "WTF?! " ++ s
        CatA (OMGError s) -> "OMG?! " ++ s
        CatB (BadError i) -> "Bad! " ++ show i
        CatB (TerribleError f) -> "Terrible! " ++ show f

main :: IO ()
main = do
    let err0 = CatA $ WTFError "Sky's falling."
    let err1 = CatB $ TerribleError 1.337

    handleError err0
    handleError err1

    handleError $ wrap $ WTFError "Yo"
    handleError $ wrap $ OMGError "this can't be right ..."