dynamic typing in haskell (from sandy maguire's thinking with types)

gistfile1.txt

data Dynamic where
  Dynamic :: Typeable t => t -> Dynamic

elimDynamic :: (forall a. Typeable a => a -> r) -> Dynamic -> r
elimDynamic f (Dynamic a) = f a

fromDynamic :: Typeable a => Dynamic -> Maybe a
fromDynamic = elimDynamic cast

liftD2 :: forall a b r. ( Typeable a , Typeable b , Typeable r )
  => Dynamic -> Dynamic -> (a -> b -> r) -> Maybe Dynamic
liftD2 d1 d2 f = fmap Dynamic . f <$> fromDynamic @a d1 <*> fromDynamic @b d2

pyPlus :: Dynamic -> Dynamic -> Dynamic pyPlus a b =
  fromMaybe (error "bad types for pyPlus") $ asum
  [ liftD2 @String @String a b (++)
  , liftD2 @Int @Int a b (+)
  , liftD2 @String @Int a b $ \strA intB -> strA ++ show intB
  , liftD2 @Int @String a b $ \intA strB -> show intA ++ strB
  ]