typeable10.hs

-- no equality
cast :: (Typeable a, Typeable b) => a -> Maybe b
cast x = r
  where
    r = if typeOf x == typeOf (fromJust r)
        then Just $ unsafeCoerce x
        else Nothing

-- with equality
eqT :: forall a b. (Typeable a, Typeable b) => Maybe (a :~: b)
eqT = if typeRep (Proxy :: Proxy a) == typeRep (Proxy :: Proxy b)
      then Just $ unsafeCoerce Refl
      else Nothing
 
cast :: forall a b. (Typeable a, Typeable b) => a -> Maybe b
cast x
  | Just Refl <- ta `eqT` tb = Just x
  | otherwise                 = Nothing
  where
    ta = typeRep :: TypeRep a
    tb = typeRep :: TypeRep b