[Either Int String, Maybe Int, MyType]
[Either Int String, MyType, Maybe Int]
...

type family (xs :: [a]) ++ (ys :: [a]) :: [a] where
  '[] ++ ys = ys
  (x : xs) ++ ys = x : (xs ++ ys)

type family Delete (x :: a) (xs :: [a]) :: [a] where
  Delete _ '[] = '[]
  Delete x (x : xs) = xs
  Delete x (y : xs) = y : Delete x xs

type family MapCons (x :: a) (xs :: [[a]]) :: [[a]] where
  MapCons _ '[] = '[]
  MapCons x (xs : xss) = (x : xs) : MapCons x xss

type family Permutations_ (as :: [a]) (xs :: [a]) :: [[a]] where
  Permutations_ _ '[] = '[]
  Permutations_ as (x : xs) = MapCons x (Permutations (Delete x as)) ++ Permutations_ as xs

type family Permutations (xs :: [a]) :: [[a]] where
  Permutations '[] = '[ '[]]
  Permutations xs = Permutations_ xs xs