As seguintes funções polimórficas podem

let id x = x;;
let compose f g x = f (g x);;
let rec fix f = f (fix f);;     (*laziness aside*)

ser escrito para tipos / tipos de construtores ou módulos / functores? eu tentei

type 'x id = Id of 'x;;
type 'f 'g 'x compose = Compose of ('f ('g 'x));;
type 'f fix = Fix of ('f (Fix 'f));;

para tipos, mas não funciona.

Aqui está uma versão Haskell para tipos:

data Id x = Id x
data Compose f g x = Compose (f (g x))
data Fix f = Fix (f (Fix f))

-- examples:
l = Compose [Just 'a'] :: Compose [] Maybe Char

type Natural = Fix Maybe   -- natural numbers are fixpoint of Maybe
n = Fix (Just (Fix (Just (Fix Nothing)))) :: Natural   -- n is 2

-- up to isomorphism composition of identity and f is f:
iso :: Compose Id f x -> f x
iso (Compose (Id a)) = a