Mendler-style iteration in Haskell

Mendler.hs

{-# LANGUAGE FunctionalDependencies #-}
{-# LANGUAGE RankNTypes #-}
module Mendler where

class Iter b t | t -> b where
  iter :: (forall x . (x -> a) -> b x -> a) -> t -> a

data ListF a b = Nil | Cons a b

instance Iter (ListF a) [a] where
  iter alg = go
    where
    go []     = alg go Nil
    go (x:xs) = alg go (Cons x xs)

-- >>> iter (\ k x -> case x of { Nil -> [] ; Cons x xs -> x : k xs }) "abcd"
-- "abcd"
-- >>> iter (\ k x -> case x of { Nil -> [] ; Cons x xs -> k xs ++ [x] }) "abcd"
-- "dcba"