type Cofree f a = Cofree a (f (Cofree f a))

type Functor f = Functor (forall a b. (a ->{} b) -> f a ->{} f b)

use Functor Functor
fmap : Functor f -> (a -> b) -> f a -> f b
fmap fn f = match fn with
  Functor map -> map f

use Cofree Cofree

namespace Cofree where

  extract : Cofree f a -> a
  extract = cases
    Cofree a _ -> a

  duplicate : Functor f -> Cofree f a -> Cofree f (Cofree f a)
  duplicate f c = match c with
    Cofree a p -> Cofree c (fmap f (duplicate f) p)