comonad_stream.hs

import Prelude hiding (head, tail)
import Control.Comonad

data Stream a = St {head :: a, tail :: Stream a}

instance Functor Stream where
    fmap f s = St (f (head s)) (fmap f (tail s))

instance Comonad Stream where
    extract = head
    extend f s = St (f s) (extend f (tail s))