Max learning various comonads

max-comonad.hs

module Main where

import Control.Comonad


data Stream a = a :> Stream a

instance Functor Stream where
  fmap f (a :> rest) = f a :> (fmap f rest) 

instance Comonad Stream where
  extract (a :> _)= a
  duplicate w@(_ :> rest) = w :> duplicate rest


data Store s a = Store (s -> a) s
instance Functor (Store s) where
  fmap g (Store f s) = Store (g . f) s

instance Comonad (Store s) where
  extract (Store f s) = f s
  duplicate (Store f s) = Store (\s' -> (Store f s')) s

experiment :: Functor f => (s -> f s) -> Store s a -> f a
experiment g (Store f s) = f <$> g s

maxstore :: Store String Int
maxstore = Store length "hello"


data Env e a = Env e a

instance Functor (Env e) where
  fmap f (Env e a) = Env e (f a)

instance Comonad (Env e) where
  extract (Env e a) = a
  duplicate w@(Env e a) = Env e w

ask :: (Env e a) -> e
ask (Env e a) = e

asks :: (e -> e') -> (Env e a) -> e'
asks f (Env e a) = f e

env :: e -> a -> Env e a
env = Env

data Settings = Settings {padAmount :: Int, padChar :: Char }

mySettings = Settings {padAmount = 6, padChar = '-'}

pad :: Env Settings String -> String
pad w = let pa = asks padAmount w
            pc = asks padChar w
            str = extract w
        in  ((replicate pa pc) :: String) ++ str

main :: IO ()
-- main = putStrLn $ show $ extract maxstore
main = putStrLn $ show $ pad (env mySettings "max")