{-# LANGUAGE FlexibleContexts #-}

module Command where

import Control.Monad.Writer

data Light = Light {

turnOn :: IO String

, turnOff :: IO String

}

simpleLamp = Light {

turnOn = putStrLn "The Light is on" >> return "on"

, turnOff = putStrLn "The Light is off" >> return "off"

}

flipUpCommand :: Light -> IO String

flipUpCommand = turnOn

flipDownCommand :: Light -> IO String

flipDownCommand = turnOff

storeAndExecute :: IO String -> WriterT[String] IO ()

storeAndExecute command = do

logEntry <- liftIO command

tell [logEntry]

commandDemo :: IO ()

commandDemo = do

putStrLn "Command -> higher order functions"

let lamp = simpleLamp

result <- execWriterT $

storeAndExecute (flipUpCommand lamp) >>

storeAndExecute (flipDownCommand lamp) >>

storeAndExecute (flipUpCommand lamp)

putStrLn $ "switch history: " ++ show result