Monad Transformers

{-# LANGUAGE GeneralizedNewtypeDeriving #-}

module Main where

import          Counter
import          Weather
import          MockWeather
import          MockCounter
import          Control.Monad.Trans.Class

-- 'someApp' works for any underlying monad 'm' 
-- and any instance of 'MonadCounter'
-- and any instance of 'MonadWeather'
someApp :: (Monad m, MonadCounter m , MonadWeather m) => m String
someApp = do
  _ <- increment
  (WeatherData weather) <- byCity "Amsterdam"
  return weather


newtype MyAppM m a = MyAppM { unMyAppM :: MockCounter (MockWeather m) a }
  deriving (Functor, Applicative, Monad, MonadCounter, MonadWeather)

instance MonadTrans MyAppM where
  lift = MyAppM . lift . lift

runMyAppM :: Int -> MyAppM m a -> m (a, Int)
runMyAppM i = runMockWeather . (`runMockCounter` i) . unMyAppM

-- set the underlying monad to 'IO'
-- and 'MonadCounter' instance to 'MockCounter'
-- and 'MonadWeather' instance to 'MockWeather'
main :: IO ()
main = runMyAppM 12 (someApp >> someApp) >>= print