example monad transformer stack

main.hs

-- | An example module.
{-# LANGUAGE GeneralizedNewtypeDeriving #-}
{-# LANGUAGE OverloadedStrings #-}

module Example (main) where

import Control.Monad.Reader(ReaderT, runReaderT)
import Control.Monad.Reader(MonadReader)
import Control.Monad.Reader(MonadIO, liftIO)
import Control.Monad.Reader(ask)
import Control.Lens

newtype User = User { userName :: String } deriving(Show)

newtype AppConfig = AppConfig {
  _appConfigUser:: User
  } deriving (Show)

class HasAppConfig a where
  getAppConfig :: Lens' a AppConfig
  getUser :: Lens' a User

  getUser = getAppConfig . getUser

instance HasAppConfig AppConfig where
  getAppConfig = id
  getUser = lens _appConfigUser (\config user -> config{_appConfigUser=user})

newtype App a = App
  {
    unApp:: (ReaderT AppConfig IO) a

  } deriving
  (
    Functor,
    Applicative,
    Monad,
    MonadReader AppConfig,
    MonadIO
  )

run :: App a -> AppConfig -> IO a
run a config = runReaderT (unApp a) config

getUserApp :: (
  MonadReader r m,
  HasAppConfig r,
  Monad m
  ) => m String
getUserApp = do
    env <- ask
    let user = env ^. getUser :: User
    return (userName user)

-- | An example function.
app :: App ()
app = do
  name <- getUserApp
  liftIO $ putStrLn name

main :: IO ()
main = do
  run app appConfig
  where
    appConfig = (AppConfig (User "huseyin")):: AppConfig