MonadTransformer.hs

{-# LANGUAGE GeneralizedNewtypeDeriving #-}
{-# LANGUAGE OverloadedStrings #-}

module Main where
    
import Control.Monad.Reader

data AppContext = AppContext { a :: String, b :: Int }

newtype AppT a = AppT { runAppT :: ReaderT AppContext IO a }
  deriving (Functor, Applicative, Monad, MonadIO, MonadReader AppContext)

-- this function simply runs a side effect
-- in this case printing a value to the console
f :: AppT ()
f = do
  ctx <- ask
  liftIO $ print $ a ctx

g :: Int -> AppT Int
g v = do
  _ <- f -- call f for the side effect
  ctx <- ask
  return (b ctx + v)

main :: IO ()
main = do
  let ctx = AppContext "string" 10
  flip runReaderT ctx . runAppT$ do
    a <- f
    liftIO $ print a
    b <- g 10
    liftIO $ print b