monad stacking example

main.hs

{-# LANGUAGE FlexibleContexts #-}
{-# LANGUAGE FlexibleInstances #-}

module Main where

import Control.Monad.Except
import Control.Monad.Reader
import Control.Monad.State
import Control.Monad.Writer

type Response = String

stack :: (MonadState Int m, MonadWriter [String] m, MonadError String m) => m Response
stack = do
  r <- get
  tell ["Hello!"]
  put (r + 1)
  r <- get
  tell ["Value: " ++ show r]
--  throwError "Error!"
  return ("this is the response" :: Response)

type A = (ExceptT String (WriterT [String] (State Int))) String
runStackWithA = runState (runWriterT (runExceptT (stack :: A)))

type B = (StateT Int (WriterT [String] (Either String))) String
runStackWithB x = runWriterT (runStateT (stack :: B) x)

type C = (WriterT [String] (StateT Int (Either String))) String
runStackWithC = runStateT (runWriterT (stack :: C))

main :: IO ()
main = do
  print ("A => " ++ show (runStackWithA 0))
  print ("B => " ++ show (runStackWithB 1))
  print ("C => " ++ show (runStackWithC 2))