readerstate.hs https://www.reddit.com/r/haskell/comments/7gl2wh/when_to_use_reader_vs_state/

readerstate.hs

module Main where

import Control.Monad.Reader
import Control.Monad.State

r1 :: IO ()
r1 = flip runReaderT 1 $ do
  ask >>= liftIO . print
  local (+1) $ ask >>= liftIO . print
  ask >>= liftIO . print

r2 :: (MonadIO m) => StateT Int m ((), Int)
r2 = flip runStateT 1 $ do
  get >>= liftIO . print
  modify (+1) >> (get >>= liftIO . print)
  get >>= liftIO . print

main :: IO ()
main = runStateT r2 0 >>= print