Simple example of the Reader Monad (in Haskell)

ReaderMonad.hs

module ReaderMonad where

import Control.Monad.Reader

stuff :: Reader Int String
stuff = do
  s <- ask
  return (show s ++ " green bottles")
    
main :: IO ()
main = print $ runReader stuff 99

-- what's the point of Reader, since we could just pass
-- the parameters to the stuff function?

-- Reader is used instead of global state, for "constants" etc
-- to avoid polluting every single function with params (which it might only pass
-- on to other functions, and not even use itself)

-- You still have to modify all these functions to use Reader, though

-- see http://lambdaman.blogspot.co.uk/2007/10/monadreader.html
-- See http://stackoverflow.com/questions/14178889/reader-monad-purpose

-- Can use 'asks' as well as 'ask' to avoid all the do-block boilerplate
-- and may create an alias for the reader if it's used in lots of places

type IntRead = Reader Int

stuff2 :: IntRead String
stuff2 = asks show

-- stuff2 = do asks $ \s -> (show s ++ " green bottles")