Reader monad example

ReaderMonad.hs

import Control.Monad.Reader

-- | Shared configuration for this application.
-- Rather trivial (and useless), it just configures how our application will
-- address the user 
--
data SalutationConfig = SalutationConfig { formal :: Bool }

-- | Returns a greeting
-- Takes in someone's name and returns a greeting string
--
greeter :: String -> Reader SalutationConfig String
greeter name = do
  -- grab the configuration out
  cfg <- ask
  -- grab the "formal" setting from the config
  let f = formal cfg
  
  -- send out the value
  return (makeSalutation f ++ name)

-- | Makes a salutation for a "formal" setting
makeSalutation :: Bool -> String
makeSalutation True = "Good day, "
makeSalutation False = "Wasaaaaaaaap, "

main :: IO ()
main = do
  -- create the configuration
  let cfg = SalutationConfig { formal = False}
  -- run the reader with the configuration for a guy named "Michael"
  let msg = runReader (greeter "Michael") $ cfg

  -- "Wasaaaaaaaaaap, Michael"
  putStrLn msg