combination of Reader and Maybe monads, using the ReaderT monad transformer

ReadingT.hs

import Control.Monad.Reader
import Control.Monad.Trans
import Data.List
import Data.Maybe
import Text.Printf

type Environment = [(String, String)]


get :: String -> ReaderT Environment Maybe String
get k = ask >>= lift . lookup k

-- here's the do-notation equivalent:
get' :: String -> ReaderT Environment Maybe String
get' k = do
  env <- ask
  lift $ lookup k env


format :: String -> String -> ReaderT Environment Maybe String
format fmt k = get k >>= return . printf fmt

business_card_lines :: Reader Environment [String]
business_card_lines = do
  let lines = [format "Name: %s" "name",
               format "Email: %s" "email",
               format "Phone: %s" "phone"]
  env <- ask
  return $ catMaybes $ map (\r -> runReaderT r env) lines

business_card :: Reader Environment String
business_card = business_card_lines >>= return . intercalate "\n"

main = putStrLn $ runReader business_card [("name", "Harold"), ("email", "hrldcpr@gmail.com")]