simple example of Reader monad

Reading.hs

import qualified Data.Maybe as Maybe
import qualified Control.Monad.Reader as Reader


type Environment = [(String, String)]


get :: String -> Environment -> String
get k env = Maybe.fromJust $ lookup k env

asks :: String -> Reader.Reader Environment String
asks k = Reader.asks $ get k


name_string :: Reader.Reader Environment String
name_string = do
  name <- asks "name"
  return $ "Name: " ++ name

phone_string :: Reader.Reader Environment String
phone_string = do
  phone <- asks "phone"
  return $ "Phone: " ++ phone

business_card :: Reader.Reader Environment String
business_card = do
  name <- name_string
  phone <- phone_string
  return $ name ++ "\n" ++ phone


card :: Environment -> String
card env = Reader.runReader business_card env


main = putStrLn $ card [("name", "Harold"), ("phone", "303.880.9599")]