第7回 スタートHaskell2演習問題「Readerモナドを使ってみよう」解答 http://wiki.haskell.jp/Workshop/StartHaskell2/exercise14

reader.hs

import Data.List
import System.Environment
import Data.Maybe
import Control.Monad.Reader

type Env = [(String, String)]

environment :: Env
environment = 
  [ ("HOME", "/home/haskell")
  , ("PATH", "/bin:/usr/bin")
  , ("LANG", "ja_JP.UTF-8")
  ]

lookupEnv :: String -> Reader Env String
lookupEnv key = do
  env <- ask
  return $ fromMaybe "" (lookup key env)

test = runReader (lookupEnv "LANG") environment

getConfPath :: Reader Env String
getConfPath = do
    env <- ask
    return $ fromMaybe "/etc" $ lookup "HOME" env

-- せっかくだから実際の環境変数を使ってみる
main :: IO ()
main = do
    env <- getEnvironment
    putStrLn $ runReader getConfPath env
    putStrLn $ runReader (lookupEnv "SHELL") env