practice: Control.Monad.Reader

test_monadreader.hs

module Main where

import Control.Monad.Reader

{-
  (1) local 関数の例
      local :: (r -> r) -> m a -> m a
      r は Reader Monad が伝搬する読み取り値の型
-}

myName step = do
  name <- ask
  return $ step ++ ", I am " ++ name

localExample :: Reader String (String, String, String)
localExample = do
  a <- myName "First"
  b <- local (++ "dy") $ myName "Second"
  c <- myName "Third"
  return (a, b, c)

{-
  (2) ユーザ定義型の例
-}

data MyConf = MyConf {
      getString :: String
    , getInt    :: Int
  } deriving (Show)

myApp :: Reader MyConf (String, Int)
myApp = do
  conf <- ask
  return (getString conf, getInt conf)

{- 実行例
  *Main> runReader localExample "HOGE"
  ("First, I am HOGE","Second, I am HOGEdy","Third, I am HOGE")

  *Main> runReader myApp $ MyConf "HOGE" 777
  ("HOGE",777)
-}