reader sample code

ReaderSample.hs

import Control.Monad.Reader

data Config = Config { verbose :: Int, debug :: Bool }

configToLevel :: Config -> Int
configToLevel config
  | debug config = 10
  | otherwise = verbose config

outputLevel :: Reader Config [Int]
outputLevel = do
  config <- ask
  return [1..(configToLevel config)]

output :: Int -> String -> Reader Config (Maybe String)
output level str = do
  levels <- outputLevel
  return $ if elem level levels then Just str else Nothing


-- *Main> runReader (output 5 "hoge") (Config 1 True)
-- Just "hoge"
-- *Main> runReader (output 5 "hoge") (Config 1 False)
-- Nothing
-- *Main> runReader (output 5 "hoge") (Config 2 False)
-- Nothing
-- *Main> runReader (output 5 "hoge") (Config 6 False)
-- Just "hoge"
-- *Main> runReader (output 5 "hoge") (Config 5 False)
-- Just "hoge"