「GenerarizedNewTypeDerivingで自分用Monadを作る」ってなに！？（具体例編） ref: http://qiita.com/aiya000/items/f6e7103c99066707f3cf

file0.txt

{-# LANGUAGE GeneralizedNewtypeDeriving #-}
{-# LANGUAGE OverloadedStrings #-}

import Data.String (IsString)
import Data.Text (Text)
import qualified Data.Text.IO as TIO

newtype MyString = MyString { unMyString :: Text }
  deriving (IsString)  -- GenerarizedNewTypeDerivingがないと、ここを書けない

txt :: MyString
txt = "アホ毛"

main :: IO ()
main = TIO.putStrLn . unMyString $ txt

file1.txt

-- | Current state of ElinParser
data ElinState = ElinState
  { _parseLogs      :: [ParseLog]
  , _parseNestLevel :: Int
  } deriving (Show)

-- | Parser with parsing logs
newtype ElinParser a = ElinParser { _runElinParser :: ParsecT Dec Text (State ElinState) a }
  deriving ( Functor, Applicative, Monad
           , Alternative, MonadPlus
           , MonadState ElinState, MonadParsec Dec Text
           )

-- | A log of ElinParser
data ParseLog = Message Text     -- ^ Simple message
              | ParsedItem Text  -- ^ Ex: '(', ''', ')' or some symbol
  deriving (Show)

-- | Run parser and extract result and logs
runElinParser :: ElinParser a -> Text -> (Either (ParseError Token Dec) a, [ParseLog])
runElinParser parser source =
  let initialState = ElinState [] 0
      bareness     = _runElinParser parser
      (result, ElinState logs _) = flip runState initialState $ runParserT bareness "" source
  in (result, reverse logs)