unfoldMの使い方

HyloM.hs

module Main where
import Control.Applicative
import Control.Exception
import Control.Monad
import Control.Monad.Loops
import System.Environment
import System.IO

main :: IO ()
main = do
  f:_ <- getArgs
  wcL f >>= print

wcL :: FilePath -> IO Int
wcL p = bracket (openFile p ReadMode)
                hClose
                (liftM length . hGetLines)

hGetLines :: Handle -> IO [String]
hGetLines h = unfoldM mb
  where mb = do
          eof <- hIsEOF h
          if eof then return Nothing
                 else Just <$> hGetLine h