HaskellでGo言語のdefer文してみる with Freeモナド ref: http://qiita.com/alucky0707/items/156b2b013e16f27cb8d0

defer.hs

{-# LANGUAGE DeriveFunctor, LambdaCase #-}
import Control.Monad
import Control.Monad.Trans
import Control.Monad.Trans.Free

import System.IO

type Defer m = FreeT (DeferF m) m

data DeferF m cont
 = Defer (Defer m ()) cont
 | Scope (Defer m ()) cont deriving (Functor)

defer :: Monad m => Defer m a -> Defer m ()
defer d = liftF $ Defer (d >> return ()) ()

scope :: Monad m => Defer m a -> Defer m ()
scope d = liftF $ Scope (d >> return ()) ()

runDefer :: Monad m => Defer m a -> m a
runDefer = runDefer' []
  where
  runDefer' :: Monad m => [Defer m ()] -> Defer m a -> m a
  runDefer' xs = runFreeT >=> \case
    Free (Defer x next) -> runDefer' (x:xs) next
    Free (Scope x next) -> runDefer' [] x >> runDefer' xs next
    Pure x -> (unless (null xs) $ runDefer' [] $ mapM_ id xs) >> return x

-- ここからは試し

test :: Defer IO ()
test = do
  h <- liftIO $ openFile "hello.txt" ReadMode
  defer $ hClose h
  liftIO $ hGetLine h >>= putStrLn

main = runDefer test

file0.go

file, err := os.Open(filename)
if err != nil {
    return err
}
//ここは関数を抜けるときに呼ばれる
defer file.Close()

file1.go

hello :: Defer IO ()
hello = do
  h <- liftIO $ openFile "hello.txt" ReadMode
  defer $ hClose h
  liftIO $ hGetLine h >>= putStrLn