Haskell:通用的IORef,MVar？

Question

我做了以下函数,该函数特定于IO monad:

memoIO :: MonadIO m => m a -> IO (m a)
memoIO action = do
  ref <- newMVar Nothing
  return $ do
    x <- maybe action return =<< liftIO (takeMVar ref)
    liftIO . putMVar ref $ Just x
    return x

用法示例:

main :: IO ()
main = do
  p <- memoIO $ putStrLn "hello"
  p
  p

打印" hello"一次.

我想(一个小小的烦恼)让它尽可能多地工作(不仅仅是IO).

我在hackage上发现了stateref,我的代码看起来像这样:

{-# LANGUAGE FlexibleContexts, FlexibleInstances, MultiParamTypeClasses, Rank2Types, UndecidableInstances #-}

import Data.MRef

class (NewMRef r m a, DefaultMRef r m a, PutMRef r m a, TakeMRef r m a) => MRef r m a
instance (NewMRef r m a, DefaultMRef r m a, PutMRef r m a, TakeMRef r m a) => MRef r m a

memo :: (MRef r m (Maybe a), Monad s) => (forall x. m x -> s x) -> s a -> m (s a)
memo liftFunc action = do
  ref <- newDefaultMRef Nothing
  return $ do
    x <- maybe action return =<< liftFunc (takeDefaultMRef ref)
    liftFunc . putDefaultMRef ref $ Just x
    return x

有没有替代stateref或更好的方式来使用它比我呢？

Answer 1

我已经MonadRef在几个不同的场合改写了一个俗气的小班,供我个人使用,有人可能在Hackage上有一个,但我找不到一个不受其他行李影响的人.

class Monad m => MonadRef m where
    type Ref m :: * -> *
    newRef :: a -> Ref m a
    writeRef :: Ref m a -> -> m ()
    readRef :: Ref m a -> m a

instance MonadRef IO where
    type Ref IO = IORef
    newRef = newIORef
    writeRef = writeIORef
    readRef = writeIORef

instance MonadRef STM where
    type Ref STM = TVar
    ...


instance MonadRef (ST s) where
    type Ref (ST s) = STRef s
    ...

然后很容易抽象出你的memoization例程(虽然你可能想IORef在这个上下文中替换为MVar.)

[编辑:澄清的废物]