Haskell：通用IORef，MVar？ [英] Haskell: generic IORef, MVar?

问题描述

我做了以下特定于IO monad的函数：

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

示例用法：

  main :: IO（）
 main = do 
p < -  memoIO $ putStrLnhello
p 
p 
  

打印 hello 一次。 / p>

我希望（一个宠物狗）能够尽可能多地工作（不仅仅是在IO中）。

我在hackage上发现了stateref，并且它的代码如下所示：

  { - ＃LANGUAGE FlexibleContexts，FlexibleInstances， MultiParamTypeClasses，Rank2Types，UndecidableInstances＃ - } 
 
 import Data.MRef 
 
 class（NewMRef rma，DefaultMRef rma，PutMRef rma，TakeMRef rma）=> MRef r m a 
实例（NewMRef r m a，DefaultMRef r m a，PutMRef r m a，TakeMRef r m a）=> MRef r m a 
 
 memo ::（MRef r m（也许a），Monad s）=> （全部×m× - > s×） - > s a  - > m（s a）
 memo liftFunc action = do 
 ref<  -  newDefaultMRef Nothing 
 return $ do 
 x<  - 也许action return =<< liftFunc（takeDefaultMRef ref）
 liftFunc。 putDefaultMRef ref $只是x 
返回x 
  

是否有替代stateref或更好的方法使用它比我做了什么？

解决方案

我已经改写了一个俗气的 MonadRef 在几个不同的场合为我自己使用，而有人可能在Hackage上有一个，但我找不到一个不受其他行李妨碍的人。

  class Monad m => MonadRef m其中
类型Ref m :: *  - > * 
 newRef :: a  - > Ref a a 
 writeRef :: Ref m a  - > - > m（）
 readRef :: Ref m a  - > ma 
 
实例MonadRef IO其中
类型Ref IO = IORef 
 newRef = newIORef 
 writeRef = writeIORef 
 readRef = writeIORef 
 
实例MonadRef STM其中
类型参考STM = TVar 
 ... 
 
 
实例MonadRef（ST s）其中
类型Ref（ST s） = STRef s 
 ... 
  

然后很容易将您的memoization例程尽管你可能想用 MVar 替换 IORef 。）

<编辑：澄清了verbage]

I made the following function which is specific for the 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

Example usage:

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

Prints "hello" once.

I would like (a pet peeve) to make it work for as many cases as possible (not just in IO).

I found stateref on hackage and with it my code looks like this:

{-# 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

Is there an alternative for stateref or a better way to use it than I did?

解决方案

I've rewritten a cheesy little MonadRef class on a few separate occasions for my own personal use and someone probably has one on Hackage, but I can't find one that is unencumbered with other baggage.

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
    ...

Then it is easy to abstract away your memoization routine (though you probably want to replace IORef in this context with an MVar.)

[Edit: clarified verbage]

这篇关于Haskell：通用IORef，MVar？的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！