Question

我遇到了一群我想尝试合并的单子。

我使用monad-coroutine，状态和镜头（因为我有深度嵌套状态）。

我有一个初步的方法，那里有一个有效的解决方案。这里的要点是我可以请求在Coroutine之外执行IO任务。

{-# LANGUAGE ExistentialQuantification #-}
{-# LANGUAGE FlexibleInstances         #-}
{-# LANGUAGE MultiParamTypeClasses     #-}
{-# LANGUAGE UndecidableInstances      #-}

module Main where

import           Control.Monad.Coroutine      (Coroutine(..), suspend, resume)
import           Control.Monad.State          (State, MonadState, MonadIO)
import           Control.Monad.State          (lift, get, put, liftIO, runState)
import           System.Environment           (getArgs)

type MyType = Coroutine IORequest (State MyState)

instance (MonadState s m) => MonadState s (Coroutine IORequest m) where
    get = lift get
    put = lift . put

io :: MonadIO m => IO a -> m a
io = liftIO

data IORequest x = forall a. RunIO (IO a) (a -> x)

instance Functor IORequest where
    fmap f (RunIO x g) = RunIO x (f . g)

data MyState = MyState { _someInt :: Int }

initialState :: MyState
initialState = MyState 1

request :: Monad m => IO a -> Coroutine IORequest m a
request x = suspend (RunIO x return)

myLogic :: MyType [String]
myLogic = do
    args <- request (io getArgs)
    request (io (print args))
    -- do a lot of useful stuff here
    return args

runMyType :: MyType [String] -> MyState -> IO ()
runMyType logic state = do
    let (req, state') = runState (resume logic) state
    case req of
        Left (RunIO cmd q') -> do
            result <- cmd
            runMyType (q' result) state'
        Right _ -> return ()

main :: IO ()
main = runMyType myLogic initialState

现在，在某个时刻，一个简单的国家变得不够，我需要ST。我开始尝试将ST置于StateT内，但出于某种原因无法想出如何在协程之外正确处理IO。当runMyType发生变化时，有没有办法提出类似的Coroutine？

type MyType s = Coroutine IORequest (StateT (MyState s) (ST s))

initialState :: ST s (MyState s)
initialState = do
    a <- newSTRef 0
    return (MyState a)

无论我试图想出什么都会引发一些关于s转义或Couldn't match type ‘s’ with ‘s2’的错误等等......也许其他一些monad堆叠顺序会有所帮助？或者它是否可能？

还有一个问题，如果你有时间：上面的MyType s和这个之间有什么区别：

type MyType = forall s. Coroutine IORequest (StateT (MyState s) (ST s))

与StateT，ST和IO协同作用

0 个答案: