在 haskell 中使用新类型隐藏嵌套状态转换器

Question

我不确定我想要完成的事情是否理智（请保持友善）。但是我有一个小游戏的想法，游戏需要有一些状态并且状态用一些随机组件更新（否则它会有点无聊）。看到 StdGen 也是某种状态，我开始像这样对我的程序建模

import Control.Monad.State
import qualified System.Random as R
type Rng = StateT StdGen IO

random :: (Random a) => Rng a
random = state R.random

randoms :: (Random a) => Int -> Rng [a]
randoms n = replicateM n random

type GameS = [Int]-- not important right now

type Game = StateT GameS Rng

mainGame :: Game ()
mainGame = do
    s <- gets
    n <- lift $ randoms 10 :: (Rng [Int]) 
    put s ++ n

mainRng :: Rng ()
mainRng = do
    liftIO $ "Inside Rng!"
    -- here do stuff that has to do with game setup and so on dependent on rng
    runStateT mainGame [1,2,3,4]


main :: IO ()
main = do
    g <- R.newStdGen
    runStateT mainRng g

好的，成功了！因此，让我们尝试将一些细节隐藏在新类型的背后。

-- change type aliases for Game and Rng to newtype
newtype Rng a {
   runR :: StateT R.StdGen IO a
} deriving (Applicative, Functor, Monad, MonadIO, MonadState R.StdGen)

newtype Game a {
   runG :: StateT GameS Rng a
} deriving (Applicative, Functor, Monad, MonadIO, MonadState GameS)

-- and create a expose newRun functions
runRng :: Rng a -> IO (a, R.StdGen)
runRng k = do
    g <- R.newStdGen
    runStateT (runR k) g

runGame :: Game a -> Rng (a, GameS)
runGame k = let initial = [1,2,3]
             in runStateT (runG k) initial

-- mainGame as before
mainGame :: Game ()
mainGame = do
   liftIO $ print "Inside game"
   s <- gets
   n <- lift $ randoms 10 :: (Rng [Int])
   put s ++ n

main :: IO ()
main = do
    final <- runRng $ do
        liftIO $ print "Inside rng moand"
        runGame mainGame
    print $ show final

这在一定程度上是有效的。在 mainGame 中，我可以执行 liftIO 和所有状态操作，除非我尝试 lift 获取一些随机数时出现错误 couldn't match type 't0 Rng' with 'Game'

我是否需要为我的 Game 和 Rng 类型实现 MonadTrans？

任何帮助都会很棒！

Answer 1

我想我明白了。如果我像这样更改 mainGame：

mainGame :: Game ()
mainGame = do
   liftIO $ print "Inside game"
   s <- gets
   n <- Game . lift $ randoms 10 :: (Rng [Int])
   put s ++ n

它按预期工作。看看 MonadTrans lift 的定义，这似乎就是我所缺少的。所以制作一个 MonadTrans 的 Game 实例可以解决我的问题。

因此对 Game 类型进行以下更改：

newtype GameT m a {
    runG :: StateT GameS m a
} deriving (Applicative, Functor, Monad, MonadIO, MonadState, MonadTrans GameS)
type Game = GameT Rng

会让我做我想做的事。我仍然不太清楚为什么我必须用签名 GameT m a 制作 GameT 才能制作 MonadTrans 的实例，在这里与类型斗争，也许有人可以 chimn

在 haskell 中使用新类型隐藏嵌套状态转换器

Hiding nested state transformers with newtype in haskell

haskell

state-monad

monad-transformers

newtype