Question

我不确定我想要实现的是理智与否（请你好）。但是我对一个小游戏有一个想法，游戏需要有一些状态，状态会更新一些随机组件（否则会有点无聊）。看到StdGen也是某种状态我开始像这样建模我的程序

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，有效！因此，让我们尝试隐藏一些新类型背后的细节。

-- 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

这很有用。在mainGame内部，我可以执行liftIO和所有状态操作，除非我尝试lift获取一些随机数字时出现错误couldn't match type 't0 Rng' with 'Game'

我是否需要为我的MonadTrans和Game类型实施Rng？

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电梯的定义，这似乎就是我所缺少的。因此，制作MonadTrans的Game实例可以解决我的问题。

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

让我做我想做的事。我仍然不确定为什么我必须使用签名GameT制作GameT m a能够制作MonadTrans的实例，并在此处使用类型，也许某些类型可以在那上面。

在haskell

1 个答案: