国家单子的传播 [英] Propagation of State Monad

问题描述

我具有以下功能，可以在游戏世界的图形"的边缘"周围走动.它改变了世界状况-尤其是玩家的位置.我需要报告一条消息，提醒玩家他们的位置也发生了变化.

I have the following function for walking around "edges" of the "graph" of my game world. It alters the state of the world--specifically, the player's location. I need to report a message alerting the player of their change in location as well.

所以我可以返回(message，newWorld)的元组，也可以使用State monad. (对吗?我是新手.)

So I could either return a tuple of (message, newWorld), or I could use a State monad. (Right? I'm new to this stuff.)

这是我尝试单子方法的尝试:

Here's my attempt at the monad approach:

walk dir = do
  world <- get
  let attempt = filter (\e -> edgeDirection e == dir) $ edges
      edges = (worldEdges world) M.! playerLoc
      playerLoc = playerLocation $ worldPlayer world
  case attempt of
    [] -> return "You can't go that way."
    (edge : _) -> do
      put world' 
      return message
        where world' = world { worldPlayer = player' }
              player' = (worldPlayer world) { playerLocation = loc }
              loc = edgeLocation edge
              message = "You go " ++ (downcase $ show dir)

问题是，现在调用 walk的函数有一个State monad要处理.我是在这里runState还是我也有使用State monads的功能，像这样:

The thing is, now the function that calls walk has a State monad to deal with. Do I runState it right there, or do I have that function use State monads too, like this:

parseWalk dir =
  let direction = case dir of
                  ('e' : _) -> Just East
                  ('n' : _) -> Just North
                  ('s' : _) -> Just South
                  ('w' : _) -> Just West
                  ('u' : _) -> Just Up
                  ('d' : _) -> Just Down
                  _         -> Nothing
  in case direction of
    Just d -> walk d
    Nothing -> return "You can't go that way"

那parseWalk的呼叫者又如何呢?我是否应该尝试让这个State monad尽可能长时间地通过?我应该在类型签名中看到很多MonadState ...吗?

Then what about parseWalk's caller, and so forth? Should I try to keep this State monad passed around as long as possible? Should I be seeing a lot of MonadState ... in my type signatures?

推荐答案

这是一种思考方式:runState需要初始状态作为其参数.您的代码中该初始状态在哪里可用?那就是状态monad需要传播的距离.

Here's one way of thinking about it: runState requires an initial state as its parameter. Where is this initial state available in your code? That's how far the state monad needs to propagate.

请注意，runState封装了有状态计算，因此，如果您有两个并排的runState计算，它们将不会看到彼此的状态(除非您将结果从一个传递到另一个).这也应该给您提示runState需要向上"的距离.

Note that runState encapsulates the stateful computation, so that if you have two runState computations side-by-side, they won't see each other's states (unless you pass the result from one to the other.) This also should give you a hint how far "up" the runState needs to be.

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