haskell - 类型族导致不明确的变量错误

Question

用例:我正在 ghcjs 前端和后端编写一个游戏，它们使用基本相同的状态，以便我可以在两侧对游戏规则进行编码并与状态变化进行通信。为此，游戏状态看起来像

data GameState = GameState {
  gameTurn :: Int,            -- | Everyone sees
  gamePhase :: GamePhase,     -- | this
  boardState :: BoardState,   -- | stuff
  -- a lot more stuff everyone can see, followed by
  usHand :: [Card],
  ussrHand :: [Card]
}

每个玩家都由我们和苏联代表。每个玩家都有一手牌，从服务器的角度来看，它是无所不知的，并且知道两个玩家手中的每张牌。但从美国玩家的角度来看，游戏状态看起来更像是这样的

data GameState = GameState {
  -- public stuff
  usHand :: [Card],
  ussrHand :: Int
}

也就是说，他可以看到自己的手牌，但只能看到对手有多少张牌。观察者看到的甚至更少。游戏规则很复杂，可能会发生很多事情，因此最好对规则进行一次编码，这样影响玩家手牌的规则，例如发新牌、迫使玩家出示牌某种类型的牌等会以适当的方式影响每手牌，具体取决于他们是谁。为此，我最终使用类型族编写了以下内容，但这不起作用

{-# LANGUAGE  TypeFamilies, RankNTypes #-}
module Test where

data Card = Card
data BoardState = BoardState
data GamePhase = GamePhase
data Country
data Player = PUS | PUSSR

data US
data USSR
data Observer
data Server

data Private = Private Int
data Public = Public [Card]

class HandType a where
  type USHand   a :: *
  type USSRHand a :: *
  toUS :: Public -> USHand a
--   toUSSR :: Public -> USSRHand a -- TODO

instance HandType Server where
  type USHand Server = Public
  type USSRHand Server = Public
  toUS (Public cs) = Public cs

instance HandType US where
  type USHand US = Public
  type USSRHand US = Private
  toUS (Public cs) = Public cs

instance HandType USSR where
  type USHand USSR = Private
  type USSRHand USSR = Public
  toUS (Public cs) = Private (length cs)

instance HandType Observer where
  type USHand Observer = Private
  type USSRHand Observer = Private
  toUS (Public cs) = Private (length cs)

data GameState a = GameState {
  gameTurn :: Int,            -- | Everyone sees
  gamePhase :: GamePhase,     -- | this
  boardState :: BoardState,   -- | stuff

  usHand :: USHand a,
  ussrHand :: USSRHand a
}

data Event a =
    PlaceInfluence Player Int Country -- | Most plays don't affect
  | PlayCard Player Card              -- | either hand
  | DealCards (USHand a) (USSRHand a) -- | This one does

-- Works
obsEvents :: [Event US]
obsEvents = [PlayCard PUS Card, PlayCard PUSSR Card, DealCards (Public [Card]) (Private 3)]

-- Works
serverEvents :: [Event Server]
serverEvents = [PlayCard PUS Card, PlayCard PUSSR Card, DealCards (Public [Card, Card]) (Public [Card])]

-- The server must send out the gamestate modified for the player's consumption.
-- serverToPlayerGS :: GameState Server -> GameState a
serverToPlayerGS (GameState turn phase bs us ussr) =
  GameState turn phase bs (toUS us) undefined -- | <- Doesn't work (line 75)

-- serverToPlayerEvent :: Event Server -> Event a
serverToPlayerEvent (PlaceInfluence p amt c) = PlaceInfluence p amt c
serverToPlayerEvent (PlayCard p c) = PlayCard p c
serverToPlayerEvent (DealCards us ussr) =
  DealCards (toUS us) undefined -- | <- Doesn't work (line 78)

第 75 行和第 78 行的错误都类似于

Couldn't match expected type ‘USHand a’
            with actual type ‘USHand a0’
NB: ‘USHand’ is a type function, and may not be injective
The type variable ‘a0’ is ambiguous
Relevant bindings include
  serverToPlayerGS :: GameState Server -> GameState a
    (bound at src/Test4.hs:74:1)
In the fourth argument of ‘GameState’, namely ‘(toUS us)’
In the expression: GameState turn phase bs (toUS us) undefined

或者如果我省略类型声明

Could not deduce (USHand a0 ~ USHand a1)
from the context (HandType a1,
                  USHand a1 ~ USHand a,
                  USHand t ~ Public)
  bound by the inferred type for ‘serverToPlayerGS’:
             (HandType a1, USHand a1 ~ USHand a, USHand t ~ Public) =>
             GameState t -> GameState a
  at src/Test4.hs:(74,1)-(75,45)
NB: ‘USHand’ is a type function, and may not be injective
The type variable ‘a0’ is ambiguous
Expected type: USHand a
  Actual type: USHand a0
When checking that ‘serverToPlayerGS’ has the inferred type
  serverToPlayerGS :: forall t a a1.
                      (HandType a1, USHand a1 ~ USHand a, USHand t ~ Public) =>
                      GameState t -> GameState a
Probable cause: the inferred type is ambiguous

我在网站上看到了一些类似的其他答案，但我不确定解释的修复如何最终导致我希望的答案，这是一种以类型检查的方式编写 serverToPlayerGS 和 serverToPlayerEvent 的方法并且很有用。

Answer 1

问题是你的类型族不是单射的:知道 USHand a是 Private例如，并没有告诉你到底是什么 a是:可能是 USSR但也可能是Observer因为两个实例都声明:

type USHand USSR = Private
type USHand Observer = Private

因为函数 toUS类型为Public -> USHand a ，a必须以某种方式进行猜测，但我们刚刚发现这是不可能的。

为了解决这个问题，您需要引入代理。代理是一种简单的数据类型，定义为:

data Proxy a = Proxy

如果你有一个函数f :: F a Haskell 无法猜测 a ，你可以把它变成 f :: Proxy a -> F a为了能够在调用站点指定哪个a你的意思是写例如 f (Proxy :: Proxy Int)在您想要的情况下 a成为Int .

您需要作用域类型变量，因为 a您将与 toUs 一起使用将来自函数的类型注释。因此，您应该在文件顶部添加这两行:

{-# LANGUAGE ScopedTypeVariables #-}
import Data.Proxy

然后更改toUS的类型来自Public -> USHand a至:

toUS :: Proxy a -> Public -> USHand a

不要忘记添加虚拟参数 _到 toUs 的所有实例声明。最后，您可以修补 serverToPlayerGS 的定义像这样:

serverToPlayerGS :: forall a. HandType a => GameState Server -> GameState a
serverToPlayerGS (GameState turn phase bs us ussr) =
  GameState turn phase bs (toUS (Proxy :: Proxy a) us) undefined