用例:我正在ghcjs前端和后端中编写游戏,它们使用基本相同的状态,以便可以在两侧编码游戏规则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]
}
每个玩家由我们和苏联代表。每个玩家都有一手牌,从服务器角度看,它是无所不能的,并且知道两个玩家手里的每张牌。但是从美国玩家的角度来看,游戏状态看起来更像这样
data GameState = GameState {
-- public stuff
usHand :: [Card],
ussrHand :: Int
}
也就是说,他可以看到自己的手,但是只能看到对手拥有多少张牌。观察者看到的甚至更少。游戏规则很复杂,并且可能会发生很多事情,因此最好对规则进行一次编码,这样会影响玩家手部的规则,例如发新牌,强迫玩家出示新的规则。诸如此类的纸牌会影响每只手,这取决于他们是谁。为此,我最终使用类型族编写了以下内容,但这是行不通的
{-# 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行的错误均与
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
我在网站上看到了其他一些类似的答案,但是我不确定所解释的修补程序最终将如何导致我希望得到的答案,这是一种以类型检查和有用的方式编写serverToPlayerGS和serverToPlayerEvent的方法。
最佳答案
问题是您的类型族不是内射性的:例如知道USHand a
是Private
并不能告诉您确切的a
是什么:它可能是USSR
但也可能是Observer
,因为两者实例声明:
type USHand USSR = Private
type USHand Observer = Private
由于函数
toUS
的类型为Public -> USHand a
,因此必须以某种方式猜测a
,而我们发现这是不可能的。为了解决此问题,您将需要引入代理。代理是一种简单的数据类型,定义为:
data Proxy a = Proxy
如果您具有Haskell无法为其猜测
f :: F a
的函数a
,则可以将其转换为f :: Proxy a -> F a
,以便能够在调用站点上指定要编写的a
含义。如果希望f (Proxy :: Proxy Int)
为a
,则为实例Int
。您将需要作用域类型变量,因为将与
a
一起使用的toUs
来自函数的类型注释。因此,您应该在文件顶部添加以下两行:{-# LANGUAGE ScopedTypeVariables #-}
import Data.Proxy
然后将
toUS
的类型从Public -> USHand a
更改为:toUS :: Proxy a -> Public -> USHand a
不要忘记在
_
的所有实例声明中添加虚拟参数toUs
。最后,您可以像这样修补serverToPlayerGS
的定义: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
关于haskell - 类型族导致模棱两可的变量错误,我们在Stack Overflow上找到一个类似的问题:https://stackoverflow.com/questions/36268734/