我可以在Haskell的警卫中使用警卫吗

Question

我在尝试在Haskell的警卫中使用警卫时遇到问题，我试图找出是否甚至不可能，但我在互联网上找不到任何答案，我希望你们可能知道答案。 这是我尝试编译代码时遇到的错误： Assignment2.hs:134:75: parse error on input '|'

我的代码：

    verticals :: Board -> (Row, Row, Row)   
    verticals ((a,b,c),(d,e,f),(g,h,i)) = ((a,d,g),(b,e,h),(c,f,i))

    symbolToPlayer :: Field -> Player   
    symbolToPlayer X = P1  
    symbolToPlayer O = P2

    showTime :: Board -> Maybe Player  
    showTime ((a,b,c),(d,e,f),(g,h,i))
                        | a==b && a==c = Just (symbolToPlayer a)          
                        | d==e && d==f = Just (symbolToPlayer d)  
                        | g==h && g==i = Just (symbolToPlayer i)
                        | otherwise = False

    hasWinner :: Board -> Maybe Player  
    hasWinner b@((a,b,c),(d,e,f),(g,h,i))       
                                   | showTime b        -> spot of error
                                   | showTime (verticals b)
                                   | a==e && a==i = Just (symbolToPlayer a)
                                   | c==e && c==g = Just (symbolToPlayer c)
                                   | otherwise = Nothing

该代码适用于井字游戏，该部分代码旨在找出哪个玩家赢了； 类型Board是自定义的，输入示例：hasWinner（（X，O，X），（X，O，O），（X，X，O））（输出应为：Just P1）。

谢谢您的帮助！

Answer 1

您的程序中至少有2个严重的语法错误和1个类型错误。

语法错误：

    | showTime b        -> spot of error
    | showTime (verticals b)

这里右边不见了。 一般形式是

    | guard = expression

没有表情就无法逃脱。 编译器等待“ =”，但找到“ |” 下一个后卫，所以它知道它的故障。

类型错误在这里：

    | g==h && g==i = Just (symbolToPlayer i)
    | otherwise = False

您必须确定该函数是否应返回Bool或Maybe Player 。

Answer 2

这个怎么样？

import Control.Monad (msum)
import Control.Applicative ((<*>), pure)

data Player = P1 | P2 | None deriving (Eq, Show)

data Field = X | O | B deriving (Eq, Show)

type Board = ((Field, Field, Field)
             ,(Field, Field, Field)
             ,(Field, Field, Field))

symbolToPlayer :: Field -> Player
symbolToPlayer X = P1
symbolToPlayer O = P2
symbolToPlayer B = None

checkThree :: (Field,Field,Field) -> Maybe Player
checkThree (a,b,c)
    | a == b && a == c = Just $ symbolToPlayer a
    | otherwise        = Nothing

winHorizontal :: Board -> Maybe Player
winHorizontal (r1, r2, r3) = msum $ map checkThree [r1, r2, r3]

winVertical :: Board -> Maybe Player
winVertical ((a,b,c), (d,e,f), (g,h,i)) =
    msum $ map checkThree [(a,d,g), (b,e,h), (c,f,i)]

winDiagonal :: Board -> Maybe Player
winDiagonal ((a,_,c), (_,e,_), (g,_,i)) =
    msum $ map checkThree [(a,e,i), (c,e,g)]

hasWinner :: Board -> Maybe Player
hasWinner b = msum $ [winHorizontal, winVertical, winHorizontal] <*> pure b

Monad和Applicative是您的朋友！