我可以在Haskell的警衛中使用警衛嗎

Question

我在嘗試在Haskell的警衛中使用警衛時遇到問題，我試圖找出是否甚至不可能，但我在互聯網上找不到任何答案，我希望你們可能知道答案。 這是我嘗試編譯代碼時遇到的錯誤： Assignment2.hs:134:75: parse error on input '|'

我的代碼：

    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

該代碼適用於井字游戲，該部分代碼旨在找出哪個玩家贏了； 類型Board是自定義的，輸入示例：hasWinner（（X，O，X），（X，O，O），（X，X，O））（輸出應為：Just P1）。

謝謝您的幫助！

Answer 1

您的程序中至少有2個嚴重的語法錯誤和1個類型錯誤。

語法錯誤：

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

這里右邊不見了。 一般形式是

    | guard = expression

沒有表情就無法逃脫。 編譯器等待“ =”，但找到“ |” 下一個后衛，所以它知道它的故障。

類型錯誤在這里：

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

您必須確定該函數是否應返回Bool或Maybe Player 。

Answer 2

這個怎么樣？

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！