do 块中的 if 语句给出错误消息

Question

我正在尝试制作一个非常简单的蛇类游戏，如果您尝试前往您已经访问过的 x,y 坐标，您将输掉游戏。

这是目前有效的代码（您可以使用箭头键移动玩家 1，使用 wasd 移动玩家 2）：

import UI.NCurses

main :: IO ()    
main = runCurses $ do
    w <- defaultWindow
    updateWindow w $ do
        drawBorder Nothing Nothing Nothing Nothing Nothing Nothing Nothing Nothing
    render
    loop w [] 1 1 0 0 10 10 0 0

loop :: Window -> [(Integer, Integer)] -> Integer -> Integer -> Integer -> Integer -> Integer -> Integer -> Integer -> Integer ->  Curses ()
loop w list p1x p1y p1oldDx p1oldDy p2x p2y p2oldDx p2oldDy = do
    e <- getEvent w (Just 100)

    let coordList = updateXY e p1oldDx p1oldDy p2oldDx p2oldDy
    let p1dX = coordList !! 0
    let p1dY = coordList !! 1
    let p2dX = coordList !! 2
    let p2dY = coordList !! 3

    updateWindow w $ do
        moveCursor (p1y+p1dY) (p1x+p1dX)
        drawString ("#")
        moveCursor (p2y+p2dY) (p2x+p2dX)
        drawString ("#")
    render

    let updatedList = list ++ [(p1y+p1dY, p1x+p1dX)] ++ [(p2y+p2dY, p2x+p2dX)]

    loop w updatedList (p1x+p1dX) (p1y+p1dY) p1dX p1dY (p2x+p2dX) (p2y+p2dY) p2dX p2dY

updateXY :: Maybe Event -> Integer -> Integer  -> Integer -> Integer -> [Integer]
updateXY e p1oldX p1oldY p2oldX p2oldY
    | e == Just (EventSpecialKey KeyLeftArrow) = [-1, 0, p2oldX, p2oldY]
    | e == Just (EventSpecialKey KeyRightArrow) = [1, 0, p2oldX, p2oldY]
    | e == Just (EventSpecialKey KeyDownArrow) = [0, 1, p2oldX, p2oldY]
    | e == Just (EventSpecialKey KeyUpArrow) = [0, -1, p2oldX, p2oldY]
    | e == Just (EventCharacter 'a') = [p1oldX, p1oldY, -1, 0]
    | e == Just (EventCharacter 'd') = [p1oldX, p1oldY, 1, 0]
    | e == Just (EventCharacter 's') = [p1oldX, p1oldY, 0, 1]
    | e == Just (EventCharacter 'w') = [p1oldX, p1oldY, 0, -1]
    | p1oldX /= 0 = [p1oldX, 0, p2oldX, p2oldY]
    | p1oldY /= 0 = [0, p1oldY, p2oldX, p2oldY]
    | p2oldX /= 0 = [p1oldX, p1oldY, p2oldX, 0]
    | p2oldY /= 0 = [p1oldX, p1oldY, 0, p2oldY]
    | otherwise = [1, 0, 1, 0] -- Starts moving in x-direction. Change to (0,0) for stand-still

因此，当您移动时，“#”字符会被放下。如果你转到一个已经有“#”的坐标，上面的代码什么都不做，所以我尝试通过在 let updatedList...:

之前添加它来改变循环函数

if length (filter (==(p1x, p1y)) list) > 0 then gameOver w "player one lost"
if length (filter (==(p2x, p2y)) list) > 0 then gameOver w "player two lost"

并添加临时 gameOver 功能：

gameOver w player = 
    updateWindow w $ do
        moveCursor (10) (10)
        drawString (player)
    render

但是当我尝试在 GHCI 中加载此文件时，我收到以下错误消息：输入解析错误 'if'

Answer 1

在 Haskell 中，if 表达式必须同时包含 then 和 else 子句。

没有 else，您会收到错误消息。

结果子句和替代子句必须具有相同的类型。

Answer 2

ifs 需要 elses。时期。当您需要命令式 "if this, then do that, else do nothing," 但没有 built-in 语法时，您可以使用 when 函数：

import Control.Monad

when (length (filter (==(p1x, p1y)) list) > 0) $ gameOver w "player one lost"
when (length (filter (==(p2x, p2y)) list) > 0) $ gameOver w "player two lost"

when的定义是

when cond action = if cond then action else pure ()

do 块中的 if 语句给出错误消息

if statement in do block gives error message

haskell

if-statement

ncurses

do-notation

io-monad