Question

我是Haskell的初学者，我遇到了一个无法解决的IO问题。

我正在进行Snake游戏，我唯一需要做的就是接受用户输入以使蛇移动。所以这是我的功能

--Get char
input :: IO Char
input =
    hSetEcho stdin False
    >> hSetBuffering stdin NoBuffering
    >> getChar

myFunction :: IO Char -> Int -> Int
myfunction myChar oldValue
     | (myChar == 'q') = 1
     | (myChar == 'w') = 2
     | (myChar == 'e') = 3
     | (myChar == 'r') = 4
     | otherwise = oldValue

-- At the beginning of the game, function is called like this
let dir = myFunction input 1 --To give the snake an initial direction

-- The game works with a function Turn that takes an Int as parameter 
-- which is the previous direction. let's call it oldDir
let newDir = myFunction input oldDir
-- Then I call Turn again with newDir as parameter

问题来自（myChar == char）部分，并且“无法将预期类型'IO Char'与实际类型'Char'匹配”

如何将IO Char与Char进行比较？

简答：我做不到。你能帮我解决更长的答案吗？这个问题可能被问了很多次，但我仍然坚持这个问题。

提前感谢您的回答！

亲切地，

Answer 1

你必须将比较提升到IO monad。例如：

myFunction :: Char -> Int -> IO Int
myFunction 'q' _ = return 1
myFunction 'w' _ = return 2
myFunction 'e' _ = return 3
myFunction 'r' _ = return 4
myFunction _ x = return x

-- Actual return type depends on what the function actually does
someOtherFunction :: Int -> IO ?
someOtherFunction oldDir = do
   dir <- input
   newDir <- myFunction dir oldDir
   ...

另一个（可能更好）选项是myFunction保持纯函数，如果需要将fmap应用于IO Char，请使用-- Note the change in the argument order myFunction :: Int -> Char -> Int myFunction _ 'q' = 1 ... myFunction x _ = x someOtherFunction oldDir :: Int -> IO ? newDir <- fmap (myFunction oldDir) input ...。

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Haskell - 将IO Char与char

1 个答案: