Haskell -比较IO Char和char

Question

我是Haskell的初学者，我陷入了一个无法解决的IO问题中。

我正在做一款蛇游戏，我唯一需要做的就是接受用户的输入来让蛇移动。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，“无法将预期类型‘myChar’与实际类型‘Char’匹配”。

如何比较IO Char和Char？

简短的回答:我做不到。你能帮我拿一下更长的答案吗？这个问题可能被问了很多次，但我仍然被困在这个问题上。

提前谢谢你的回答！

真诚地，

Answer 1

您必须将比较提升到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)选项是将myFunction保持为纯函数，如果需要将其应用于IO Char，则使用fmap。

-- 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
   ...