如何将 IO 字符串与字符串进行比较，如果为 false，则打印消息并返回值？

问：

我不知道以前是否有人问过这个问题，但我没有从谷歌搜索中得到任何东西。 我想在 Haskell 中拥有以下代码的等效项：

def inputThing(string: str) -> bool:
  a=input("Input: ")
  if a=="Hello":
    return True
  else:
    print("Not hello")
    return False

我想出的是这样的：

import System.IO ( hFlush, stdout )
inputThing :: String -> IO Bool
inputThing string = do
  putStr "Input:"
  hFlush stdout
  input <- getLine
  if string == input then True else putStrLn "Not hello" >> False

我简化的最后一行（或者更确切地说是 linter 建议的）

(string == input) || (putStrLn "Not hello" >> False)

但是我收到各种错误，这些错误似乎都与IO类型与否有关：

thing.hs:7:3: error:
    • Couldn't match expected type ‘IO Bool’ with actual type ‘Bool’
    • In a stmt of a 'do' block:
        (string == input) || (putStrLn "Not hello" >> False)
      In the expression:
        do putStr "Input:"
           hFlush stdout
           input <- getLine
           (string == input) || (putStrLn "Not hello" >> False)
      In an equation for ‘inputThing’:
          inputThing string
            = do putStr "Input:"
                 hFlush stdout
                 input <- getLine
                 ....
  |
7 |   (string == input) || (putStrLn "Not hello" >> False)
  |   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

thing.hs:7:25: error:
    • Couldn't match expected type ‘Bool’ with actual type ‘IO b0’
    • In the second argument of ‘(||)’, namely
        ‘(putStrLn "Not hello" >> False)’
      In a stmt of a 'do' block:
        (string == input) || (putStrLn "Not hello" >> False)
      In the expression:
        do putStr "Input:"
           hFlush stdout
           input <- getLine
           (string == input) || (putStrLn "Not hello" >> False)
  |
7 |   (string == input) || (putStrLn "Not hello" >> False)
  |                         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

thing.hs:7:49: error:
    • Couldn't match expected type ‘IO b0’ with actual type ‘Bool’
    • In the second argument of ‘(>>)’, namely ‘False’
      In the second argument of ‘(||)’, namely
        ‘(putStrLn "Not hello" >> False)’
      In a stmt of a 'do' block:
        (string == input) || (putStrLn "Not hello" >> False)
  |
7 |   (string == input) || (putStrLn "Not hello" >> False)
  |                                                 ^^^^^

我能想到的最好的是：

(fmap ((==) string) input) || (putStrLn "Not hello" >> return False)

这仍然不起作用：

thing.hs:7:3: error:
    • Couldn't match expected type ‘IO Bool’ with actual type ‘Bool’
    • In a stmt of a 'do' block:
        (fmap ((==) string) input)
          || (putStrLn "Not hello" >> return False)
      In the expression:
        do putStr "Input:"
           hFlush stdout
           input <- getLine
           (fmap ((==) string) input)
             || (putStrLn "Not hello" >> return False)
      In an equation for ‘inputThing’:
          inputThing string
            = do putStr "Input:"
                 hFlush stdout
                 input <- getLine
                 ....
  |
7 |   (fmap ((==) string) input) || (putStrLn "Not hello" >> return False)
  |   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

thing.hs:7:4: error:
    • Couldn't match expected type ‘Bool’ with actual type ‘[Bool]’
    • In the first argument of ‘(||)’, namely
        ‘(fmap ((==) string) input)’
      In a stmt of a 'do' block:
        (fmap ((==) string) input)
          || (putStrLn "Not hello" >> return False)
      In the expression:
        do putStr "Input:"
           hFlush stdout
           input <- getLine
           (fmap ((==) string) input)
             || (putStrLn "Not hello" >> return False)
  |
7 |   (fmap ((==) string) input) || (putStrLn "Not hello" >> return False)
  |    ^^^^^^^^^^^^^^^^^^^^^^^^

thing.hs:7:23: error:
    • Couldn't match type ‘Char’ with ‘[Char]’
      Expected: [String]
        Actual: String
    • In the second argument of ‘fmap’, namely ‘input’
      In the first argument of ‘(||)’, namely
        ‘(fmap ((==) string) input)’
      In a stmt of a 'do' block:
        (fmap ((==) string) input)
          || (putStrLn "Not hello" >> return False)
  |
7 |   (fmap ((==) string) input) || (putStrLn "Not hello" >> return False)
  |                       ^^^^^

thing.hs:7:34: error:
    • Couldn't match expected type ‘Bool’ with actual type ‘IO Bool’
    • In the second argument of ‘(||)’, namely
        ‘(putStrLn "Not hello" >> return False)’
      In a stmt of a 'do' block:
        (fmap ((==) string) input)
          || (putStrLn "Not hello" >> return False)
      In the expression:
        do putStr "Input:"
           hFlush stdout
           input <- getLine
           (fmap ((==) string) input)
             || (putStrLn "Not hello" >> return False)
  |
7 |   (fmap ((==) string) input) || (putStrLn "Not hello" >> return False)
  |                                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

在这一点上，我想“肯定有一种我没有想到的更简单的方法来做到这一点”，但如前所述，谷歌搜索并没有让我走得太远，所以我决定发布这个。

我非常确定您无法将 IO 字符串转换为字符串 - 但我猜有一种方法可以获得 IO 布尔值，该布尔值返回 IO 字符串与字符串的比较 - 但是，如图所示，我没有得到它。