问题

以下是你要的翻译部分：

我不确定以前是否有人提过这个问题，但是我从Google上没有找到任何信息。
我想在Haskell中实现以下代码的等效版本：

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

我想到的是这样的实现：

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

我将最后一行简化为（或者更确切地说是linter建议的）：

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

但我得到了各种错误，似乎都与事物是IO类型还是非IO类型有关：

thing.hs:7:3: 错误：
• 无法匹配预期类型‘IO Bool’与实际类型‘Bool’
• 在‘do’块的语句中：
   (string == input) || (putStrLn "Not hello" >> False)
 在表达式中：
   do putStr "Input:"
      hFlush stdout
      input <- getLine
      (string == input) || (putStrLn "Not hello" >> False)
 在‘inputThing’的等式中：
   inputThing string
     = do putStr "Input:"
          hFlush stdout
          input <- getLine
          ....
|
7 |   (string == input) || (putStrLn "Not hello" >> False)
|   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

thing.hs:7:25: 错误：
• 无法匹配预期类型‘Bool’与实际类型‘IO b0’
• 在‘(||)’的第二个参数中，即
   ‘(putStrLn "Not hello" >> False)’
 在‘do’块的语句中：
   (string == input) || (putStrLn "Not hello" >> False)
 在表达式中：
   do putStr "Input:"
      hFlush stdout
      input <- getLine
      (string == input) || (putStrLn "Not hello" >> False)
|
7 |   (string == input) || (putStrLn "Not hello" >> False)
|                         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

thing.hs:7:49: 错误：
• 无法匹配预期类型‘IO b0’与实际类型‘Bool’
• 在‘(>>)’的第二个参数中，即‘False’
 在‘(||)’的第二个参数中：
   ‘(putStrLn "Not hello" >> False)’
 在‘do’块的语句中：
   (string == input) || (putStrLn "Not hello" >> False)
 在表达式中：
   do putStr "Input:"
      hFlush stdout
      input <- getLine
      (string == input) || (putStrLn "Not hello" >> False)
|
7 |   (string == input) || (putStrLn "Not hello" >> False)
|                                                 ^^^^^

我能想到的最好的方法是：

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

但这仍然不起作用：

thing.hs:7:3: 错误：
• 无法匹配预期类型‘IO Bool’与实际类型‘Bool’
• 在‘do’块的语句中：
   (fmap ((==) string) input)
     || (putStrLn "Not hello" >> return False)
 在表达式中：
   do putStr "Input:"
      hFlush stdout
      input <- getLine
      (fmap ((==) string) input)
        || (putStrLn "Not hello" >> return False)
 在‘inputThing’的等式中：
   inputThing string
     = do putStr "Input:"
          hFlush stdout
          input <- getLine
          ....
|
7 |   (fmap ((==) string) input) || (putStrLn "Not hello" >> return False)
|   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

thing.hs:7:4: 错误：
• 无法匹配预期类型‘Bool’与实际类型‘[Bool]’
• 在‘(||)’的第一个参数中，即
   ‘(fmap ((==) string) input)’
 在‘do’块的语句中：
   (fmap ((==) string) input)
     || (putStrLn "Not hello" >> return False)
 在表达式中：
   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: 错误：
• 无法匹配类型‘Char’与‘[Char]’
 预期：[String]
 实际：String
• 在‘fmap’的第二个参数中，即‘input’
 在‘fmap’的第一个参数中，即‘(fmap ((==) string) input)’
 在‘(&&)’的第一个参数中，即
   ‘(fmap ((==) string) input)’
 在‘do’块的语句中：
   (fmap ((==) string)

<details>
<summary>英文:</summary>

I don&#39;t know if this has been asked before, but I haven&#39;t got anywhere from Googling.
I want to have the following code&#39;s equivalent in Haskell:
```python
def inputThing(string: str) -&gt; bool:
  a=input(&quot;Input: &quot;)
  if a==&quot;Hello&quot;:
    return True
  else:
    print(&quot;Not hello&quot;)
    return False

What I came up with is something like this:

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

that last line I simplified to (or rather the linter suggested)

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

But I get various errors that all seem to have something to do with things being IO typed or not:

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)
  |                                                 ^^^^^

The best I could think of was:

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

This still doesn't work:

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)
  |                                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

At this point I thought "Surely there's a much simpler way of doing this I haven't thought of" but as previously mentioned, Googling wasn't getting me far, so I decided to post this.

I am very certain it is the case that you cannot convert an IO String to a String - but I'm guessing there's a way to get a IO Bool that returns the comparison of an IO String to a String - however, as shown, I didn't get anywhere with it.

答案1

得分: 3

True 和 False 的类型是 Bool，但结果需要是 IO Bool 类型。这种情况下，存在一个易于理解的转换方式（与 IO Bool 转为 Bool 不同，后者不可行）：这就是 return 的作用！因此，它实际上看起来更类似于 Python：

if string == input
  then return True
  else putStrLn "Not hello" >> return False

或者，根据您对代码布局的偏好：

if string == input then
    return True
else do
    putStrLn "Not hello"
    return False

或者更简洁的，适用于 applicative 风格：

if string == input
  then pure True
  else False <$ putStrLn "Not hello"

   if string == input
then return True
else putStrLn "Not hello" >> return False

   if string == input then
return True
else do
putStrLn "Not hello"
return False

   if string == input
then pure True
else False <$ putStrLn "Not hello"

答案2

得分: 1

The linter put you on the wrong path. All you need to do is add a pure or two:

(代码部分未翻译)

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

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