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true-hacker
Asked:2020-05-07 10:30:39 +0000 UTC2020-05-07 10:30:39 +0000 UTC

Haskell 和 Monads

  • 772

播放这段代码时：

half x = if even x
           then Just (x `div` 2)
           else Nothing

printMaybe m = case m of
  Nothing -> putStrLn "List was empty!"
  Just x -> print x

main = do
  let halfN = Just 4 >>= half
  halfN >>= printMaybe

发生错误：

main.hs:9:1: error:
    * Couldn't match expected type `IO t0' with actual type `Maybe ()'
    * In the expression: main
      When checking the type of the IO action `main'
  |
9 | main = do
  | ^
main.hs:11:13: error:
    * Couldn't match type `IO' with `Maybe'
      Expected type: Maybe a0 -> Maybe ()
        Actual type: Maybe a0 -> IO ()
    * In the second argument of `(>>=)', namely `printMaybe'
      In a stmt of a 'do' block: halfN >>= printMaybe
      In the expression:
        do let halfN = Just 4 >>= half
           halfN >>= printMaybe
   |
11 |   halfN >>= printMaybe

这是解决此问题的一种方法：

half x = if even x
           then Just (x `div` 2)
           else Nothing

printMaybe m = case m of
  Nothing -> putStrLn "List was empty!"
  Just x -> print x

main = do
  let halfN = Just 4 >>= half
  printMaybe $ halfN

我无法弄清楚为什么原始程序不起作用。首先，我们得到函数中数字的一半half（以 monad 的形式Maybe），然后我们将这个值“推”到输出函数 -printMaybe中。

为什么原始版本的代码不起作用？

io

1 个回答

  1. Best Answer

    >>=这不是“推”运算符，而是一元绑定

    (>>=) :: Monad m => m a -> (a -> m b) -> m b

    并且，从签名中可以看出，它只绑定相同类型的容器Maybewith Maybe，IOwithIO等。您是否尝试连接

    halfN :: Integral a => Maybe a

    和

    printMaybe :: Show a => Maybe a -> IO ()

    那些。不同类型的容器 -Maybe和IO, 内容类型不兼容Integral a => avs. Show a => Maybe a.

    printMaybe $ halfN

    这是使用此功能的正确方法，替代方法（根据单子第一定律等效，但在这种情况下完全不合适）可能是

    return halfN >>= printMaybe

    同样适用于 and Just 4 >>= half，最好将其替换为half 4因为 forMonad Maybe

    return = Just

    正确使用示例

    *Main> half 12
Just 6
*Main> half 12 >>= half
Just 3
*Main> [1..3] >>= show
"123"
*Main> [1..3] >>= \x -> replicate x x
[1,2,2,3,3,3]
*Main> getLine >>= print -- далее ввод с клавиатуры
12
"12"
*Main> getLine >>= \x -> print (half (read x)) -- далее ввод с клавиатуры
12
Just 6

    还有一件事：不要忘记指定函数签名，这会让你更容易理解你做了什么（或没做什么）

    • 4

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