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Martin Hope
true-hacker
Asked: 2020-05-09 23:41:06 +0000 UTC
  • -1

Javascript是一种事件驱动的编程语言。

function first() {
  console.log('first');
}

function second() {
  setTimeout(function() {
      console.log('second');
  }, 1000);
}

second()
first()

First- 同步功能。Second- 一个同步函数，其中包含一个异步函数，其执行将在一秒后结束。

结论：

first

second

为什么，如果first和second同步函数，第一个完成它的工作first？

*这是编程语言及其架构、结构和操作的特性吗？（或相同的功能setTimeout）。

javascript
Martin Hope
true-hacker
Asked: 2020-05-07 10:30:39 +0000 UTC
  • 1

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