本文介绍了'in'关键字是什么？的处理方法，对大家解决问题具有一定的参考价值，需要的朋友们下面随着小编来一起学习吧！ 问题描述 29岁程序员，3月因学历无情被辞！ 在Haskell中，为什么不在'do'块中使用'in'和'let'，但你必须以其他方式？ 例如，在 afunc :: Int - > Int afunc a = let x = 9 in a * x amfunc :: IO Int - > IO Int amfunc a = do let x = 9 a'< - a return（a'* x） 这个规则很容易记住，但我不明白其原因。解决方案您正在提供用于定义 afunc 和 amfunc 的表达式。让 - 表达式和do-blocks都是表达式。但是，尽管let表达式引入了一个新的绑定，它围绕在'in'关键字后面给出的表达式，但do块并不是由表达式组成的：它是一系列语句。在do-block中有三种形式的语句： 一个计算结果被绑定到某个变量 x 中 $ b $ > putStrLnhello let x = 3 + 5 let语句引入了一个新的绑定，就像let-expressions一样。这个新绑定的作用域覆盖了do-block中所有剩余的语句。 总之，let表达式中的'in'后面是一个表达式，而let表达式后面是一系列语句。当然，我可以使用let表达式来表达特定语句的计算，但是这个绑定的范围不会超出这个语句到后面的语句。考虑： 做putStrLnhello let x = 3 + 5 in putStrLneight putStrLn（show x） 以上代码在GHC中会导致以下错误消息： 不在范围内：`x' $ b而 $ b $ pre $ 做putStrLnhello let x = 3 + 5 putStrLn 八 putStrLn（显示x） 正常工作。 In Haskell, why do you not use 'in' with 'let' inside of a do-block, but you must otherwise?For example, in the somewhat contrived examples below:afunc :: Int -> Intafunc a = let x = 9 in a * xamfunc :: IO Int -> IO Intamfunc a = do let x = 9 a' <- a return (a' * x)It's an easy enough rule to remember, but I just don't understand the reason for it. 解决方案 You are providing expressions to define both afunc and amfunc. Let-expressions and do-blocks are both expressions. However, while a let-expression introduces a new binding that scopes around the expression given after the 'in' keyword, a do-block isn't made of expressions: it is a sequence of statements. There are three forms of statements in a do-block:a computation whose result is bound to some variable x, as inx <- getChara computation whose result is ignored, as inputStrLn "hello"A let-statement, as inlet x = 3 + 5A let-statement introduces a new binding, just as let-expressions do. The scope of this new binding extends over all the remaining statements in the do-block.In short, what comes after the 'in' in a let-expression is an expression, whereas what comes after a let expression is a sequence of statements. I can of course express a computation of a particular statement using a let-expression, but then the scope of the binding would not extend beyond that statement to statements that follow. Consider:do putStrLn "hello" let x = 3 + 5 in putStrLn "eight" putStrLn (show x)The above code causes the following error message in GHC:Not in scope: `x'whereasdo putStrLn "hello" let x = 3 + 5 putStrLn "eight" putStrLn (show x)works fine. 这篇关于'in'关键字是什么？的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持！ 上岸，阿里云！
