关于`let 5 = 10` [英] About `let 5 = 10`

问题描述

如果我说 let 5 = 10 ，为什么 5 + 1 返回 6 代替 11 ？

解决方案

你说

  let 5 = 10 
  

它不是重新定义5，它是一种模式匹配，当您说


  foo 5 = undefined 
 ... foo 10 ... 
  

失败，如果它匹配。

在let表达式中，匹配是懒惰的。这意味着匹配只在被它所绑定的变量进行评估时完成。这使我们可以写下如下内容：

$ p $ let foo = undefined in 10


在你的表达式中，没有变量被绑定，所以该模式永远不会匹配。

可以说这样的模式没有变量在let-bindings中没有意义，应该由编译器检测，但该语言不禁止它们。

If I say let 5 = 10, why does 5 + 1 return 6 instead of 11?

解决方案

When you say

let 5 = 10

it's not a redefinition of 5, it's a pattern matching, the same which occurs when you say

foo 5 = undefined
 ... foo 10 ...

The pattern simply fails if it's ever matched.

In let-expressions the match is lazy. This means the match is only being done when a variable bound by it is evaluated. This allows us to write things like

 let foo = undefined in 10

In your expression, no variable is bound, so the pattern is never matched.

Arguably such patterns with no variables make no sense in let-bindings and should be detected by the compiler, but the language doesn't forbid them.

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