有时会出现FS0037错误，非常令人困惑 [英] Error FS0037 sometimes, very confusing

问题描述

如果我编写以下F#代码，则编译器将发出错误消息.

If I write the following F# code, the compiler issues an error.

let a = 123
let a = 123

产生的错误是:

错误FS0037:值'a'的重复定义

error FS0037: Duplicate definition of value 'a'

如果我在这样的函数中编写相同的代码:

If I write the same code in a function like this:

let fctn = 
    let a =123
    let a =123
    a

它不会产生任何错误.

我不明白区别.谁能解释一下?

I don't understand the difference. Can anyone please explain?

我在模块级别编写的第一个代码.

Edit : first code I write in module level.

推荐答案

我同意这令人困惑.问题是let用作局部变量(在函数内)和用作全局定义(在模块内)时，行为会有所不同.

I agree this is confusing. The problem is that let behaves differently when it is used as a local variable (within a function) and when it is used as a global definition (within a module).

全局定义(在模块中)被编译为静态类的静态成员，因此名称只能使用一次.这意味着顶级用途:

Global definitions (in a module) are compiled as static members of a static class and so a name can be used only once. This means that top-level use of:

let a = 10
let a = 11

...是一个错误，因为F#必须产生两个同名的静态成员.

... is an error, because F# would have to produce two static members of the same name.

本地定义(在函数或其他嵌套作用域内部)被编译为IL，并且变量名实际上消失了(IL使用堆栈代替).在这种情况下，F#允许变量阴影化，并且您可以隐藏现有名称的变量.这可以在函数内部，甚至可以是do块:

Local definitions (inside a function or other nested scope) is compiled to IL and the variable name essentially disappears (the IL uses stack instead). In this case, F# allows variable shadowing and you can hide variable of existing name. This can be inside a function, or even just a do block:

do
  let a = 10
  let a = 11
  ()

这有点令人困惑，因为变量阴影仅在局部作用域内起作用，而在顶级作用域内不起作用.当您知道事物是如何编译时，这才有意义.

This is a bit confusing, because variable shadowing only works inside local scopes but not at the top level. It makes sense when you know how things are compiled though..

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