let和var在Swift REPL中无效的const重声明

问题描述

在Swift REPL中，我可以使用 let 分配一个常量，但是为什么以后可以使用 var 对其进行修改？

In Swift REPL I can assign a constant with let, but why can I modify it later using var?

let name = "al"

var name = "bob"

Swift在这里没有抱怨，但是不是常量吗？

Swift is not complaining here, but wasn't name a constant?

推荐答案

在Swift中重新声明变量（在相同范围内）无效：

Redeclaring a variable (in the same scope) is not valid in Swift:

$ cat test.swift
let name = "al"
var name = "bob"

$ swiftc test.swift
test.swift:2:5: error: invalid redeclaration of 'name'
var name = "bob"
    ^
test.swift:1:5: note: 'name' previously declared here
let name = "al"
    ^

但是，Swift REPL的行为有所不同：

However, the Swift REPL behaves differently:

$ swift
Welcome to Apple Swift version 3.1 (swiftlang-802.0.53 clang-802.0.42). Type :help for assistance.
  1> let name = "al"
name: String = "al"
  2> var name = "bob"
name: String = "bob"

这是有意的，如
：

...较新的定义替换了所有后续引用的现有定义

... The newer definition replaces the existing definition for all subsequent references

---

注意：您必须分别输入行。如果将这两个
复制，行到粘贴缓冲区中，启动REPL并将
与 粘贴在一起，然后结果是

---

Note: You have to enter the lines separately. If you copy thosetwo lines into the paste buffer, start the REPL and pastethem with then the result is

$ swift
Welcome to Apple Swift version 3.1 (swiftlang-802.0.53 clang-802.0.42). Type :help for assistance.
  1> let name = "al"
  2. var name = "bob"
error: repl.swift:2:5: error: invalid redeclaration of 'name'
var name = "bob"
    ^

显然，这两个语句现在在相同的作用域
中求值（第二行有一个继续提示）并产生错误。
这两个语句都在同一行中发生相同的情况：

Apparently the two statements are now evaluated in the same scope(the second line has a continuation prompt) and produce an error.The same happens with both statements in a single line:

$ swift
Welcome to Apple Swift version 3.1 (swiftlang-802.0.53 clang-802.0.42). Type :help for assistance.
  1> let name = "al" ; var name = "bob"
error: repl.swift:1:23: error: invalid redeclaration of 'name'
let name = "al" ; var name = "bob"
                      ^

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