swift - if let 可以在 Swift 3.1 中以逗号分隔吗？

Question

在Implement Navigation开始篇Developing iOS Apps (Swift) Apple的教程，有以下代码片段:

if let sourceViewController = sender.sourceViewController as? MealViewController,
   meal = sourceViewController.meal {}

本教程使用 Swift 3.1，此代码段似乎对我不起作用。仅当我在 meal = sourceViewController.meal 前面加上 let 关键字时它才有效，如下所示:

if let sourceViewController = sender.sourceViewController as? MealViewController,
   let meal = sourceViewController.meal {}

我的问题纯粹出于好奇:if let a = b, c = d 在任何 Swift 版本中是否曾经是有效语法？

Answer 1

是的，以下代码使用 Xcode 8 和“传统模式”(即 swift 2.3):

let a: Int? = 1
let b: Int? = 2

if let x = a, y = b { }

从 Swift 3 开始，它必须写成

if let x = a, let y = b { }

由于 SE-0099 Restructuring Condition Clauses ，特别是(强调):

The root problem lies in the condition grammar: commas are used both to separate items within a clause (e.g. in if let x = a, y = b {) and to separate mixed kinds of clauses (e.g. if let x = a, case y? = b {). This proposal resolves this problem by retaining commas as separators between clauses (as used elsewhere in Swift) and limits clauses to single items.