Swift:具有多种模式的开关盒无法绑定(bind)到变量

Question

在官方Swift Programming Language指南，它对开关盒有这样的说法: “...如果 case 包含与控制表达式匹配的多个模式，则这些模式都不能包含常量或变量绑定(bind)。”

包含多个模式是什么意思？

Answer 1

这意味着具有多个模式的 case 标签不能声明变量。

这是允许的:

let somePoint = (1, 1)
switch somePoint {
// Case with multiple patterns without binding
case (0, _),
     (_, 0):
    println("(\(somePoint.0), \(somePoint.1)) is on an axis")
default:
    println("(\(somePoint.0), \(somePoint.1)) is not of an axis")
}

这也是允许的:

let somePoint = (1, 1)
switch somePoint {
// Case with single patterns with binding
case (0, let y):
    println("(0, \(y)) is on an axis")
case (let x, 0):
    println("(\(x), 0) is on an axis")
default:
    println("(\(somePoint.0), \(somePoint.1)) is not of an axis")
}

但是，这是被禁止的:

let somePoint = (1, 1)
switch somePoint {
// Case with multiple patterns that have bindings
case (0, let y),
     (let x, 0):
    println("(\(x), \(y)) is on an axis")
default:
    println("(\(somePoint.0), \(somePoint.1)) is not of an axis")
}

以上会产生错误:

error: 'case' labels with multiple patterns cannot declare variables