Swift：具有多个模式的切换大小写无法绑定到变量

Question

在官方的《Swift编程语言指南》中，它有关于开关情况的说法：“
…如果情况包含与控件表达式匹配的多个模式，则这些模式均不能包含常量或变量绑定。”

包含多个模式是什么意思？

Answer 1

这意味着具有多个模式的案例标签无法声明变量。

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")
}

上面产生一个错误：

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