Question

我在我的代码中犯了一个错误，我认为应该阻止它编译和运行，但事实并非如此。所以我很好奇，为什么这会编译：

func function1() {
    print("function1")

    func function2() {
        print("function2")
    }
}

function1() // prints "function1"

Answer 1

因为Swift支持嵌套函数。

...您还可以在其他函数体内定义函数，称为嵌套函数。

默认情况下，嵌套函数对外界是隐藏的，但它们的封闭函数仍然可以调用它们。封闭函数也可以返回其嵌套函数之一，以允许嵌套函数在另一个范围内使用。

https://developer.apple.com/library/content/documentation/Swift/Conceptual/Swift_Programming_Language/Functions.html#//apple_ref/doc/uid/TP40014097-CH10-ID178

它们有很多用途，但一个常见的用例是定义一个公共函数，它使用嵌套函数通过递归计算结果。例如，作为这种设计的结果，您可以添加一次参数检查（在外部函数中），这样您就不必在每次递归调用时重复检查。

func factorial(_ i: Int) -> Int {
    // This check is only done once, rather than with every recursion
    print("Checking to verify that i is non-negative")
    guard 0 <= i else {
        fatalError("Can't compute factorial of the negative number: \(i)")
    }

    func factorialRecurser(_ i: Int) -> Int {
        print("Computing factorial of \(i)")
        switch i {
            case 0, 1: return 1;
            default: return i * factorialRecurser(i - 1)
        }
    }

    return factorialRecurser(i)
}

print(factorial(10))

结果如下：

Checking to verify that i is non-negative
Computing factorial of 10
Computing factorial of 9
Computing factorial of 8
Computing factorial of 7
Computing factorial of 6
Computing factorial of 5
Computing factorial of 4
Computing factorial of 3
Computing factorial of 2
Computing factorial of 1
3628800

将此与更天真的解决方案进行比较：

func naiveFactorial(_ i: Int) -> Int {
    // This check is needlessly repeated
    print("Checking to verify that i is non-negative")
    guard 0 <= i else {
        fatalError("Can't compute factorial of the negative number: \(i)")
    }

    print("Computing factorial of \(i)")
    switch i {
        case 0, 1: return 1;
        default: return i * naiveFactorial(i - 1)
    }
}

print(naiveFactorial(10))

结果如下：

Checking to verify that i is non-negative
Computing factorial of 10
Checking to verify that i is non-negative
Computing factorial of 9
Checking to verify that i is non-negative
Computing factorial of 8
Checking to verify that i is non-negative
Computing factorial of 7
Checking to verify that i is non-negative
Computing factorial of 6
Checking to verify that i is non-negative
Computing factorial of 5
Checking to verify that i is non-negative
Computing factorial of 4
Checking to verify that i is non-negative
Computing factorial of 3
Checking to verify that i is non-negative
Computing factorial of 2
Checking to verify that i is non-negative
Computing factorial of 1
3628800

查看冗余执行非负检查的次数。

为什么编译？在另一个函数

1 个答案: