如何在Swift中使用inout参数和Void的返回类型返回函数？

Question

I have attempted to run this code in an Xcode 6.0 playground and in an iOS project through the viewController viewDidLoad function, and in both settings the program crashes the compiler. I have read about people having similar issues when returning inout functions when the program is run through a playground, however the issue was resolved when they ran their program through a project. Is something incorrect with my code, and if so what is wrong, or am I running the code incorrectly in the playground or project?

// testingPlayground
// July 18, 2015


func chooseFunction(isYNegative lessThanZero: Bool) -> (inout Int) -> Void {
    func increaseY(inout #yValue: Int){ // Increases yValue
        yValue += 1
    }

    func decreaseY(inout #yValue: Int){ // Decreases yValue
        yValue -= 1
    }

    return lessThanZero ? increaseY : decreaseY // Returns either the increase or decrease yValue function
}


var yValue = -1
var changeYFunction = chooseFunction(isYNegative: yValue < 0)


while yValue != 0 {
    changeYFunction(&yValue) // Increments/Decrements yValue
}

Answer 1

The code works fine for me after removing the # from both of the #yValue: Int parameters inside chooseFunction (having the # would imply that an argument name must be given, which you don't do).

Alternatively, you need to specify the parameter name in the return type of chooseFunction and use it when you call the returned function, ie:

func chooseFunction(isYNegative lessThanZero: Bool) -> ((inout yValue: Int) -> Void) {

And:

changeYFunction(yValue: &yValue)

In other words, the problem is that you are not consistent with whether or not the returned functions require a name for the argument or not.

edit : As yet another alternative, you could consider refactoring the whole thing, eg, use the shorthand for curried functions:

func stepper(increase increase: Bool)(inout _ y: Int) {
    increase ? ++y : --y
}
var y = -5
let step = stepper(increase: y < 0)
while y != 0 {
    step(&y)
}

In fact, even the following works, although the hairy syntax required makes me wary:

func stepper(increase increase: Bool)(inout _ y: Int)() {
    increase ? ++y : --y
}
var y = -5
let stepY = stepper(increase: y < 0)(&y)
while y != 0 {
    stepY()
}