'Cannot convert the expression's type' with optional in closure body

问题

I have come across (most definitely) a feature that I don't quite understand. I have a closure, that takes in a Float and has no return value, aka (number: Float) -> Void. In that closure, I perform a call on optional value, not forcing its unwrapping, because I like the behavior since the optional is a delegate (= if it's nil, don't do anything).

But when I do that, I get an error: Cannot convert the expression's type '...' to type Float.

Interestingly enough, when I simply add println(), the error disappears. I have simplified my case to tiny little example for illustration:

var optional: [String : Int]?
let closure: (number: Int) -> Void = { (number) -> Void in
    optional?.updateValue(number, forKey: "key")
    //println() <-- when this gets uncommented, error disappears
}

My bet would be that the compiler maybe doesn't like that in some cases I'm not handling the float number, but since I am not returning the value then it should just disappear right? What am I missing?

回答1:

An expression containing a single expression is inferred to return that result, hence:

let closure: (number: Int) -> Void = { (number) -> Void in
    optional?.updateValue(number, forKey: "key")
}

is equivalent to:

let closure: (number: Int) -> Void = { (number) -> Void in
    return optional?.updateValue(number, forKey: "key")
}

You now have conflicting return types between Void and Int? (remember, updateValue returns the old value)

Splitting it up with an explicit return clarifies the inferred typing.

let closure: (number: Int) -> Void = { (number) -> Void in
    optional?.updateValue(number, forKey: "key")
    return
}

回答2:

I think your closure won't like returning nil because Void can't be converted to nil.

If you do that in a playground:

var n: Void = ()
n = nil

You'll get this error: error: type 'Void' does not conform to protocol 'NilLiteralConvertible'

来源：https://stackoverflow.com/questions/28882441/cannot-convert-the-expressions-type-with-optional-in-closure-body

标签

swift

closures

optional