Swift: closure as parameter reports error

Question

just writing a simple swift app and this error came up.

protocol FormDelegate {
    func formDidFinish(form: Form)
}

class Form {
    var delegate: FormDelegate?

    func testClosure(sender: () -> Void) {
    }
}

let form = Form()
form.testClosure {
//      let removeCommentToGetRidOfError = true
    form.delegate?.formDidFinish(form) // error: Cannot convert the expression's type '() -> () -> $T2' to type '()'
}

but when i insert the let statement, everything works. Any clue what's going on?

Kirsteins · Accepted Answer · 2014-09-19 11:35:20Z

3

Problem is that closures have auto return, when there are no explicit return. In this case the return value is Void? as there is optional chaining involved. You can fix this by returning as last statement:

form.testClosure {
    form.delegate?.formDidFinish(form)
    return
}

or make testClosure return Void?

class Form {
    var delegate: FormDelegate?

    func testClosure(sender: () -> Void?) {
    }
}

answered Sep 19, 2014 at 11:35

Kirsteins

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3 Comments

warly Over a year ago

that makes a lot of sense. now even the error message make sense. thanks :)

Craig Otis Over a year ago

Wow, that's interesting. How does Void differ from Void?, from a conceptual standpoint? Does it?

Kirsteins Over a year ago

Void? is optional, just like Int? etc... It can be nil (None) or Void (Some). It main use is in optional chaining method calls to tell whether methods returning Void where ever called.

mustafa · Accepted Answer · 2014-09-19 11:35:51Z

1

If the closure has one expression swift tries to return that expreeions result. There is great blog post about this feature ( or bug? ) in swift. link

answered Sep 19, 2014 at 11:35

mustafa

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