Cannot assign value of type '(_) -> ()' to type '((String, String, String, Int) -> ())?'

Question

I have a closure defined like this,

public var onLogCompletion:((_ printLog:String,_ fileName:String,_ functionName:String,_ lineNumber:Int) -> ())? = nil

Which is updated like this,

fileprivate func printerCompletion(printLog:String, fileName:String, functionName: String, lineNumber:Int) -> Void {
    if onLogCompletion != nil {
        onLogCompletion!(printLog, getFileName(name: fileName), functionName, lineNumber)
    }
}

And using it like this,

    Printer.log.onLogCompletion = { (log) in
        //print(log)
        //print(log.0)
    }

Error:

Cannot assign value of type '(_) -> ()' to type '((String, String, String, Int) -> ())?'

But this is giving me above error and not sure what to do?

The same is working fine with Swift 3.x.

Someone, please tell me the reason to downvote my question? This is completely different than the previously asked. — Hemang, Jun 19 '17 at 07:11

Nirav D · Accepted Answer · 2017-06-19T07:25:13.580

2

The reason its not working in Swift 4 is because of Distinguish between single-tuple and multiple-argument function types(SE-0110).

If you still want to work in a way you are doing in Swift 3 than you need to set the function type's argument list to enclosed with Double parentheses like this.

public var onLogCompletion:(((String,String,String,Int)) -> ())? = nil

Now you all set to go

Printer.log.onLogCompletion = { (log) in
    //print(log)
    //print(log.0)
}

edited Jun 19 '17 at 07:25

answered Jun 19 '17 at 07:13

Nirav D

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Awesome! Good find Nirav D. – Hemang Jun 19 '17 at 08:32

Cannot assign value of type '(_) -> ()' to type '((String, String, String, Int) -> ())?'

1 Answers1