Why can a swift function declared as needing parameters, be passed without them?

Question

Code written Swift 2.0 on Xcode 7 beta

My question concerns passing functions are parameters. I have two functions: getImagesFromImgur and randomImageHandle. getImagesFromImgur is called at ViewDidLoad like this:

self.getImagesFromImgur(apiAction: "gallery.json", 
    handleResponse: self.randomImageHandle)

The getImagesFromImgur function looks like this:

func getImagesFromImgur(apiAction: String, handleResponse: 
    (NSData?, NSURLResponse?, NSError?) -> Void) {
...

    let task = session.dataTaskWithRequest(request, completionHandler:
        handleResponse)
...
}

The randomImgurHandle that is passed to getImagesFromImgur function in viewDidLoad looks like this:

func randomImageHandle(data: NSData?, response: NSURLResponse?, 
    error: NSError?) -> Void {
    ...
    //code that edits the UI
    ...
}

So my question is, hqow can I pass self.randomImageHandle to self.getImagesFromImgur without specifying any argument values? I know dataTaskWithRequest passes the parameters to the handleResponse, and while I understand that, I'm confused on why the compiler doesn't force me to specify parameters.

EDIT: I'm passing a handle like this so I can have specific handles to match imgur's Api.

Answer 1

In Swift, functions are first-class types, and you can pass them around as variables. What you're actually doing is that getImagesFromImgur is expecting a function that has the exact signature (NSData?, NSURLResponse?, NSError?) -> Void (you don't need the Void here, BTW). The compiler checks for you that you passed in a method that has that signature, and thus you don't need to specify parameters.