Member operator '%' must have at least one argument of type 'ViewController’

Question

I am trying to create a simple Swift 3 template with a custom function for calculating percentage using postfix unary operator in an Xcode app. This may seem like a duplicat

Answer 1

I declared the ’operator' at file scope

No, you didn't. You defined it in the scope of the UIViewController definition:

postfix operator %

class ViewController: UIViewController {

    // ...

    static postfix func % (percentage: Int) -> Double {
        return (Double(percentage) / 100)
    }
}

One can define operators as static member functions of a type in Swift 3, but only if they take at least one argument of that type.

Move the declaration to the file scope to fix the problem:

postfix operator %

postfix func % (percentage: Int) -> Double {
    return (Double(percentage) / 100)
}

class ViewController: UIViewController {

    // ...

}

Answer 2

other alternative if you want to use closures in Swift 3:

import UIKit

typealias Filter = (CIImage) -> CIImage

infix operator >>>
func >>> (filter1: @escaping Filter, filter2: @escaping Filter) -> Filter{
    return { image in
        filter2( filter1( image))
    }
}

class ViewController: UIViewController {
    //...
}

Eidhof, Chris; Kugler, Florian; Swierstra, Wouter. Functional Swift: Updated for Swift 3 (Kindle Location 542). GbR Florian Kugler & Chris Eidhof. Kindle Edition.