Swift Generic Protocol Function Parameters

Question

This seems like it should work to me. All I am trying to do is make the Rule protocol able to performRule on whatever struct adopts that Rule protocol

Answer 1

Saying <NSNumber> defines a new generic placeholder type in your performRule(value:) method, which, as you've named it NSNumber, will shadow Foundation's NSNumber class – meaning that the value: parameter is of type your generic placeholder, not Foundation's NSNumber.

If you want it so that types conforming to Rule can choose their own type for the parameter of the performRule(value:) method – then you want an associated type, not a generic placeholder.

protocol NumberCalculation {
    var number : NSNumber { get set }
}

protocol Rule {

    // define associated type that conforming types must satisfy
    // by providing a type to replace it with
    associatedtype Value
    var invalidMessage : String { get set }
    func performRule(value: Value) -> Bool
}

struct GreaterThanRule : Rule, NumberCalculation {

    var invalidMessage: String
    var number : NSNumber

    init(valueMustBeGreaterThan value: NSNumber, withInvalidMessage message : String = "") {
        number = value
        invalidMessage = message
    }

    // satisfy associated type implicitly by annotating the type of the parameter
    // as NSNumber – the compiler will infer that Value == NSNumber.
    func performRule(value: NSNumber) -> Bool {
        number.decimalValue // works
        value.decimalValue // also works!
        return true
    }
}