Protocol extension function overrides the default value in implementations

Evolution Discussion
1

I got this by discussions in another thread of swift-evolution. The result
is interesting. I don't know if it should be treat as a bug.

protocol Good {

    func printSomething(something:String)

}

extension Good {

    func printSomething() { self.printSomething("Good") }

}

struct Name:Good {

    func printSomething(something: String = "John") {

        print(something)

    }

}

struct Talk:Good {

    func printSomething(something: String = "Hahaha") {

        print(something)

    }

}

var protocols:[Good] = [Name(), Talk()]

for p in protocols {

    p.printSomething() // Good, Good instead of John, Hahaha

}

As you can see, the final result is "Good, Good", not as I expected "John,
Hahaha", is that right or is it a bug?

zhaoxin

2

is that right or is it a bug?

It's not a real bug, but I'd call it "unexpected behavior" — and I guess it will be changed sooner or later.
In the meantime: When you include the extension method in the protocol, everything should be fine.

Tino

3

This is expected behavior.

for p in protocols {
    p.printSomething() // Good, Good instead of John, Hahaha
}

is calling method from extension:

extension Good {
    func printSomething() { self.printSomething("Good") }
}

this method (for class Name) is calling method guaranteed by protocol :

func printSomething(something: String = "John") {
        print(something)
    }

and this method is printing Good, because it has a argument. (Default
Parameter Value - John - is not triggered)

More about it:
* video "Protocol-Oriented Programming in Swift" (WWDC 2015 - Session 408)
* Swift Book, section Function, subsection Default Parameter Values

Greg

···

2016-01-16 12:51 GMT+00:00 肇鑫 via swift-evolution <swift-evolution@swift.org>:

I got this by discussions in another thread of swift-evolution. The result
is interesting. I don't know if it should be treat as a bug.

protocol Good {

    func printSomething(something:String)

}

extension Good {

    func printSomething() { self.printSomething("Good") }

}

struct Name:Good {

    func printSomething(something: String = "John") {

        print(something)

    }

}

struct Talk:Good {

    func printSomething(something: String = "Hahaha") {

        print(something)

    }

}

var protocols:[Good] = [Name(), Talk()]

for p in protocols {

    p.printSomething() // Good, Good instead of John, Hahaha

}

As you can see, the final result is "Good, Good", not as I expected "John,
Hahaha", is that right or is it a bug?

zhaoxin

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swift-evolution mailing list
swift-evolution@swift.org
https://lists.swift.org/mailman/listinfo/swift-evolution

4

Isn’t is just because “equivalent modulo default parameters” ranks lower in overload resolution? That may or may not be on purpose or desired, I don’t really know. As long as it’s documented, I guess.

-Sune

···

On 16 Jan 2016, at 16:06, Tino Heth via swift-evolution <swift-evolution@swift.org> wrote:

is that right or is it a bug?

It's not a real bug, but I'd call it "unexpected behavior" — and I guess it will be changed sooner or later.
In the meantime: When you include the extension method in the protocol, everything should be fine.

Tino
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swift-evolution mailing list
swift-evolution@swift.org
https://lists.swift.org/mailman/listinfo/swift-evolution