Mutating protocol extension function forces class to be declared with var

[the5avage]

Hi,
I'm a bit confused by the behavior of the following code. Why is it ok to call mutatingProtocolFunction() on an object declared with let, but not mutatingExtensionFunction()?

protocol hasMutatingFunction {
    mutating func mutatingProtocolFunction()
}

extension hasMutatingFunction {
    mutating func mutatingExtensionFunction() {
        mutatingProtocolFunction()
        mutatingProtocolFunction()
    }
}

class C : hasMutatingFunction {
    var count: Int = 0

    func increment() {
        count += 1
    }

    func mutatingProtocolFunction() {
        count += 5
    }
}

let c = C()
c.increment() // works
c.mutatingProtocolFunction() // works
c.mutatingExtensionFunction() //  error: cannot use mutating member on immutable value: 'countdown' is a 'let' constant

[Jens]

Mutating Function on Class Type (Possible bug?)

It's "correct" behavior because the default implementation is allowed to go self = someCompletelyDifferentObjectOfTheSameType, and that would reassign the reference at the call site. See SR-142 for some more discussion. Even beyond this, mutating and classes don't play so well together. It may not be the best model for what you're trying to do.

And there's an interesting related discussion here.

That is:

protocol HasMutatingFunction {
    mutating func mutatingProtocolFunction()
    init()
}

extension HasMutatingFunction {
    mutating func mutatingExtensionFunction() {
        mutatingProtocolFunction()
        mutatingProtocolFunction()
        self = Self.init() // <--- Note that this is OK here.
    }
}

class C : HasMutatingFunction {
    var count: Int = 0
    required init() { }
    func increment() { count += 1 }
    func mutatingProtocolFunction() {
        count += 5
        // self = Self.init() // <--- But not here.
    }
}

[the5avage]

Thanks for the links and explanation. How does stdlib's Sequence and IteratorProtocol solve this? next() is a mutating function, there are a lot of extensions (e.g. drop(while:)) and i can implement a custom Sequence with structs and classes...

I found this workaround. It's a bit awkward, but at least it works with structs and classes.

protocol hasMutatingFunction {
    mutating func mutatingProtocolFunction()
}

extension hasMutatingFunction {
    mutating func mutatingExtensionFunction() {
        _mutatingExtensionFunction(this: &self)
    }
}

extension hasMutatingFunction where Self: AnyObject {
    func mutatingExtensionFunction() {
        var this = self
        _mutatingExtensionFunction(this: &this)
    }
}

private func _mutatingExtensionFunction<T: hasMutatingFunction>(this: inout T) {
        this.mutatingProtocolFunction()
        this.mutatingProtocolFunction()
}