When you start a project, and you try to plan for the future. Protocols help you do that — you use protocols to define what objects can do, what their interface is, and your vision is a neatly-scoped protocol-oriented future.

However, as your product goes, so do your protocols. One property here, another method there. And now you’ve got protocols that do a lot of things. So many things, that not everything in the protocol definition needs to be done by all things implementing that protocol.

Now what do you do?

The Right Answer™️, and Its Cost

There’s an easy-to-say-yet-possibly-costly right answer. If your protocol has a method that not all who conform to it need to implement, it shouldn’t be in the protocol. Refactor that thing out. Either

  • It doesn’t need to be in a protocol, anywhere,
  • It should be in its own protocol, and then only some things should inherit it
  • Or it can be part of a protocol composition that combines things…

All of these are great, reasonable answers.

All of these might require considerable work, depending on the size of the project, and the number of things conforming to that protocol. If time or cost is a factor, what are the alternatives?

A Simple Example.

Let’s imagine a protocol for traffic lights. You define it like this:

protocol TrafficLight {
    var color: LightColor { get }
    
    mutating func turnRed()
    mutating func turnYellow()
    mutating func turnGreen()
}

Then, you spend years conforming to this protocol, and build your city. Until one day, Mayor Quimby decries that, to increase traffic flow, there will be no more green lights – only red & yellow

Lenny waiting for a red light to turn yellow, then flooring it

Approach 1: Ignore it

By far, the easiest thing to do is to just ignore it.

struct FasterTrafficLight: TrafficLight {
    // ...
    mutating func turnGreen() {
        // Don't turn green anymore.
    }
}

While this meets the protocol, it also fails silently. This can lead to very hard-to find bugs. For instance, if the TrafficLight controller waited for the light to turn green before turning it red again, the light could never turn red, and you wouldn’t know why.

Let’s try something that gives us an error.

Approach 2: Crash

The quickest way to an error is a crash. Let’s try that.

struct FasterTrafficLight: TrafficLight {
    // ...
    mutating func turnGreen() {
        fatalError("Faster traffic lights don't turn green")
    }
}

This works well: anytime someone tells this light – which shouldn’t turn green – to turn green, it will crash the program. We’ll know immediately that someone is using this method in a way they’re not supposed to.

Unfortunately, so will users. Crashing is an effective way of stopping your program in an invalid state, but someone using the protocol won’t know that this is an invalid method to use. You could use a different crash method that only works on debug builds, like assertionFailure, but you might still crash the app during normal usage.

Approach 3: Annotating the method

Maybe we could annotate the method somehow, so we can catch misuse at compile time, not run time? For instance, what if we label the method as unavailable:

struct FasterTrafficLight: TrafficLight {
    // ...
    @available(*, unavailable)
    mutating func turnGreen() {
        // Don't turn green anymore.
    }
}

This… flat out doesn’t work. With this method marked unavailable, it no longer conforms to the TrafficLight protocol.

What about deprecated?

struct FasterTrafficLight: TrafficLight {
    // ...
    @available(*, deprecated)
    mutating func turnGreen() {
        // Don't turn green anymore.
    }
}

This is better. It compiles, and if we use FasterTrafficLight directly, the compiler will warn us.

var light = FasterTrafficLight()
light.turnGreen() // Warning: turnGreen() is deprecated

This is pretty good. It tells other developers that they shouldn’t use this method, and if used directly, the compiler will warn you that you’re about to do something wrong.

However, it’s not exactly what we want. Anything that uses the protocol instead of the concrete type won’t see a warning, even if the underlying type is using the deprecated method.

var light: TrafficLight = FasterTrafficLight()
light.turnGreen() // This is fine 🐶☕️🔥

Conclusion

None of these are perfect solutions. None outperform removing turnYellow from the TrafficLight protocol and auditing its usage. But they’re all significantly cheaper to implement.

Personally, I like the idea of both marking it deprecated and using assertionFailure to crash debug builds. That’s the right level of annoying for me, but figure out what works best for your project and your team. And when you can, take the time to do that proper refactor.