Holocron

Holocron

Swift Type Erasure

Type Erasure is a term that I've run across a few times as I've been developing in Swift, but have never really understood (until recently) what it is or when it can be useful. Hopefully I can change that by writing about it here!

The Motivation

Let's start with a hypothetical situation where we have a ForceWielder protocol and various implementers of that protocol.

protocol ForceWielder {
    associatedtype Ability

    func attack() -> Ability
}

struct ObiWan: ForceWielder {
    func attack() -> LightsaberCombat {
        let form = "Soresu"
        print("Attacking with \(form) form")
        return LightsaberCombat(form: form)
    }
}

struct Anakin: ForceWielder {
    func attack() -> LightsaberCombat {
        let form = "Djem So"
        print("Attacking with \(form) form")
        return LightsaberCombat(form: form)
    }
}

struct Emperor: ForceWielder {
    func attack() -> ForceLightning {
        let voltage = 50
        print("Zapping with \(voltage) volts")
        return ForceLightning( voltage: voltage )
    }
}

struct ForceLightning { let voltage: Int }
struct LightsaberCombat { let form: String }

Now we want to take a sequence of force wielders and perform their attacks, so attempt to create the sequence:

let forceWielders: [ForceWielder] = [Anakin(), ObiWan()]
// error: protocol 'ForceWielder' can only be used as a generic constraint
// because it has Self or associated type requirements

Uh oh, seems like the ForceWielder protocol is not usable in this situation because it has an associated type. Ok, but why? The compiler is confused because there is type ambiguity in two different ways: the protocol itself can be implemented by a number of concrete types, and the associated type can also be defined as different types.

We'll try using the Type Erasure pattern to make it so that we have a new type that is generic over the Ability associated type, but is a concrete implementer of the ForceWielder protocol. Something like this:

The Pattern

final class AnyForceWielder<Ability>: ForceWielder {

    init<Concrete: ForceWielder>(_ concrete: Concrete)
        where Concrete.Ability == Ability {
            // do something here
    }

    func attack() -> Ability {
        // do something here
    }
}

So we have a class that we can initialize by sending in a concrete ForceWielder, but we need to figure out how to forward the attack function on to the concrete instance that comes in. For this, we'll create a container class that's generic over ForceWielder and inherits from a base class that's generic over Ability. The inheritance is what "erases" the ForceWielder type so that the super class only knows about Ability. Our wrapper will now look like this:

final class AnyForceWielder<Ability>: ForceWielder {
    private let box: _AnyForceWielderBase<Ability>

    init<Concrete: ForceWielder>(_ concrete: Concrete)
        where Concrete.Ability == Ability {
            self.box = _AnyForceWielderBox(concrete)
    }

    func attack() -> Ability {
        return box.attack()
    }
}

We are using the concrete class to instantiate an _AnyForceWielderBox (which knows about ForceWielder) but we are storing it as its superclass that only knows about Ability. Then we call attack on that stored _AnyForceWielderBase<Ability>. Let's look at the code for these new classes...

private class _AnyForceWielderBase<Ability>: ForceWielder {
    init() {
        guard type(of: self) != _AnyForceWielderBase.self else {
            fatalError("Must be instantiated from a subclass")
        }
    }
    func attack() -> Ability { fatalError("Method must be overriden") }
}

private final class _AnyForceWielderBox<Concrete: ForceWielder>: _AnyForceWielderBase<Concrete.Ability> {
    private var concrete: Concrete

    init(_ concrete: Concrete) { self.concrete = concrete }

    override func attack() -> Concrete.Ability {
        return concrete.attack()
    }
}

Using the New Type

Now we can create that sequence we wanted before...

let duelists = [AnyForceWielder(Anakin()), AnyForceWielder(ObiWan())]

Note that the type of duelists is inferred to be [AnyForceWielder<LightsaberCombat>].

duelists.map() { $0.attack() }
// Attacking with Djem So form
// Attacking with Soresu form

It also works with abilities some consider to be... unnatural.

let zappers = [AnyForceWielder(Emperor())]
zappers.map { $0.attack() }
// Zapping with 50 volts

Also note that we are still getting type safety. If we were to try to combine Jedi and Sith...

let both = [AnyForceWielder(Anakin()), AnyForceWielder(Emperor())]
// error: heterogeneous collection literal could only be inferred to '[Any]';
// add explicit type annotation if this is intentional

There has Got to be a Simpler Way

Ok, you got me. The pattern above was hard to understand but is pretty robust. Apple even uses it in the standard library. But if you just have a method or two that you need exposed (as in our example), you can just save it off instead of worrying about the two extra classes.

struct AnyForceWielder<Ability>: ForceWielder {
    private let _attack: () -> Ability

    init<Concrete: ForceWielder>(_ concrete: Concrete)
        where Concrete.Ability == Ability {
            self._attack = concrete.attack
    }

    func attack() -> Ability {
        return _attack()
    }
}
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