Swift protocol witnesses example 3: Multiple implementations

Tue Mar 16 2021

This is part of a series on protocol witnesses in Swift.

<— Example 2: Instance-bound implementation

Protocol witnesses, as we have discussed them so far, already offer a notable benefit over protocols. With normal protocols, a class can only conform to a protocol in one way. With protocol witnesses, on the other hand, we create an explicit instance for each “way” that a class conforms to a protocol, so we can create arbitrarily many different “implementations” of a single class to a single protocol.

To demonstrate this, we’ll modify our CanFoo example:

struct CanFoo {
  let foo: () -> Void
}

class Fooer {
  func foo(debug: Bool = false) {}

  lazy var canFoo = CanFoo { self.foo() }
  lazy var canFooDebug = CanFoo { self.foo(debug: true) }
}

class FooClient {
  var fooer: CanFoo!
}

let client = FooClient()
let fooer = Fooer()
client.fooer = (debug) ? fooer.canFooDebug : fooer.canFoo

Admittedly, the debug scenario is a little contrived, but the point is: the same class (Fooer) conforms to the same protocol (CanFoo) in two different ways, and because those two different ways are stored in two different explicit protocol witness instances, we can pick which one we’d like. This is simply not possible with normal protocols; we are only allowed to write extension Fooer: CanFoo once. If we want different implementations, we need different classes.

So far so good, but also so far this has had nothing to do with generics. Isn’t this series about protocols with generics? Why yes! That brings us to our next example.

Example 4: With generics —>