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

<-- Example 4: With generics

As we mentioned in a previous example, a limitation of normal protocols is that each class can only implement a protocol once. Here is an example of a protocol-like concept that is only implementable using a protocol witness.

Say I want to create an abstract expression of the notion that one type can be converted to another. We could define a protocol:

protocol Convertible {
  associatedtype To
  func convert() -> To
}

And we can conform a type to this protocol; for example, we can make Int convertible to String:

extension Int: Convertible {
  func convert() -> String {
    return “\(self)”
  }
}

let s: String = 1.convert()

But if we also want to make the same type convertible to another type, we’re out of luck. For example, we may want to do this:

extension Int: Convertible {
    func convert() -> Float {
        return Float(self)
    }
}

let f: Float = 1.convert()

But because we already created one conformance of Int to Convertible, we’re not allowed to create another. The compiler tells us Redundant conformance of 'Int' to protocol 'Convertible’.

Convertible is a silly example, but I have come across many real use cases in my code where I wanted to create type-specific implementations of a protocol. Luckily, with protocol witnesses, this is easy:

struct Converting<From, To> {
  let convert: (From) -> To
}

extension Converting where From == Int, To == String {
  static let intToString = Self { "\($0)" }
}

extension Converting where From == Int, To == Float {
  static let intToFloat = Self { Float($0) }
}

let s: String = Converting.intToString.convert(1)
let f: Float = Converting.intToFloat.convert(1)

Your next question might be: can I actually replace all my protocols with protocol witnesses? One particular issue that we haven’t discussed yet is whether we can model protocol inheritance in protocol witness world. It can be quite a crucial point, depending on your needs. As always, let’s see in the next example.

Example 6: Composed protocols -->