Pokemon-CopyOnWrite.swift

// Output: 
// AnyPokemon<🔥>(Ponyta())
// AnyPokemon<🔥>(Charmander())

protocol Pokemon {
  associatedtype Power
  mutating func attack() -> Power
}

struct Pikachu: Pokemon {
  func attack() -> 🌩 {
    return 🌩()
  }
}

struct Charmander: Pokemon {
  func attack() -> 🔥 {
    return 🔥()
  }
}

struct Ponyta: Pokemon {
  func attack() -> 🔥 {
    return 🔥()
  }
}

// power types
struct 🔥 {}
struct 🌩 {}

// MARK: - Type Erasure

/*
 
 We need a generic base that can represent any pokemon with a specific power.
 This class is private so we can use value-semantics still.
 We could use it instead of 'struct AnyPokemon'.
 We use fatalError because we don't have enough information to implement yet.
 
 */
private class _AnyPokemonWithPower<Power>: NonObjectiveCBase, Pokemon, CustomStringConvertible {
  func attack() -> Power { fatalError() }
  var description: String { fatalError() }
  func clone() -> _AnyPokemonWithPower { fatalError() }
}

/*

 This Subclass of _AnyPokemon allows us to use the implementation from a
 specific Pokemon.

 */
private final class _AnyPokemonOfType<P: Pokemon>: _AnyPokemonWithPower<P.Power> {
  private var pokemon: P
  init(_ pokemon: P) {
    self.pokemon = pokemon
  }
  override func attack() -> P.Power {
    return self.pokemon.attack()
  }
  override var description: String {
    return "AnyPokemon<\(P.Power.self)>(\(self.pokemon))"
  }
  override func clone() -> _AnyPokemonWithPower<P.Power> {
    return _AnyPokemonOfType(pokemon)
  }
}

/*

 This is the actual AnyPokemon struct, it uses _AnyPokemonOfType to
 get an _AnyPokemonWithPower with the appropriate Power.

 */

struct AnyPokemon<Power>: Pokemon, CustomStringConvertible {
  private var base: _AnyPokemonWithPower<Power>
  init<P: Pokemon where P.Power == Power>(_ pokemon: P) {
    self.base = _AnyPokemonOfType(pokemon)
  }
  private mutating func copyOnWrite<T>(
    @noescape apply: _AnyPokemonWithPower<Power> -> T
  ) -> T {
    if !isUniquelyReferenced(&self.base) {
      self.base = self.base.clone()
    }
    return apply(self.base)
  }
  mutating func attack() -> Power {
    return self.copyOnWrite { base in
      base.attack()
    }
  }
  var description: String {
    return self.base.description
  }
}

// MARK: - now we can use it!
let pokemon: [AnyPokemon<🔥>] = [
  AnyPokemon(Ponyta()),
  AnyPokemon(Charmander())
]
pokemon.forEach { 🐣 in
  print("\(🐣)")
}