type-erasure.playground.swift

protocol Pokemon {
    associatedtype PokemonType
    var name: String { get }
    var type: PokemonType { get }
}

struct Electric {
    // something
}

struct Flame {
    // something
}

struct Pikachu: Pokemon {
    typealias PokemonType = Electric
    let name = "ピカチュウ"
    let type = Electric()
}

struct Hitokage: Pokemon {
    typealias PokemonType = Flame
    let name = "ヒトカゲ"
    let type = Flame()
}

struct AnyPokemon: Pokemon {
    typealias PokemonType = Void // Type Erasure

    private let _name: () -> String
    private let _type: () -> PokemonType

    init<X>(base: X) where X: Pokemon {
        _name = { return base.name }
        _type = { return base.type }
    }

    var name: String {
        return _name()
    }
    var type: PokemonType {
        return _type()
    }
}

func battle(pokemon1: AnyPokemon, pokemon2: AnyPokemon) {
    print("\(pokemon1.name) vs. \(pokemon2.name)")
    let pokemons: [AnyPokemon] = [pokemon1, pokemon2]
    if let winner = pokemons.shuffled().first {
        print("Winner: \(winner.name)")
    }
}

// -- battle --

let pikachu: AnyPokemon = AnyPokemon(base: Pikachu())
let hitokage: AnyPokemon = AnyPokemon(base: Hitokage())

battle(pokemon1: pikachu, pokemon2: hitokage)