PlayTypeErasure.swift

//: Playground - noun: a place where people can play

import Foundation

enum Element {
    case Water, Fire, Thunder
}

protocol Magic {
    typealias MagicType
    func chant(element: MagicType)
}

// これができない
//class Mage: Magic {
//    func chant(element: MagicType) {}
//}

class AnyMage <MagicType>: Magic {
    let _chant: MagicType -> Void
    
    required init<U: Magic where U.MagicType == MagicType> (mage: U) {
        self._chant = mage.chant
    }
    
    func chant(element: MagicType) {
        chant(element)
    }
}

class WaterMage: Magic {
    func chant(element: Element) { print("💦") }
}

class FireMage: Magic {
    func chant(element: Element) { print("🔥") }
}

let waterMage = AnyMage(mage: WaterMage())
let fireMage = AnyMage(mage: FireMage())

// なぜできない！！！
//waterMage.chant(.Water)
//fireMage.chant(.Fire)