Opaque Types

protocol Advance {
    associatedtype Element
    mutating func next() -> Element
}


struct Cylinder<Cartridge>: Advance {
    var array: [Cartridge]
    mutating func next() -> Cartridge {
        array.removeLast()
    }
}

struct Bullet {}
struct GunpowderChip {
    let bullet: Bullet
}

struct MachineGunExplicit<B> {
    var clip: Cylinder<B>
    mutating func fire() -> B {
        clip.next()
    }
}

protocol Gun {
    associatedtype Bullet
    mutating func fire() -> Bullet
}

struct MachineGunImplicit<C: Advance> : Gun {
    var clip: C
    mutating func fire() -> C.Element {
        clip.next()
    }
}

struct CraftBench {
    func makeSomeAdvane() -> some Advance {
        let bullets = Array(repeating: Bullet(), count: 20)
        let clip = Cylinder(array: bullets)
        return clip
    }
    func makeSomeGun() -> some Gun {
        let clip = makeSomeAdvane()
        let gun = MachineGunImplicit(clip: clip)
        return gun
    }
}

struct ShootingRange {


    func run() {
        var gun = CraftBench().makeSomeGun()
        (0...10).forEach { _ in
            let b = gun.fire() // the b has wonderful type 'let b: (some Gun).Bullet'
            print(b)
        }
    }

}
/*
Bullet()
Bullet()
Bullet()
Bullet()
Bullet()
Bullet()
Bullet()
Bullet()
Bullet()
Bullet()
Bullet()
Compiler knows that underlying type of 'b' is the Bullet.
*/