Builder.swift

protocol Vehicle {

    var name: String { get }

    associatedtype FuelType
    func fillGasTank(with fuel: FuelType)
}

struct Car: Vehicle {

    let name = "car"

    func fillGasTank(with fuel: Gasoline) {
        print("Fill \(name) with \(fuel.name)")
    }
}

struct Bus: Vehicle {

    let name = "bus"

    func fillGasTank(with fuel: Diesel) {
        print("Fill \(name) with \(fuel.name)")
    }
}

struct Gasoline {
    let name = "gasoline"
}

struct Diesel {
    let name = "diesel"
}

@resultBuilder
struct Builder {
    static func buildEither<V>(first component: V) -> V where V: Vehicle {
        component
    }
    
    static func buildEither<V>(second component: V) -> V where V: Vehicle {
        component
    }
    
    static func buildBlock<V>(_ component: V) -> V where V: Vehicle {
        component
    }
}

@Builder func createSomeVehicle(isPublicTransport: Bool) -> some Vehicle {
    if isPublicTransport {
        Bus()
    } else {
        Car()
    }
}