An implementation of the Builder pattern in Swift

Pizza.swift

import Foundation

enum Cheese : String {
    case Mozzarella = "Mozzarella"
    case Provolone = "Provolone"
    case Pecorino = "Pecorino"
}

class PizzaBuilder {
    var size: Int = 0
    var cheese: Cheese?
    var pepperoni: Bool = false
    var mushrooms: Bool = false
}

struct Pizza {
    let size: Int
    let cheese: Cheese?
    let pepperoni: Bool
    let mushrooms: Bool

    init?(block: (PizzaBuilder -> Void)) {
        let builder = PizzaBuilder()
        block(builder)
        self.init(builder: builder)
    }

    init?(builder: PizzaBuilder) {
        if contains([12, 14, 16], builder.size) {
            self.size = builder.size
            self.cheese = builder.cheese
            self.pepperoni = builder.pepperoni
            self.mushrooms = builder.mushrooms
        } else {
            return nil
        }
    }

    func description() -> String {
        var description = "Pizza { size: \(self.size) "
        func add(property: Bool, name: String) {
            if property {
                description += " \(name)"
            }
        }
        if let cheese = self.cheese {
            description += " \(cheese.rawValue)"
        } else {
            description += " No cheese"
        }
        add(pepperoni, "pepperoni")
        add(mushrooms, "mushrooms")
        description += " }"
        return description
    }
}

Pizza() { builder in
    builder.size = 12
    builder.cheese = .Mozzarella
    builder.pepperoni = true
}

Pizza() {
    $0.size = 12
    $0.mushrooms = true
}

Pizza() {
    $0.cheese = .Provolone
    $0.pepperoni = true
}