Sample Kotlin DSL for making a pizza

PizzaDsl.kt

import kotlin.contracts.ExperimentalContracts
import kotlin.contracts.InvocationKind
import kotlin.contracts.contract

@Target(AnnotationTarget.CLASS, AnnotationTarget.TYPE)
@DslMarker
annotation class PizzaMarker

class Pizza(
    val name: String,
    val sauce: String,
    val toppings: Toppings
) {
    override fun toString() =
        "Name: $name\nSauce: $sauce\nToppings: $toppings\n\n"
}

@PizzaMarker
class Toppings {
    private val pizzaToppings: MutableList<String> = mutableListOf()

    fun cheese(): Toppings {
        pizzaToppings.add("Cheese")
        return this
    }

    fun pepperoni(): Toppings {
        pizzaToppings.add("Pepperoni")
        return this
    }

    fun mushrooms(): Toppings {
        pizzaToppings.add("Mushrooms")
        return this
    }

    override fun toString() = pizzaToppings.joinToString()
}

@PizzaMarker
class PizzaBuilder {
    var name: String = ""
    var sauce: String = ""
    var toppings: Toppings? = null

    fun build(): Pizza {
        if (toppings == null) throw IllegalArgumentException("Your pizza must have toppings")
        return Pizza(
            name = name,
            sauce = sauce,
            toppings = toppings!!
        )
    }
}

@OptIn(ExperimentalContracts::class)
fun makePizza(init: (@PizzaMarker PizzaBuilder).() -> Unit): Pizza {
    contract { callsInPlace(init, InvocationKind.EXACTLY_ONCE) }
    return PizzaBuilder().apply { init() }.build()
}

@OptIn(ExperimentalContracts::class)
fun PizzaBuilder.toppings(block: Toppings.() -> Toppings) {
    contract { callsInPlace(block, InvocationKind.EXACTLY_ONCE) }
    Toppings().apply { block() }.also { this.toppings = it }
}

fun main() {
    val cheesePizza = makePizza {
        name = "Cheese pizza"
        sauce = "Marinara"

        toppings {
            cheese()
        }
    }

    val pepperoniPizza = makePizza {
        name = "Pepperoni pizza"
        sauce = "Tomato"

        toppings {
//            toppings { // This violates the @PizzaMarker
//                pepperoni()
//            }
            cheese()
            pepperoni()
            mushrooms()
        }
//        cheese() // This violates encapsulation
    }

//    val veggiePizza = makePizza {
//        name = "Veggie pizza"
//        sauce = "Tomato"
//
//        // you have to have toppings
//    }

    println(cheesePizza)
    println(pepperoniPizza)
//    println(veggiePizza)
}