How to implement the Builder pattern in Kotlin?

In Kotlin, the Builder pattern can be implemented using a class with an internal DSL or with a nested Builder class. The apply function is often used for convenient object configuration.

Example of a classic Builder with a nested class:

class User private constructor(
    val name: String?,
    val age: Int?
) {
    data class Builder(
        var name: String? = null,
        var age: Int? = null
    ) {
        fun name(name: String) = apply { this.name = name }
        fun age(age: Int) = apply { this.age = age }
        fun build() = User(name, age)
    }
}

// Usage:
val user = User.Builder()
    .name("Ivan")
    .age(30)
    .build()

You can also use a function with a lambda and apply for a more idiomatic Kotlin style:

data class User(var name: String? = null, var age: Int? = null)

fun user(block: User.() -> Unit) = User().apply(block)

// Usage:
val user = user {
    name = "Ivan"
    age = 30
}