https://craftinginterpreters.com/representing-code.html#the-visitor-pattern

visitor_pattern.kt

abstract class Pastry {
    abstract fun accept(visitor: PastryVisitor)
}

class Beignet : Pastry() {
    override fun accept(visitor: PastryVisitor) {
        visitor.visitBeignet(this)
    }
}

class Cruller : Pastry() {
    override fun accept(visitor: PastryVisitor) {
        visitor.visitCruller(this)
    }
}

/**
 * Each operation that can be performed on pastries is a new class that implements that interface:
 *  - cooking
 *  - storing
 */
interface PastryVisitor {
    fun visitBeignet(beignet: Beignet)
    fun visitCruller(cruller: Cruller)
}

fun main() {
    val somePastry: Pastry = Beignet()

    val visitor = BreakfastVisitor()
    somePastry.accept(visitor)
}

// ------------

class BreakfastVisitor : PastryVisitor {
    override fun visitBeignet(beignet: Beignet) {
        println("visiting beignet!!!")
    }

    override fun visitCruller(cruller: Cruller) {
        println("visiting cruller!!!")
    }
}