Arrow Optics Example - https://arrow-kt.io/docs/optics/lens/

build.gradle

plugins {
    id 'org.jetbrains.kotlin.jvm' version '1.3.41'
    id "org.jetbrains.kotlin.kapt" version "1.3.41"
}

group 'com.caseykulm.learnyouanarrow'
version '1.0-SNAPSHOT'

def arrow_version = "0.10.0-SNAPSHOT"

repositories {
    mavenCentral()
    maven { url "https://dl.bintray.com/arrow-kt/arrow-kt/" }
    maven { url 'https://oss.jfrog.org/artifactory/oss-snapshot-local/' }
}

dependencies {
    implementation "org.jetbrains.kotlin:kotlin-stdlib-jdk8"
    implementation "io.arrow-kt:arrow-core:$arrow_version"
    implementation "io.arrow-kt:arrow-syntax:$arrow_version"
    implementation "io.arrow-kt:arrow-optics:$arrow_version"
    kapt    "io.arrow-kt:arrow-meta:$arrow_version"

    testImplementation('org.junit.jupiter:junit-jupiter:5.5.1')
    testImplementation "org.jetbrains.kotlin:kotlin-test"
    testImplementation "org.jetbrains.kotlin:kotlin-test-junit"
}

compileKotlin {
    kotlinOptions.jvmTarget = "1.8"
}
compileTestKotlin {
    kotlinOptions.jvmTarget = "1.8"
}

JustKotlinThings.kt

import arrow.optics.Lens
import arrow.optics.optics

@optics data class Street(val number: Int, val name: String) { companion object }
@optics data class Address(val city: String, val street: Street) { companion object }
@optics data class Company(val name: String, val address: Address) { companion object }
@optics data class Employee(val name: String, val company: Company) { companion object }

fun main() {
    withCopy()
//    withArrowOptics()
}

private fun createEmployee(): Employee {
    return Employee(
        name = "John Doe",
        company = Company(
            name = "Arrow",
            address = Address(
                city = "Functional city",
                street = Street(
                    number = 23,
                    name = "lambda street"
                )
            )
        )
    )
}

private fun withCopy() {
    var employee = createEmployee()

    println("Employee Before: $employee")

    employee = employee.copy(
        company = employee.company.copy(
            address = employee.company.address.copy(
                street = employee.company.address.street.copy(
                    name = employee.company.address.street.name.capitalize()
                )
            )
        )
    )

    println("Employee After: $employee")
}

private fun withArrowOptics() {
    var employee = createEmployee()

    println("Employee Before: $employee")

    val employeeStreetName: Lens<Employee, String> =
        Employee.company compose Company.address compose Address.street compose Street.name

    employee = employeeStreetName.modify(employee, String::capitalize)

    println("Employee After: $employee")
}