nw-fp-kotlin

FP in Kotlin -- Functional Software Crafter Skill

Cross-references: fp-principles | fp-domain-modeling | property-based-testing

When to Choose Kotlin

• Best for: gentlest FP onboarding for Java teams | Android | coroutines | pragmatic FP with great IDE support
• Not ideal for: teams wanting compiler-enforced purity | full effect systems | higher-kinded type abstractions

[STARTER] Quick Setup

mkdir order-service && cd order-service
gradle init --type kotlin-application --dsl kotlin
# Add arrow-core, arrow-fx-coroutines, kotest-property to build.gradle.kts
./gradlew build && ./gradlew test

[STARTER] Type System for Domain Modeling

Choice Types (Sealed Hierarchies)

sealed interface PaymentMethod {
    data class CreditCard(val cardNumber: String, val expiryDate: String) : PaymentMethod
    data class BankTransfer(val accountNumber: String) : PaymentMethod
    data object Cash : PaymentMethod
}

Exhaustive when expressions ensure all cases handled.

Record Types and Domain Wrappers

data class Customer(
    val customerId: CustomerId,
    val customerName: CustomerName,
    val customerEmail: EmailAddress
)

@JvmInline
value class OrderId(val value: Int)

@JvmInline
value class EmailAddress private constructor(val value: String) {
    companion object {
        fun from(raw: String): Either<ValidationError, EmailAddress> =
            if ("@" in raw) EmailAddress(raw).right()
            else InvalidEmail(raw).left()
    }
}

Value classes inlined by compiler -- zero allocation overhead.

[INTERMEDIATE] Validated Construction with Raise DSL

import arrow.core.raise.Raise
import arrow.core.raise.ensure

context(Raise<ValidationError>)
fun validateEmail(raw: String): EmailAddress {
    ensure(raw.contains("@")) { InvalidEmail(raw) }
    return EmailAddress.unsafeCreate(raw)
}