lidge-jun/kotlin-patterns

Kotlin Development Patterns

Idiomatic Kotlin patterns and best practices for building robust, efficient, and maintainable applications.

When to Use

• Writing new Kotlin code
• Reviewing Kotlin code
• Refactoring existing Kotlin code
• Designing Kotlin modules or libraries
• Configuring Gradle Kotlin DSL builds

How It Works

This skill enforces idiomatic Kotlin conventions across seven key areas: null safety using the type system and safe-call operators, immutability via val and copy() on data classes, sealed classes and interfaces for exhaustive type hierarchies, structured concurrency with coroutines and Flow, extension functions for adding behaviour without inheritance, type-safe DSL builders using @DslMarker and lambda receivers, and Gradle Kotlin DSL for build configuration.

Examples

Null safety with Elvis operator:

fun getUserEmail(userId: String): String {
    val user = userRepository.findById(userId)
    return user?.email ?: "[email protected]"
}

Sealed class for exhaustive results:

sealed class Result<out T> {
    data class Success<T>(val data: T) : Result<T>()
    data class Failure(val error: AppError) : Result<Nothing>()
    data object Loading : Result<Nothing>()
}

Structured concurrency with async/await:

suspend fun fetchUserWithPosts(userId: String): UserProfile =
    coroutineScope {
        val user = async { userService.getUser(userId) }
        val posts = async { postService.getUserPosts(userId) }
        UserProfile(user = user.await(), posts = posts.await())
    }

Core Principles

1. Null Safety

Kotlin's type system distinguishes nullable and non-nullable types. Use non-nullable types by default and handle nulls with safe calls and the Elvis operator.

// Prefer non-nullable types with fallback
fun getUser(id: String): User {
    return userRepository.findById(id)
        ?: throw UserNotFoundException("User $id not found")
}

// Use safe calls and Elvis operator
fun getUserEmail(userId: String): String {
    val user = userRepository.findById(userId)
    return user?.email ?: "[email protected]"
}

// Avoid force-unwrapping (can throw NPE)
val email = user?.email // Safe; returns null if user is null

2. Immutability by Default

Prefer val over var, immutable collections over mutable ones. Use copy() to create modified versions.

// Immutable data with copy()
data class User(val id: String, val name: String, val email: String)

fun updateEmail(user: User, newEmail: String): User =
    user.copy(email = newEmail)

// Prefer immutable collections
val users: List<User> = listOf(user1, user2)
val filtered = users.filter { it.email.isNotBlank() }

// Avoid mutable state and mutable collections unless necessary
// var currentUser: User? = null  // Don't do this
// val mutableUsers = mutableListOf<User>()  // Only when truly needed

3. Expression Bodies and Single-Expression Functions

Use expression bodies for concise, readable functions. Use when as an expression for exhaustive pattern matching.

// Expression body
fun isAdult(age: Int): Boolean = age >= 18
fun formatFullName(first: String, last: String): String = "$first $last".trim()
fun User.displayName(): String = name.ifBlank { email.substringBefore('@') }

// When as expression for exhaustive matching
fun statusMessage(code: Int): String = when (code) {
    200 -> "OK"
    404 -> "Not Found"
    500 -> "Internal Server Error"
    else -> "Unknown status: $code"
}

// Avoid unnecessary block bodies
// fun isAdult(age: Int): Boolean { return age >= 18 }  // Use expression instead

4. Data Classes and Value Classes

Use data classes for value objects. Use value classes (@JvmInline) for type-safe zero-overhead wrappers.

// Data class with auto-generated copy, equals, hashCode, toString
data class CreateUserRequest(
    val name: String,
    val email: String,
    val role: Role = Role.USER,
)

// Value class for type safety (zero runtime overhead)
@JvmInline
value class UserId(val value: String) {
    init { require(value.isNotBlank()) { "UserId cannot be blank" } }
}

@JvmInline
value class Email(val value: String) {
    init { require('@' in value) { "Invalid email: $value" } }
}

fun getUser(id: UserId): User = userRepository.findById(id)

Sealed Classes and Interfaces

Exhaustive Type Hierarchies

Use sealed classes for exhaustive pattern matching. Combine with when to ensure all cases are covered.

// Sealed class for exhaustive when expressions
sealed class Result<out T> {
    data class Success<T>(val data: T) : Result<T>()
    data class Failure(val error: AppError) : Result<Nothing>()
    data object Loading : Result<Nothing>()
}

fun <T> Result<T>.getOrNull(): T? = when (this) {
    is Result.Success -> data
    is Result.Failure -> null
    is Result.Loading -> null
}

fun <T> Result<T>.getOrThrow(): T = when (this) {
    is Result.Success -> data
    is Result.Failure -> throw error.toException()
    is Result.Loading -> throw IllegalStateException("Still loading")
}

Sealed Interfaces for API Responses

sealed interface ApiError {
    val message: String

    data class NotFound(override val message: String) : ApiError
    data class Unauthorized(override val message: String) : ApiError
    data class Validation(override val message: String, val field: String) : ApiError
    data class Internal(override val message: String, val cause: Throwable? = null) : ApiError
}

fun ApiError.toStatusCode(): Int = when (this) {
    is ApiError.NotFound -> 404
    is ApiError.Unauthorized -> 401
    is ApiError.Validation -> 422
    is ApiError.Internal -> 500
}

Scope Functions

When to Use Each

// let: Transform nullable or scoped result
val length: Int? = name?.let { it.trim().length }

// apply: Configure an object, returns the object itself
val user = User().apply {
    name = "Alice"
    email = "[email protected]"
}

// also: Side effects, returns the object itself
val user = createUser(request).also { logger.info("Created user: ${it.id}") }

// run: Execute a block with receiver, returns the result
val result = connection.run {
    prepareStatement(sql)
    executeQuery()
}

// with: Non-extension form of run
val csv = with(StringBuilder()) {
    appendLine("name,email")
    users.forEach { appendLine("${it.name},${it.email}") }
    toString()
}

Avoid Deeply Nested Scope Functions

Chain safe calls instead. Use let only when necessary.

// Prefer direct null-safe chain over nested lets
val city = user?.address?.city
city?.let { process(it) }

// Avoid: Nesting scope functions (hard to read)
// user?.let { u -> u.address?.let { a -> a.city?.let { c -> process(c) } } }

Extension Functions

Add behavior without inheritance. Keep extensions domain-specific and scoped when appropriate.

// Domain-specific extensions
fun String.toSlug(): String =
    lowercase()
        .replace(Regex("[^a-z0-9\\s-]"), "")
        .replace(Regex("\\s+"), "-")
        .trim('-')

fun Instant.toLocalDate(zone: ZoneId = ZoneId.systemDefault()): LocalDate =
    atZone(zone).toLocalDate()

// Collection extensions
fun <T> List<T>.second(): T = this[1]
fun <T> List<T>.secondOrNull(): T? = getOrNull(1)

// Scoped extensions (avoid polluting global namespace)
class UserService {
    private fun User.isActive(): Boolean =
        status == Status.ACTIVE && lastLogin.isAfter(Instant.now().minus(30, ChronoUnit.DAYS))

    fun getActiveUsers(): List<User> = userRepository.findAll().filter { it.isActive() }
}

Coroutines and Flow

Use coroutineScope and async/await for structured concurrency. Use Flow for cold, reactive streams with proper cancellation and error handling. See references/code-examples.md for detailed examples of structured concurrency, Flow operators, and cancellation patterns.

Delegation

Use by for property delegation (lazy, observable, map-backed) and interface delegation to reuse implementations.

// Lazy initialization
val expensiveData: List<User> by lazy { userRepository.findAll() }

// Observable property with logging
var name: String by Delegates.observable("initial") { _, old, new ->
    logger.info("Name changed from '$old' to '$new'")
}

// Interface delegation for wrapper classes
class LoggingUserRepository(
    private val delegate: UserRepository,
) : UserRepository by delegate {
    override suspend fun findById(id: String): User? {
        logger.info("Finding user: $id")
        return delegate.findById(id).also { logger.info("Found: ${it?.name ?: "null"}") }
    }
}

See references/code-examples.md for more delegation patterns.

DSL Builders

Create type-safe domain-specific languages using @DslMarker and lambda receivers. Builders provide readable, fluent syntax.

// Type-safe builder with @DslMarker
@DslMarker
annotation class ServerConfigDsl

@ServerConfigDsl
class ServerConfigBuilder {
    var host: String = "0.0.0.0"
    var port: Int = 8080

    fun build(): ServerConfig = ServerConfig(host, port)
}

fun serverConfig(init: ServerConfigBuilder.() -> Unit): ServerConfig =
    ServerConfigBuilder().apply(init).build()

// Usage: fluent, type-safe configuration
val config = serverConfig {
    host = "127.0.0.1"
    port = 443
}

See references/code-examples.md for HTML DSL and configuration DSL examples.

Sequences for Lazy Evaluation

Use sequences for large collections with multiple chained operations to avoid creating intermediate lists. See references/code-examples.md for sequence examples and infinite sequence patterns.

Gradle Kotlin DSL

Use Kotlin DSL in build.gradle.kts for type-safe build configuration. See references/code-examples.md for a complete example with Ktor, Exposed, Koin, and testing dependencies.

Error Handling Patterns

Use Result<T> or custom sealed classes for domain operations. Use require() and check() for preconditions.

// Use Result<T> for recoverable errors
suspend fun createUser(request: CreateUserRequest): Result<User> = runCatching {
    require(request.name.isNotBlank()) { "Name cannot be blank" }
    require('@' in request.email) { "Invalid email format" }
    userRepository.save(User(id = UUID.randomUUID().toString(), name = request.name, email = request.email))
}

val displayName = createUser(request)
    .map { it.name }
    .getOrElse { "Unknown" }

// Preconditions and postconditions
fun withdraw(account: Account, amount: Money): Account {
    require(amount.value > 0) { "Amount must be positive: $amount" }
    check(account.balance >= amount) { "Insufficient balance: ${account.balance} < $amount" }
    return account.copy(balance = account.balance - amount)
}

Collection Operations

Key patterns for filtering, grouping, aggregation, and association. See references/code-examples.md for advanced examples.

// Chained operations with filter, map, sort
val activeAdminEmails: List<String> = users
    .filter { it.role == Role.ADMIN && it.isActive }
    .sortedBy { it.name }
    .map { it.email }

// Grouping and aggregation
val usersByRole: Map<Role, List<User>> = users.groupBy { it.role }
val oldestByRole: Map<Role, User?> = users.groupBy { it.role }
    .mapValues { (_, users) -> users.minByOrNull { it.createdAt } }

// Associate for efficient map creation
val usersById: Map<UserId, User> = users.associateBy { it.id }

// Partition for splitting into two groups
val (active, inactive) = users.partition { it.isActive }

Quick Reference: Kotlin Idioms

| Idiom | Description | |-------|-------------| | val over var | Prefer immutable variables | | data class | For value objects with equals/hashCode/copy | | sealed class/interface | For restricted type hierarchies | | value class | For type-safe wrappers with zero overhead | | Expression when | Exhaustive pattern matching | | Safe call ?. | Null-safe member access | | Elvis ?: | Default value for nullables | | let/apply/also/run/with | Scope functions for clean code | | Extension functions | Add behavior without inheritance | | copy() | Immutable updates on data classes | | require/check | Precondition assertions | | Coroutine async/await | Structured concurrent execution | | Flow | Cold reactive streams | | sequence | Lazy evaluation | | Delegation by | Reuse implementation without inheritance |

Anti-Patterns to Avoid

// Avoid: Force-unwrapping nullable types
// val name = user!!.name  // Can throw NPE

// Prefer: Safe calls and Elvis operator
val name = user?.name ?: "Unknown"

// Avoid: Mutable data classes
// data class MutableUser(var name: String, var email: String)

// Prefer: Use copy() for updates
val updated = user.copy(name = "New Name")

// Avoid: Using exceptions for control flow
// try { val user = findUser(id) } catch (e: NotFoundException) { }

// Prefer: Nullable return or Result type
val user: User? = findUserOrNull(id)
val result: Result<User> = runCatching { findUser(id) }

// Avoid: GlobalScope for coroutines
// GlobalScope.launch { }

// Prefer: Structured concurrency with proper scope
coroutineScope { launch { } }

// Avoid: Deeply nested scope functions
// user?.let { u -> u.address?.let { a -> a.city?.let { c -> process(c) } } }

// Prefer: Direct null-safe chains
user?.address?.city?.let { process(it) }