Kotlin Patterns

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

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>()}

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())    }

// Good: Use non-nullable types by defaultfun getUser(id: String): User {    return userRepository.findById(id)        ?: throw UserNotFoundException("User $id not found")}
// Good: Safe calls and Elvis operatorfun getUserEmail(userId: String): String {    val user = userRepository.findById(userId)    return user?.email ?: "[email protected]"}
// Bad: Force-unwrapping nullable typesfun getUserEmail(userId: String): String {    val user = userRepository.findById(userId)    return user!!.email // Throws NPE if null}

// Good: Immutable datadata class User(    val id: String,    val name: String,    val email: String,)
// Good: Transform with copy()fun updateEmail(user: User, newEmail: String): User =    user.copy(email = newEmail)
// Good: Immutable collectionsval users: List<User> = listOf(user1, user2)val filtered = users.filter { it.email.isNotBlank() }
// Bad: Mutable statevar currentUser: User? = null // Avoid mutable global stateval mutableUsers = mutableListOf<User>() // Avoid unless truly needed

// Good: Expression bodyfun isAdult(age: Int): Boolean = age >= 18
fun formatFullName(first: String, last: String): String =    "$first $last".trim()
fun User.displayName(): String =    name.ifBlank { email.substringBefore('@') }
// Good: When as expressionfun statusMessage(code: Int): String = when (code) {    200 -> "OK"    404 -> "Not Found"    500 -> "Internal Server Error"    else -> "Unknown status: $code"}
// Bad: Unnecessary block bodyfun isAdult(age: Int): Boolean {    return age >= 18}

// Good: Data class with copy, equals, hashCode, toStringdata class CreateUserRequest(    val name: String,    val email: String,    val role: Role = Role.USER,)
// Good: Value class for type safety (zero overhead at runtime)@JvmInlinevalue class UserId(val value: String) {    init {        require(value.isNotBlank()) { "UserId cannot be blank" }    }}
@JvmInlinevalue class Email(val value: String) {    init {        require('@' in value) { "Invalid email: $value" }    }}
fun getUser(id: UserId): User = userRepository.findById(id)

// Good: Sealed class for exhaustive whensealed 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 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}

// let: Transform nullable or scoped resultval length: Int? = name?.let { it.trim().length }
// apply: Configure an object (returns the object)val user = User().apply {    name = "Alice"    email = "[email protected]"}
// also: Side effects (returns the object)val user = createUser(request).also { logger.info("Created user: ${it.id}") }
// run: Execute a block with receiver (returns result)val result = connection.run {    prepareStatement(sql)    executeQuery()}
// with: Non-extension form of runval csv = with(StringBuilder()) {    appendLine("name,email")    users.forEach { appendLine("${it.name},${it.email}") }    toString()}

// Bad: Nesting scope functionsuser?.let { u ->    u.address?.let { addr ->        addr.city?.let { city ->            println(city) // Hard to read        }    }}
// Good: Chain safe calls insteadval city = user?.address?.citycity?.let { println(it) }

// Good: Domain-specific extensionsfun 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()
// Good: Collection extensionsfun <T> List<T>.second(): T = this[1]
fun <T> List<T>.secondOrNull(): T? = getOrNull(1)
// Good: Scoped extensions (not 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() }}

// Good: Structured concurrency with coroutineScopesuspend fun fetchUserWithPosts(userId: String): UserProfile =    coroutineScope {        val userDeferred = async { userService.getUser(userId) }        val postsDeferred = async { postService.getUserPosts(userId) }
        UserProfile(            user = userDeferred.await(),            posts = postsDeferred.await(),        )    }
// Good: supervisorScope when children can fail independentlysuspend fun fetchDashboard(userId: String): Dashboard =    supervisorScope {        val user = async { userService.getUser(userId) }        val notifications = async { notificationService.getRecent(userId) }        val recommendations = async { recommendationService.getFor(userId) }
        Dashboard(            user = user.await(),            notifications = try {                notifications.await()            } catch (e: CancellationException) {                throw e            } catch (e: Exception) {                emptyList()            },            recommendations = try {                recommendations.await()            } catch (e: CancellationException) {                throw e            } catch (e: Exception) {                emptyList()            },        )    }

// Good: Cold flow with proper error handlingfun observeUsers(): Flow<List<User>> = flow {    while (currentCoroutineContext().isActive) {        val users = userRepository.findAll()        emit(users)        delay(5.seconds)    }}.catch { e ->    logger.error("Error observing users", e)    emit(emptyList())}
// Good: Flow operatorsfun searchUsers(query: Flow<String>): Flow<List<User>> =    query        .debounce(300.milliseconds)        .distinctUntilChanged()        .filter { it.length >= 2 }        .mapLatest { q -> userRepository.search(q) }        .catch { emit(emptyList()) }

// Good: Respect cancellationsuspend fun processItems(items: List<Item>) {    items.forEach { item ->        ensureActive() // Check cancellation before expensive work        processItem(item)    }}
// Good: Cleanup with try/finallysuspend fun acquireAndProcess() {    val resource = acquireResource()    try {        resource.process()    } finally {        withContext(NonCancellable) {            resource.release() // Always release, even on cancellation        }    }}

// Lazy initializationval expensiveData: List<User> by lazy {    userRepository.findAll()}
// Observable propertyvar name: String by Delegates.observable("initial") { _, old, new ->    logger.info("Name changed from '$old' to '$new'")}
// Map-backed propertiesclass Config(private val map: Map<String, Any?>) {    val host: String by map    val port: Int by map    val debug: Boolean by map}
val config = Config(mapOf("host" to "localhost", "port" to 8080, "debug" to true))

// Good: Delegate interface implementationclass LoggingUserRepository(    private val delegate: UserRepository,    private val logger: Logger,) : UserRepository by delegate {    // Only override what you need to add logging to    override suspend fun findById(id: String): User? {        logger.info("Finding user by id: $id")        return delegate.findById(id).also {            logger.info("Found user: ${it?.name ?: "null"}")        }    }}

// Good: DSL with @DslMarker@DslMarkerannotation class HtmlDsl
@HtmlDslclass HTML {    private val children = mutableListOf<Element>()
    fun head(init: Head.() -> Unit) {        children += Head().apply(init)    }
    fun body(init: Body.() -> Unit) {        children += Body().apply(init)    }
    override fun toString(): String = children.joinToString("\n")}
fun html(init: HTML.() -> Unit): HTML = HTML().apply(init)
// Usageval page = html {    head { title("My Page") }    body {        h1("Welcome")        p("Hello, World!")    }}

data class ServerConfig(    val host: String = "0.0.0.0",    val port: Int = 8080,    val ssl: SslConfig? = null,    val database: DatabaseConfig? = null,)
data class SslConfig(val certPath: String, val keyPath: String)data class DatabaseConfig(val url: String, val maxPoolSize: Int = 10)
class ServerConfigBuilder {    var host: String = "0.0.0.0"    var port: Int = 8080    private var ssl: SslConfig? = null    private var database: DatabaseConfig? = null
    fun ssl(certPath: String, keyPath: String) {        ssl = SslConfig(certPath, keyPath)    }
    fun database(url: String, maxPoolSize: Int = 10) {        database = DatabaseConfig(url, maxPoolSize)    }
    fun build(): ServerConfig = ServerConfig(host, port, ssl, database)}
fun serverConfig(init: ServerConfigBuilder.() -> Unit): ServerConfig =    ServerConfigBuilder().apply(init).build()
// Usageval config = serverConfig {    host = "0.0.0.0"    port = 443    ssl("/certs/cert.pem", "/certs/key.pem")    database("jdbc:postgresql://localhost:5432/mydb", maxPoolSize = 20)}

// Good: Use sequences for large collections with multiple operationsval result = users.asSequence()    .filter { it.isActive }    .map { it.email }    .filter { it.endsWith("@company.com") }    .take(10)    .toList()
// Good: Generate infinite sequencesval fibonacci: Sequence<Long> = sequence {    var a = 0L    var b = 1L    while (true) {        yield(a)        val next = a + b        a = b        b = next    }}
val first20 = fibonacci.take(20).toList()

// Check for latest versions: https://kotlinlang.org/docs/releases.htmlplugins {    kotlin("jvm") version "2.3.10"    kotlin("plugin.serialization") version "2.3.10"    id("io.ktor.plugin") version "3.4.0"    id("org.jetbrains.kotlinx.kover") version "0.9.7"    id("io.gitlab.arturbosch.detekt") version "1.23.8"}
group = "com.example"version = "1.0.0"
kotlin {    jvmToolchain(21)}
dependencies {    // Ktor    implementation("io.ktor:ktor-server-core:3.4.0")    implementation("io.ktor:ktor-server-netty:3.4.0")    implementation("io.ktor:ktor-server-content-negotiation:3.4.0")    implementation("io.ktor:ktor-serialization-kotlinx-json:3.4.0")
    // Exposed    implementation("org.jetbrains.exposed:exposed-core:1.0.0")    implementation("org.jetbrains.exposed:exposed-dao:1.0.0")    implementation("org.jetbrains.exposed:exposed-jdbc:1.0.0")    implementation("org.jetbrains.exposed:exposed-kotlin-datetime:1.0.0")
    // Koin    implementation("io.insert-koin:koin-ktor:4.2.0")
    // Coroutines    implementation("org.jetbrains.kotlinx:kotlinx-coroutines-core:1.10.2")
    // Testing    testImplementation("io.kotest:kotest-runner-junit5:6.1.4")    testImplementation("io.kotest:kotest-assertions-core:6.1.4")    testImplementation("io.kotest:kotest-property:6.1.4")    testImplementation("io.mockk:mockk:1.14.9")    testImplementation("io.ktor:ktor-server-test-host:3.4.0")    testImplementation("org.jetbrains.kotlinx:kotlinx-coroutines-test:1.10.2")}
tasks.withType<Test> {    useJUnitPlatform()}
detekt {    config.setFrom(files("config/detekt/detekt.yml"))    buildUponDefaultConfig = true}

// Good: Use Kotlin's Result or a custom sealed classsuspend fun createUser(request: CreateUserRequest): Result<User> = runCatching {    require(request.name.isNotBlank()) { "Name cannot be blank" }    require('@' in request.email) { "Invalid email format" }
    val user = User(        id = UserId(UUID.randomUUID().toString()),        name = request.name,        email = Email(request.email),    )    userRepository.save(user)    user}
// Good: Chain resultsval displayName = createUser(request)    .map { it.name }    .getOrElse { "Unknown" }

// Good: Preconditions with clear messagesfun 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)}

// Good: Chained operationsval activeAdminEmails: List<String> = users    .filter { it.role == Role.ADMIN && it.isActive }    .sortedBy { it.name }    .map { it.email }
// Good: Grouping and aggregationval usersByRole: Map<Role, List<User>> = users.groupBy { it.role }
val oldestByRole: Map<Role, User?> = users.groupBy { it.role }    .mapValues { (_, users) -> users.minByOrNull { it.createdAt } }
// Good: Associate for map creationval usersById: Map<UserId, User> = users.associateBy { it.id }
// Good: Partition for splittingval (active, inactive) = users.partition { it.isActive }

// Bad: Force-unwrapping nullable typesval name = user!!.name
// Bad: Platform type leakage from Javafun getLength(s: String) = s.length // Safefun getLength(s: String?) = s?.length ?: 0 // Handle nulls from Java
// Bad: Mutable data classesdata class MutableUser(var name: String, var email: String)
// Bad: Using exceptions for control flowtry {    val user = findUser(id)} catch (e: NotFoundException) {    // Don't use exceptions for expected cases}
// Good: Use nullable return or Resultval user: User? = findUserOrNull(id)
// Bad: Ignoring coroutine scopeGlobalScope.launch { /* Avoid GlobalScope */ }
// Good: Use structured concurrencycoroutineScope {    launch { /* Properly scoped */ }}
// Bad: Deeply nested scope functionsuser?.let { u ->    u.address?.let { a ->        a.city?.let { c -> process(c) }    }}
// Good: Direct null-safe chainuser?.address?.city?.let { process(it) }