Kotlin Patterns

fun getUserEmail(userId: String): String {
    val user = userRepository.findById(userId)
    return user?.email ?: "unknown@example.com"
}

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 default
fun getUser(id: String): User {
    return userRepository.findById(id)
        ?: throw UserNotFoundException("User $id not found")
}

// Good: Safe calls and Elvis operator
fun getUserEmail(userId: String): String {
    val user = userRepository.findById(userId)
    return user?.email ?: "unknown@example.com"
}

// Bad: Force-unwrapping nullable types
fun getUserEmail(userId: String): String {
    val user = userRepository.findById(userId)
    return user!!.email // Throws NPE if null
}

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

// Bad: Mutable state
var currentUser: User? = null // Avoid mutable global state
val mutableUsers = mutableListOf<User>() // Avoid unless truly needed

// Good: 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('@') }

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

// Bad: Unnecessary block body
fun isAdult(age: Int): Boolean {
    return age >= 18
}

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

// Good: Value class for type safety (zero overhead at runtime)
@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)

// Good: Sealed class for exhaustive when
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 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 result
val length: Int? = name?.let { it.trim().length }

// apply: Configure an object (returns the object)
val user = User().apply {
    name = "Alice"
    email = "alice@example.com"
}

// 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 run
val csv = with(StringBuilder()) {
    appendLine("name,email")
    users.forEach { appendLine("${it.name},${it.email}") }
    toString()
}

// Bad: Nesting scope functions
user?.let { u ->
    u.address?.let { addr ->
        addr.city?.let { city ->
            println(city) // Hard to read
        }
    }
}

// Good: Chain safe calls instead
val city = user?.address?.city
city?.let { println(it) }

// Good: 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()

// Good: Collection extensions
fun <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 coroutineScope
suspend 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 independently
suspend 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 handling
fun 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 operators
fun 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 cancellation
suspend fun processItems(items: List<Item>) {
    items.forEach { item ->
        ensureActive() // Check cancellation before expensive work
        processItem(item)
    }
}

// Good: Cleanup with try/finally
suspend fun acquireAndProcess() {
    val resource = acquireResource()
    try {
        resource.process()
    } finally {
        withContext(NonCancellable) {
            resource.release() // Always release, even on cancellation
        }
    }
}

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

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

// Map-backed properties
class 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 implementation
class 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"}")
        }
    }
}