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())
    }

// Bien: Usar tipos no-nullable por defecto
fun getUser(id: String): User {
    return userRepository.findById(id)
        ?: throw UserNotFoundException("User $id not found")
}

// Bien: Llamadas seguras y operador Elvis
fun getUserEmail(userId: String): String {
    val user = userRepository.findById(userId)
    return user?.email ?: "unknown@example.com"
}

// Mal: Desempaquetar forzadamente tipos nullable
fun getUserEmail(userId: String): String {
    val user = userRepository.findById(userId)
    return user!!.email // Lanza NPE si es null
}

// Bien: Datos inmutables
data class User(
    val id: String,
    val name: String,
    val email: String,
)

// Bien: Transformar con copy()
fun updateEmail(user: User, newEmail: String): User =
    user.copy(email = newEmail)

// Bien: Colecciones inmutables
val users: List<User> = listOf(user1, user2)
val filtered = users.filter { it.email.isNotBlank() }

// Mal: Estado mutable
var currentUser: User? = null // Evitar estado global mutable
val mutableUsers = mutableListOf<User>() // Evitar a menos que sea realmente necesario

// Bien: Cuerpo de expresión
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('@') }

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

// Mal: Cuerpo de bloque innecesario
fun isAdult(age: Int): Boolean {
    return age >= 18
}

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

// Bien: Value class para type safety (cero overhead en tiempo de ejecución)
@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)

// Bien: Sealed class para when exhaustivo
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: Transformar resultado nullable o delimitado
val length: Int? = name?.let { it.trim().length }

// apply: Configurar un objeto (retorna el objeto)
val user = User().apply {
    name = "Alice"
    email = "alice@example.com"
}

// also: Efectos secundarios (retorna el objeto)
val user = createUser(request).also { logger.info("Created user: ${it.id}") }

// run: Ejecutar un bloque con receptor (retorna resultado)
val result = connection.run {
    prepareStatement(sql)
    executeQuery()
}

// with: Forma no-extensión de run
val csv = with(StringBuilder()) {
    appendLine("name,email")
    users.forEach { appendLine("${it.name},${it.email}") }
    toString()
}

// Mal: Anidar funciones de scope
user?.let { u ->
    u.address?.let { addr ->
        addr.city?.let { city ->
            println(city) // Difícil de leer
        }
    }
}

// Bien: Encadenar llamadas seguras en su lugar
val city = user?.address?.city
city?.let { println(it) }

// Bien: Extensiones específicas del dominio
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()

// Bien: Extensiones de colecciones
fun <T> List<T>.second(): T = this[1]

fun <T> List<T>.secondOrNull(): T? = getOrNull(1)

// Bien: Extensiones delimitadas (sin contaminar el namespace global)
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() }
}

// Bien: Concurrencia estructurada con 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(),
        )
    }

// Bien: supervisorScope cuando los hijos pueden fallar independientemente
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()
            },
        )
    }

// Bien: Flow frío con manejo de errores adecuado
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())
}

// Bien: Operadores de Flow
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()) }

// Bien: Respetar la cancelación
suspend fun processItems(items: List<Item>) {
    items.forEach { item ->
        ensureActive() // Verificar cancelación antes del trabajo costoso
        processItem(item)
    }
}

// Bien: Limpieza con try/finally
suspend fun acquireAndProcess() {
    val resource = acquireResource()
    try {
        resource.process()
    } finally {
        withContext(NonCancellable) {
            resource.release() // Siempre liberar, incluso al cancelar
        }
    }
}

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

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

// Propiedades respaldadas por Map
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))

// Bien: Delegar implementación de interfaz
class LoggingUserRepository(
    private val delegate: UserRepository,
    private val logger: Logger,
) : UserRepository by delegate {
    // Solo sobreescribir lo que necesitas agregar logging
    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"}")
        }
    }
}

// Bien: DSL con @DslMarker
@DslMarker
annotation class HtmlDsl

@HtmlDsl
class 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)

// Uso
val page = html {
    head { title("My Page") }
    body {
        h1("Welcome")
        p("Hello, World!")
    }
}