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())
    }

// İyi: Varsayılan olarak non-nullable tipler kullan
fun getUser(id: String): User {
    return userRepository.findById(id)
        ?: throw UserNotFoundException("User $id not found")
}

// İyi: Safe call'lar ve Elvis operatörü
fun getUserEmail(userId: String): String {
    val user = userRepository.findById(userId)
    return user?.email ?: "unknown@example.com"
}

// Kötü: Nullable tipleri zorla açma
fun getUserEmail(userId: String): String {
    val user = userRepository.findById(userId)
    return user!!.email // null ise NPE fırlatır
}

// İyi: Immutable veri
data class User(
    val id: String,
    val name: String,
    val email: String,
)

// İyi: copy() ile dönüştürme
fun updateEmail(user: User, newEmail: String): User =
    user.copy(email = newEmail)

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

// Kötü: Mutable state
var currentUser: User? = null // Mutable global state'ten kaçın
val mutableUsers = mutableListOf<User>() // Gerçekten gerekmedikçe kaçın

// İyi: 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('@') }

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

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

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

// İyi: Tip güvenliği için value class (runtime'da sıfır maliyet)
@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)

// İyi: Exhaustive when için sealed class
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: Nullable'ı veya scope edilmiş sonucu dönüştür
val length: Int? = name?.let { it.trim().length }

// apply: Bir nesneyi yapılandır (nesneyi döndürür)
val user = User().apply {
    name = "Alice"
    email = "alice@example.com"
}

// also: Yan etkiler (nesneyi döndürür)
val user = createUser(request).also { logger.info("Created user: ${it.id}") }

// run: Receiver ile block çalıştır (sonucu döndürür)
val result = connection.run {
    prepareStatement(sql)
    executeQuery()
}

// with: run'ın extension olmayan formu
val csv = with(StringBuilder()) {
    appendLine("name,email")
    users.forEach { appendLine("${it.name},${it.email}") }
    toString()
}

// İyi: Domain'e özgü extension'lar
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()

// İyi: Koleksiyon extension'ları
fun <T> List<T>.second(): T = this[1]

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

// İyi: Scope edilmiş extension'lar (global namespace'i kirletmez)
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() }
}

// İyi: coroutineScope ile yapılandırılmış eşzamanlılık
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(),
        )
    }

// İyi: child'lar bağımsız başarısız olabildiğinde supervisorScope
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()
            },
        )
    }

// İyi: Uygun hata işleme ile cold flow
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())
}

// İyi: Flow operatörleri
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()) }