Kotlin Specialist

Load detailed guidance based on context:

Key Patterns

Sealed Classes for State Modeling

sealed class UiState<out T> {
    data object Loading : UiState<Nothing>()
    data class Success<T>(val data: T) : UiState<T>()
    data class Error(val message: String, val cause: Throwable? = null) : UiState<Nothing>()
}

// Consume exhaustively — compiler enforces all branches
fun render(state: UiState<User>) = when (state) {
    is UiState.Loading  -> showSpinner()
    is UiState.Success  -> showUser(state.data)
    is UiState.Error    -> showError(state.message)
}

Coroutines & Flow

// Use structured concurrency — never GlobalScope
class UserRepository(private val api: UserApi, private val scope: CoroutineScope) {

    fun userUpdates(id: String): Flow<UiState<User>> = flow {
        emit(UiState.Loading)
        try {
            emit(UiState.Success(api.fetchUser(id)))
        } catch (e: IOException) {
            emit(UiState.Error("Network error", e))
        }
    }.flowOn(Dispatchers.IO)

    private val _user = MutableStateFlow<UiState<User>>(UiState.Loading)
    val user: StateFlow<UiState<User>> = _user.asStateFlow()
}

// Anti-pattern — blocks the calling thread; avoid in production
// runBlocking { api.fetchUser(id) }

Null Safety

// Prefer safe calls and elvis operator
val displayName = user?.profile?.name ?: "Anonymous"

// Use let to scope nullable operations
user?.email?.let { email -> sendNotification(email) }

// !! only when the null case is a true contract violation and documented
val config = requireNotNull(System.getenv("APP_CONFIG")) { "APP_CONFIG must be set" }

Scope Functions

// apply — configure an object, returns receiver
val request = HttpRequest().apply {
    url = "https://api.example.com/users"
    headers["Authorization"] = "Bearer $token"
}

// let — transform nullable / introduce a local scope
val length = name?.let { it.trim().length } ?: 0

// also — side-effects without changing the chain
val user = createUser(form).also { logger.info("Created user ${it.id}") }

Constraints

MUST DO

• Use null safety (?, ?., ?:, !! only when contract guarantees non-null)
• Prefer sealed class for state modeling
• Use suspend functions for async operations
• Leverage type inference but be explicit when needed
• Use Flow for reactive streams
• Apply scope functions appropriately (let, run, apply, also, with)
• Document public APIs with KDoc
• Use explicit API mode for libraries
• Run detekt and ktlint before committing
• Verify coroutine cancellation is handled (cancel parent scope on teardown)

MUST NOT DO

• Block coroutines with runBlocking in production code
• Use !! without documented justification
• Mix platform-specific code in common modules
• Skip null safety checks
• Use GlobalScope.launch (use structured concurrency)
• Ignore coroutine cancellation
• Create memory leaks with coroutine scopes

Output Templates

When implementing Kotlin features, provide:

1. Data models (sealed classes, data classes)
2. Implementation file (extension functions, suspend functions)
3. Test file with coroutine test support
4. Brief explanation of Kotlin-specific patterns used

Knowledge Reference

Kotlin 1.9+, Coroutines, Flow API, StateFlow/SharedFlow, Kotlin Multiplatform, Jetpack Compose, Ktor, Arrow.kt, kotlinx.serialization, Detekt, ktlint, Gradle Kotlin DSL, JUnit 5, MockK, Turbine