kotlin

name	kotlin
description	Kotlin conventions with null safety, coroutines, and functional patterns. Trigger: When working with .kt files, coroutines, or Kotlin-specific patterns.
license	Apache-2.0
allowed-tools	Read, Edit, Write, Glob, Grep, Bash
metadata	{"author":"@dallay","version":"1.0"}

Kotlin Skill

Conventions for writing idiomatic, safe, and maintainable Kotlin code.

When to Use

• Creating or modifying .kt files
• Working with coroutines and Flow
• Implementing domain models and services
• Writing unit tests with Kotest

Critical Patterns

1. Null Safety - NEVER Use !!

STRICTLY AVOID the !! operator. Use safe alternatives:

// ❌ NEVER do this
val name = user!!.name

// ✅ Safe call with elvis
val name = user?.name ?: "Unknown"

// ✅ requireNotNull for asserting
val name = requireNotNull(user?.name) { "User name is required" }

// ✅ let for scoped operations
user?.let {
    sendEmail(it.email)
}

// ✅ takeIf/takeUnless for conditional
val activeUser = user.takeIf { it.isActive }

2. Data Classes for Models

ALWAYS use data classes for immutable models:

data class User(
    val id: UserId,
    val email: Email,
    val name: String,
    val isActive: Boolean = true,
    val createdAt: Instant = Instant.now(),
)

// ✅ Value classes for type safety
@JvmInline
value class UserId(val value: UUID)

@JvmInline
value class Email(val value: String) {
    init {
        require(value.contains("@")) { "Invalid email format" }
    }
}

3. Sealed Classes for State

Use sealed classes for restricted hierarchies:

sealed interface UiState<out T> {
    data class Success<T>(val data: T) : UiState<T>
    data class Failure(val error: DomainError) : UiState<Nothing>
    data object Loading : UiState<Nothing>
}

// Usage with when (exhaustive)
fun handleState(state: UiState<User>) = when (state) {
    is UiState.Success -> displayUser(state.data)
    is UiState.Failure -> showError(state.error)
    UiState.Loading -> showSpinner()
}

// Domain errors
sealed interface DomainError {
    data class NotFound(val id: String) : DomainError
    data class Validation(val field: String, val message: String) : DomainError
    data object Unauthorized : DomainError
}

4. Imports and Fully-Qualified Names

STRICTLY AVOID inline fully-qualified class or static references. Favor top-level imports and short names.

// ❌ NEVER do this (inline FQCN)
val id = java.util.UUID.randomUUID()
fun update(@io.swagger.v3.oas.annotations.parameters.RequestBody req: Request)

// ✅ ALWAYS do this
import java.util.UUID
import io.swagger.v3.oas.annotations.parameters.RequestBody

val id = UUID.randomUUID()
fun update(@RequestBody req: Request)

Exception: Fully-qualified names are ALLOWED and encouraged in KDoc for unambiguous linking.

/**
 * Processes a [com.example.project.domain.Entity].
 */

See no-fully-qualified-references.md for detailed rules.

Testing with Kotest

This project uses Kotest for expressive, idiomatic Kotlin tests.

Spec Styles: Prefer FunSpec for simple tests, DescribeSpec for BDD-style grouping:

// FunSpec - simple flat structure
class UserServiceTest : FunSpec({
    val repository = mockk<UserRepository>()
    val service = UserService(repository)

    test("should create user with valid data") {
        coEvery { repository.save(any()) } returns testUser

        val result = service.create(validUserData)

        result.shouldNotBeNull()
        result.email shouldBe validUserData.email
    }

    test("should throw when email already exists") {
        coEvery { repository.findByEmail(any()) } returns existingUser

        shouldThrow<ConflictException> {
            service.create(validUserData)
        }.message shouldContain "already exists"
    }

})

// DescribeSpec - BDD-style grouping
class EmailValueObjectTest : DescribeSpec({
    describe("Email") {
        context("when created with valid format") {
            it("should create successfully") {
                val email = Email("[e-mail]")
                email.value shouldBe "[e-mail]"
            }
        }

        context("when created with invalid format") {
            it("should throw IllegalArgumentException") {
                shouldThrow<IllegalArgumentException> {
                    Email("invalid-email")
                }
            }
        }
    }
})

Common Matchers:

// Equality and nullability
result shouldBe expected
result shouldNotBe null
result.shouldNotBeNull()
result.shouldBeNull()

// Collections
list shouldHaveSize 3
list shouldContain element
list.shouldContainAll(a, b, c)
list.shouldBeEmpty()

// Exceptions
shouldThrow<NotFoundException> { service.findById(unknownId) }
shouldNotThrow { service.findById(validId) }

// String matchers
name shouldStartWith "John"
error.message shouldContain "not found"

5. Error Handling with Result

Prefer Result<T> over exceptions for business logic:

// ✅ Return Result for operations that can fail
fun findUser(id: UserId): Result<User> = runCatching {
    userRepository.findById(id)
        ?: throw NotFoundException("User not found")
}

// ✅ Chain operations
fun processUser(id: UserId): Result<ProcessedUser> {
    return findUser(id)
        .mapCatching { user -> validate(user) }
        .mapCatching { validated -> enrich(validated) }
        .onFailure { logger.error(it) { "Failed to process user $id" } }
}

// ✅ Handle result
findUser(userId).fold(
    onSuccess = { user -> Response.ok(user) },
    onFailure = { error -> Response.error(error.message) },
)

6. Coroutines and Flow

Embrace structured concurrency:

// ✅ Suspend functions for async operations
suspend fun fetchUser(id: UserId): User {
    return withContext(Dispatchers.IO) {
        userRepository.findById(id)
    }
}

// ✅ Flow for streams
fun observeUsers(): Flow<List<User>> = flow {
    while (true) {
        emit(userRepository.findAll())
        delay(5.seconds)
    }
}.flowOn(Dispatchers.IO)

// ✅ Proper scope management
class UserService(
    private val scope: CoroutineScope,
) {
    fun startSync() {
        scope.launch {
            observeUsers().collect { users ->
                processUsers(users)
            }
        }
    }
}

7. Extension Functions

Use for enhancing existing types:

// ✅ Domain-specific extensions
fun String.toSlug(): String =
    lowercase()
        .replace(Regex("[^a-z0-9\\s-]"), "")
        .replace(Regex("\\s+"), "-")

fun Instant.isRecent(threshold: Duration = 24.hours): Boolean =
    this.isAfter(Instant.now().minus(threshold))

// ✅ Null-safe extensions
fun String?.orEmpty(): String = this ?: ""
fun <T> List<T>?.orEmpty(): List<T> = this ?: emptyList()

Naming Conventions

Element	Convention	Example
Classes/Interfaces	PascalCase	UserService, WorkspaceRepository
Functions/Variables	camelCase	findById, userName
Constants	UPPER_SNAKE_CASE	MAX_RETRY_COUNT, DEFAULT_TIMEOUT
Test Methods	Backticks	`should return user when exists`
Booleans	is, has, are prefix	isActive, hasPermission, areValid

Code Style

// ✅ Expression body for simple functions
fun double(x: Int): Int = x * 2

// ✅ Trailing commas (helps with diffs)
data class Config(
    val host: String,
    val port: Int,
    val timeout: Duration,  // ← trailing comma
)

// ✅ Named arguments for 3+ parameters
createUser(
    name = "John",
    email = "[e-mail]",
    role = Role.USER,
)

// ✅ Prefer val over var
val items = mutableListOf<Item>()  // val for reference, mutable for content

Logging

import io.github.oshai.kotlinlogging.KotlinLogging

private val logger = KotlinLogging.logger {}

class UserService {
    fun processUser(user: User) {
        logger.info { "Processing user: ${user.id}" }

        try {
            // logic
        } catch (e: ProcessingException) {
            logger.error(e) { "Failed to process user: ${user.id}" }
            throw e
        }
    }
}

// ❌ NEVER log sensitive data
logger.info { "User logged in: password=${user.password}" }  // WRONG!

Anti-Patterns

❌ !! operator - Use ?., ?:, requireNotNull, let ❌ Catching generic Exception - Catch specific exceptions ❌ var when val works - Prefer immutability ❌ Inheritance over composition - Prefer composition ❌ Wildcard imports - Except java.util.*, io.mockk.* ❌ Mutable public properties - Use private set or immutable data

Rules

• No Fully Qualified References: Avoid inline fully-qualified names; favor imports.

Commands

# Run tests
./gradlew test

# Run specific test class
./gradlew test --tests "com.example.project.user.UserServiceTest"

# Run tests by tag
./gradlew test -PincludeTags=unit

# Lint with Detekt
./gradlew detektAll

# Build
./gradlew build

Resources

• Kotlin Coding Conventions
• Kotlin Coroutines Guide
• Kotlin Flow