feat: add kotlin commands and skill pack

2026-04-10 11:23:32 +08:00 · 2026-03-10 21:25:52 -07:00
parent 82f9f58d28
commit 135eb4c98d
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--- a/skills/kotlin-patterns/SKILL.md
+++ b/skills/kotlin-patterns/SKILL.md
@@ -0,0 +1,711 @@
+---
+name: kotlin-patterns
+description: Idiomatic Kotlin patterns, best practices, and conventions for building robust, efficient, and maintainable Kotlin applications with coroutines, null safety, and DSL builders.
+origin: ECC
+---
+
+# Kotlin Development Patterns
+
+Idiomatic Kotlin patterns and best practices for building robust, efficient, and maintainable applications.
+
+## When to Use
+
+- Writing new Kotlin code
+- Reviewing Kotlin code
+- Refactoring existing Kotlin code
+- Designing Kotlin modules or libraries
+- Configuring Gradle Kotlin DSL builds
+
+## How It Works
+
+This skill enforces idiomatic Kotlin conventions across seven key areas: null safety using the type system and safe-call operators, immutability via `val` and `copy()` on data classes, sealed classes and interfaces for exhaustive type hierarchies, structured concurrency with coroutines and `Flow`, extension functions for adding behaviour without inheritance, type-safe DSL builders using `@DslMarker` and lambda receivers, and Gradle Kotlin DSL for build configuration.
+
+## Examples
+
+**Null safety with Elvis operator:**
+```kotlin
+fun getUserEmail(userId: String): String {
+    val user = userRepository.findById(userId)
+    return user?.email ?: "unknown@example.com"
+}
+```
+
+**Sealed class for exhaustive results:**
+```kotlin
+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>()
+}
+```
+
+**Structured concurrency with async/await:**
+```kotlin
+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())
+    }
+```
+
+## Core Principles
+
+### 1. Null Safety
+
+Kotlin's type system distinguishes nullable and non-nullable types. Leverage it fully.
+
+```kotlin
+// Good: Use non-nullable types by default
+fun getUser(id: String): User {
+    return userRepository.findById(id)
+        ?: throw UserNotFoundException("User $id not found")
+}
+
+// Good: Safe calls and Elvis operator
+fun getUserEmail(userId: String): String {
+    val user = userRepository.findById(userId)
+    return user?.email ?: "unknown@example.com"
+}
+
+// Bad: Force-unwrapping nullable types
+fun getUserEmail(userId: String): String {
+    val user = userRepository.findById(userId)
+    return user!!.email // Throws NPE if null
+}
+```
+
+### 2. Immutability by Default
+
+Prefer `val` over `var`, immutable collections over mutable ones.
+
+```kotlin
+// Good: Immutable data
+data class User(
+    val id: String,
+    val name: String,
+    val email: String,
+)
+
+// Good: Transform with copy()
+fun updateEmail(user: User, newEmail: String): User =
+    user.copy(email = newEmail)
+
+// Good: Immutable collections
+val users: List<User> = listOf(user1, user2)
+val filtered = users.filter { it.email.isNotBlank() }
+
+// Bad: Mutable state
+var currentUser: User? = null // Avoid mutable global state
+val mutableUsers = mutableListOf<User>() // Avoid unless truly needed
+```
+
+### 3. Expression Bodies and Single-Expression Functions
+
+Use expression bodies for concise, readable functions.
+
+```kotlin
+// Good: Expression body
+fun isAdult(age: Int): Boolean = age >= 18
+
+fun formatFullName(first: String, last: String): String =
+    "$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
+}
+```
+
+### 4. Data Classes for Value Objects
+
+Use data classes for types that primarily hold data.
+
+```kotlin
+// 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)
+```
+
+## Sealed Classes and Interfaces
+
+### Modeling Restricted Hierarchies
+
+```kotlin
+// 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 Interfaces for API Responses
+
+```kotlin
+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
+}
+```
+
+## Scope Functions
+
+### When to Use Each
+
+```kotlin
+// 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()
+}
+```
+
+### Anti-Patterns
+
+```kotlin
+// 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) }
+```
+
+## Extension Functions
+
+### Adding Functionality Without Inheritance
+
+```kotlin
+// 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() }
+}
+```
+
+## Coroutines
+
+### Structured Concurrency
+
+```kotlin
+// 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()
+            },
+        )
+    }
+```
+
+### Flow for Reactive Streams
+
+```kotlin
+// 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()) }
+```
+
+### Cancellation and Cleanup
+
+```kotlin
+// 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
+        }
+    }
+}
+```
+
+## Delegation
+
+### Property Delegation
+
+```kotlin
+// 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))
+```
+
+### Interface Delegation
+
+```kotlin
+// 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"}")
+        }
+    }
+}
+```
+
+## DSL Builders
+
+### Type-Safe Builders
+
+```kotlin
+// Good: DSL with @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)
+
+// Usage
+val page = html {
+    head { title("My Page") }
+    body {
+        h1("Welcome")
+        p("Hello, World!")
+    }
+}
+```
+
+### Configuration DSL
+
+```kotlin
+data class ServerConfig(
+    val host: String = "0.0.0.0",
+    val port: Int = 8080,
+    val ssl: SslConfig? = null,
+    val database: DatabaseConfig? = null,
+)
+
+data class SslConfig(val certPath: String, val keyPath: String)
+data class DatabaseConfig(val url: String, val maxPoolSize: Int = 10)
+
+class ServerConfigBuilder {
+    var host: String = "0.0.0.0"
+    var port: Int = 8080
+    private var ssl: SslConfig? = null
+    private var database: DatabaseConfig? = null
+
+    fun ssl(certPath: String, keyPath: String) {
+        ssl = SslConfig(certPath, keyPath)
+    }
+
+    fun database(url: String, maxPoolSize: Int = 10) {
+        database = DatabaseConfig(url, maxPoolSize)
+    }
+
+    fun build(): ServerConfig = ServerConfig(host, port, ssl, database)
+}
+
+fun serverConfig(init: ServerConfigBuilder.() -> Unit): ServerConfig =
+    ServerConfigBuilder().apply(init).build()
+
+// Usage
+val config = serverConfig {
+    host = "0.0.0.0"
+    port = 443
+    ssl("/certs/cert.pem", "/certs/key.pem")
+    database("jdbc:postgresql://localhost:5432/mydb", maxPoolSize = 20)
+}
+```
+
+## Sequences for Lazy Evaluation
+
+```kotlin
+// Good: Use sequences for large collections with multiple operations
+val result = users.asSequence()
+    .filter { it.isActive }
+    .map { it.email }
+    .filter { it.endsWith("@company.com") }
+    .take(10)
+    .toList()
+
+// Good: Generate infinite sequences
+val fibonacci: Sequence<Long> = sequence {
+    var a = 0L
+    var b = 1L
+    while (true) {
+        yield(a)
+        val next = a + b
+        a = b
+        b = next
+    }
+}
+
+val first20 = fibonacci.take(20).toList()
+```
+
+## Gradle Kotlin DSL
+
+### build.gradle.kts Configuration
+
+```kotlin
+// Check for latest versions: https://kotlinlang.org/docs/releases.html
+plugins {
+    kotlin("jvm") version "2.3.10"
+    kotlin("plugin.serialization") version "2.3.10"
+    id("io.ktor.plugin") version "3.4.0"
+    id("org.jetbrains.kotlinx.kover") version "0.9.7"
+    id("io.gitlab.arturbosch.detekt") version "1.23.8"
+}
+
+group = "com.example"
+version = "1.0.0"
+
+kotlin {
+    jvmToolchain(21)
+}
+
+dependencies {
+    // Ktor
+    implementation("io.ktor:ktor-server-core:3.4.0")
+    implementation("io.ktor:ktor-server-netty:3.4.0")
+    implementation("io.ktor:ktor-server-content-negotiation:3.4.0")
+    implementation("io.ktor:ktor-serialization-kotlinx-json:3.4.0")
+
+    // Exposed
+    implementation("org.jetbrains.exposed:exposed-core:1.0.0")
+    implementation("org.jetbrains.exposed:exposed-dao:1.0.0")
+    implementation("org.jetbrains.exposed:exposed-jdbc:1.0.0")
+    implementation("org.jetbrains.exposed:exposed-kotlin-datetime:1.0.0")
+
+    // Koin
+    implementation("io.insert-koin:koin-ktor:4.2.0")
+
+    // Coroutines
+    implementation("org.jetbrains.kotlinx:kotlinx-coroutines-core:1.10.2")
+
+    // Testing
+    testImplementation("io.kotest:kotest-runner-junit5:6.1.4")
+    testImplementation("io.kotest:kotest-assertions-core:6.1.4")
+    testImplementation("io.kotest:kotest-property:6.1.4")
+    testImplementation("io.mockk:mockk:1.14.9")
+    testImplementation("io.ktor:ktor-server-test-host:3.4.0")
+    testImplementation("org.jetbrains.kotlinx:kotlinx-coroutines-test:1.10.2")
+}
+
+tasks.withType<Test> {
+    useJUnitPlatform()
+}
+
+detekt {
+    config.setFrom(files("config/detekt/detekt.yml"))
+    buildUponDefaultConfig = true
+}
+```
+
+## Error Handling Patterns
+
+### Result Type for Domain Operations
+
+```kotlin
+// Good: Use Kotlin's Result or a custom sealed class
+suspend fun createUser(request: CreateUserRequest): Result<User> = runCatching {
+    require(request.name.isNotBlank()) { "Name cannot be blank" }
+    require('@' in request.email) { "Invalid email format" }
+
+    val user = User(
+        id = UserId(UUID.randomUUID().toString()),
+        name = request.name,
+        email = Email(request.email),
+    )
+    userRepository.save(user)
+    user
+}
+
+// Good: Chain results
+val displayName = createUser(request)
+    .map { it.name }
+    .getOrElse { "Unknown" }
+```
+
+### require, check, error
+
+```kotlin
+// Good: Preconditions with clear messages
+fun withdraw(account: Account, amount: Money): Account {
+    require(amount.value > 0) { "Amount must be positive: $amount" }
+    check(account.balance >= amount) { "Insufficient balance: ${account.balance} < $amount" }
+
+    return account.copy(balance = account.balance - amount)
+}
+```
+
+## Collection Operations
+
+### Idiomatic Collection Processing
+
+```kotlin
+// Good: Chained operations
+val activeAdminEmails: List<String> = users
+    .filter { it.role == Role.ADMIN && it.isActive }
+    .sortedBy { it.name }
+    .map { it.email }
+
+// Good: Grouping and aggregation
+val usersByRole: Map<Role, List<User>> = users.groupBy { it.role }
+
+val oldestByRole: Map<Role, User?> = users.groupBy { it.role }
+    .mapValues { (_, users) -> users.minByOrNull { it.createdAt } }
+
+// Good: Associate for map creation
+val usersById: Map<UserId, User> = users.associateBy { it.id }
+
+// Good: Partition for splitting
+val (active, inactive) = users.partition { it.isActive }
+```
+
+## Quick Reference: Kotlin Idioms
+
+| Idiom | Description |
+|-------|-------------|
+| `val` over `var` | Prefer immutable variables |
+| `data class` | For value objects with equals/hashCode/copy |
+| `sealed class/interface` | For restricted type hierarchies |
+| `value class` | For type-safe wrappers with zero overhead |
+| Expression `when` | Exhaustive pattern matching |
+| Safe call `?.` | Null-safe member access |
+| Elvis `?:` | Default value for nullables |
+| `let`/`apply`/`also`/`run`/`with` | Scope functions for clean code |
+| Extension functions | Add behavior without inheritance |
+| `copy()` | Immutable updates on data classes |
+| `require`/`check` | Precondition assertions |
+| Coroutine `async`/`await` | Structured concurrent execution |
+| `Flow` | Cold reactive streams |
+| `sequence` | Lazy evaluation |
+| Delegation `by` | Reuse implementation without inheritance |
+
+## Anti-Patterns to Avoid
+
+```kotlin
+// Bad: Force-unwrapping nullable types
+val name = user!!.name
+
+// Bad: Platform type leakage from Java
+fun getLength(s: String) = s.length // Safe
+fun getLength(s: String?) = s?.length ?: 0 // Handle nulls from Java
+
+// Bad: Mutable data classes
+data class MutableUser(var name: String, var email: String)
+
+// Bad: Using exceptions for control flow
+try {
+    val user = findUser(id)
+} catch (e: NotFoundException) {
+    // Don't use exceptions for expected cases
+}
+
+// Good: Use nullable return or Result
+val user: User? = findUserOrNull(id)
+
+// Bad: Ignoring coroutine scope
+GlobalScope.launch { /* Avoid GlobalScope */ }
+
+// Good: Use structured concurrency
+coroutineScope {
+    launch { /* Properly scoped */ }
+}
+
+// Bad: Deeply nested scope functions
+user?.let { u ->
+    u.address?.let { a ->
+        a.city?.let { c -> process(c) }
+    }
+}
+
+// Good: Direct null-safe chain
+user?.address?.city?.let { process(it) }
+```
+
+**Remember**: Kotlin code should be concise but readable. Leverage the type system for safety, prefer immutability, and use coroutines for concurrency. When in doubt, let the compiler help you.