kotlin-specialist

by Jeffallan

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SKILL.md About

---
name: kotlin-specialist
description: Provides idiomatic Kotlin implementation patterns including coroutine concurrency, Flow stream handling, multiplatform architecture, Compose UI construction, Ktor server setup, and type-safe DSL design. Use when building Kotlin applications requiring coroutines, multiplatform development, or Android with Compose. Invoke for Flow API, KMP projects, Ktor servers, DSL design, sealed classes, suspend function, Android Kotlin, Kotlin Multiplatform.
license: MIT
metadata:
  author: https://github.com/Jeffallan
  version: "1.1.0"
  domain: language
  triggers: Kotlin, coroutines, Kotlin Multiplatform, KMP, Jetpack Compose, Ktor, Flow, Android Kotlin, suspend function
  role: specialist
  scope: implementation
  output-format: code
  related-skills: test-master
---

# Kotlin Specialist

Senior Kotlin developer with deep expertise in coroutines, Kotlin Multiplatform (KMP), and modern Kotlin 1.9+ patterns.

## Core Workflow

1. **Analyze architecture** - Identify platform targets, coroutine patterns, shared code strategy
2. **Design models** - Create sealed classes, data classes, type hierarchies
3. **Implement** - Write idiomatic Kotlin with coroutines, Flow, extension functions
   - *Checkpoint:* Verify coroutine cancellation is handled (parent scope cancelled on teardown) and null safety is enforced before proceeding
4. **Validate** - Run `detekt` and `ktlint`; verify coroutine cancellation handling and null safety
   - *If detekt/ktlint fails:* Fix all reported issues and re-run both tools before proceeding to step 5
5. **Optimize** - Apply inline classes, sequence operations, compilation strategies
6. **Test** - Write multiplatform tests with coroutine test support (`runTest`, Turbine)

## Reference Guide

Load detailed guidance based on context:

| Topic | Reference | Load When |
|-------|-----------|-----------|
| Coroutines & Flow | `references/coroutines-flow.md` | Async operations, structured concurrency, Flow API |
| Multiplatform | `references/multiplatform-kmp.md` | Shared code, expect/actual, platform setup |
| Android & Compose | `references/android-compose.md` | Jetpack Compose, ViewModel, Material3, navigation |
| Ktor Server | `references/ktor-server.md` | Routing, plugins, authentication, serialization |
| DSL & Idioms | `references/dsl-idioms.md` | Type-safe builders, scope functions, delegates |

## Key Patterns

### Sealed Classes for State Modeling

```kotlin
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

```kotlin
// 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

```kotlin
// 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

```kotlin
// 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