kotlin-expert

name

kotlin-expert

description

Advanced Kotlin patterns for AmethystMultiplatform. Flow state management (StateFlow/SharedFlow), sealed hierarchies (classes vs interfaces), immutability (@Immutable, data classes), DSL builders (type-safe fluent APIs), inline functions (reified generics, performance). Use when working with: (1) State management patterns (StateFlow/SharedFlow/MutableStateFlow), (2) Sealed classes or sealed interfaces, (3) @Immutable annotations for Compose, (4) DSL builders with lambda receivers, (5) inline/reified functions, (6) Kotlin performance optimization. Complements kotlin-coroutines agent (async patterns) - this skill focuses on Amethyst-specific Kotlin idioms.

Kotlin Expert

Advanced Kotlin patterns for AmethystMultiplatform. Covers Flow state management, sealed hierarchies, immutability, DSL builders, and inline functions with real codebase examples.

Mental Model

Kotlin in Amethyst:

State Management (Hot Flows)
    ├── StateFlow<T>           # Single value, always has value, replays to new subscribers
    ├── SharedFlow<T>          # Event stream, configurable replay, multiple subscribers
    └── MutableStateFlow<T>    # Private mutable, public via .asStateFlow()

Type Safety (Sealed Hierarchies)
    ├── sealed class           # State variants with data (AccountState.LoggedIn/LoggedOut)
    └── sealed interface       # Generic result types (SignerResult<T>)

Compose Performance (@Immutable)
    ├── @Immutable             # 173+ event classes - prevents recomposition
    └── data class             # Structural equality, copy(), immutable by convention

DSL Patterns
    ├── Builder classes        # Fluent APIs (TagArrayBuilder)
    ├── Lambda receivers       # inline fun tagArray { ... }
    └── Method chaining        # return this

Performance
    ├── inline fun             # Eliminate lambda overhead
    ├── reified type params    # Runtime type info (OptimizedJsonMapper)
    └── value class            # Zero-cost wrappers (NOT USED yet in Amethyst)

Delegation:

• kotlin-coroutines agent: Deep async (structured concurrency, channels, operators)
• kotlin-multiplatform skill: expect/actual, source sets
• This skill: Amethyst Kotlin idioms, state patterns, type safety

---

1. Flow State Management

StateFlow: State that Changes

Mental model: StateFlow is a "hot" observable state holder. Always has a value, new collectors immediately get current state.

Amethyst pattern:

// AccountManager.kt:48-50
class AccountManager {
    private val _accountState = MutableStateFlow<AccountState>(AccountState.LoggedOut)
    val accountState: StateFlow<AccountState> = _accountState.asStateFlow()

    fun login(key: String) {
        _accountState.value = AccountState.LoggedIn(...)
    }
}

Key principles:

1. Private mutable, public immutable: _accountState (MutableStateFlow) private, accountState (StateFlow) public
2. Always has value: Initial value required (LoggedOut)
3. Single value: Replays ONE most recent value to new subscribers
4. Hot: Stays in memory, all collectors share same instance

See: AccountManager.kt:48-50, RelayConnectionManager.kt:49-52

SharedFlow: Event Streams

Mental model: SharedFlow is a "hot" broadcast stream for events. Configurable replay buffer, doesn't require initial value.

Amethyst pattern:

// RelayConnectionManager.kt:52-53
val connectedRelays: StateFlow<Set<NormalizedRelayUrl>> = client.connectedRelaysFlow()
val availableRelays: StateFlow<Set<NormalizedRelayUrl>> = client.availableRelaysFlow()

When to use StateFlow vs SharedFlow:

Scenario	Use StateFlow	Use SharedFlow
UI state	✅ Current screen data, login status	❌
One-time events	❌	✅ Navigation, snackbars, toasts
Always has value	✅	❌ Optional
Replay count	1 (latest only)	Configurable (0, 1, n)
Backpressure	Conflates (drops old)	Configurable buffer

Best practice:

// State: Use StateFlow
private val _uiState = MutableStateFlow(UiState.Loading)
val uiState: StateFlow<UiState> = _uiState.asStateFlow()

// Events: Use SharedFlow
private val _navigationEvents = MutableSharedFlow<NavEvent>(replay = 0)
val navigationEvents: SharedFlow<NavEvent> = _navigationEvents.asSharedFlow()

Flow Anti-Patterns

❌ Exposing mutable state:

val accountState: MutableStateFlow<AccountState>  // BAD: Can be mutated externally

✅ Expose immutable:

val accountState: StateFlow<AccountState> = _accountState.asStateFlow()  // GOOD

---

❌ SharedFlow for state:

val loginState = MutableSharedFlow<LoginState>()  // BAD: State might get lost

✅ StateFlow for state:

val loginState = MutableStateFlow(LoginState.LoggedOut)  // GOOD: Always has value

See: references/flow-patterns.md for comprehensive examples.

---

2. Sealed Hierarchies

Sealed Classes: State Variants

Mental model: Sealed classes represent a closed set of variants that share common data/behavior.

Amethyst pattern:

// AccountManager.kt:36-46
sealed class AccountState {
    data object LoggedOut : AccountState()

    data class LoggedIn(
        val signer: NostrSigner,
        val pubKeyHex: String,
        val npub: String,
        val nsec: String?,
        val isReadOnly: Boolean
    ) : AccountState()
}

// Usage
when (state) {
    is AccountState.LoggedOut -> showLogin()
    is AccountState.LoggedIn -> showFeed(state.pubKeyHex)
}  // Exhaustive - compiler enforces all cases

Key principles:

1. Closed hierarchy: All subclasses known at compile-time
2. Exhaustive when: Compiler ensures all cases handled
3. Shared data: Sealed class can hold common properties
4. Single inheritance: Subclass can't extend another class

When to use:

• Modeling UI states (Loading, Success, Error)
• Login states (LoggedOut, LoggedIn)
• Result types with different data per variant

Sealed Interfaces: Generic Result Types

Mental model: Sealed interfaces for contracts with multiple implementations that need generics or multiple inheritance.

Amethyst pattern:

// SignerResult.kt:25-46
sealed interface SignerResult<T : IResult> {
    sealed interface RequestAddressed<T : IResult> : SignerResult<T> {
        class Successful<T : IResult>(val result: T) : RequestAddressed<T>
        class Rejected<T : IResult> : RequestAddressed<T>
        class TimedOut<T : IResult> : RequestAddressed<T>
        class ReceivedButCouldNotPerform<T : IResult>(
            val message: String?
        ) : RequestAddressed<T>
    }
}

// Usage with generics
fun handleResult(result: SignerResult<SignResult>) {
    when (result) {
        is SignerResult.RequestAddressed.Successful -> processEvent(result.result.event)
        is SignerResult.RequestAddressed.Rejected -> showRejected()
        is SignerResult.RequestAddressed.TimedOut -> showTimeout()
    }
}

Key principles:

1. Multiple inheritance: Subtype can implement other interfaces
2. Variance: Supports out/in modifiers for generics
3. No constructor: Can't hold state directly (subtypes can)
4. Nested hierarchies: Can create sub-sealed hierarchies

Sealed Class vs Sealed Interface

Feature	Sealed Class	Sealed Interface
Constructor	✅ Can hold common state	❌ No constructor
Inheritance	❌ Single parent only	✅ Multiple interfaces
Generics	❌ No variance	✅ Covariance/contravariance
Use case	State variants	Result types, contracts

Decision tree:

Need to hold common data in base?
    YES → sealed class
    NO → sealed interface

Need generics with variance (out/in)?
    YES → sealed interface
    NO → Either works

Subtypes need multiple inheritance?
    YES → sealed interface
    NO → Either works

Amethyst examples:

• sealed class AccountState - state variants with different data
• sealed interface SignerResult<T> - generic result types with variance

See: references/sealed-class-catalog.md for all sealed types in quartz.

---

3. Immutability & Compose Performance

@Immutable Annotation

Mental model: @Immutable tells Compose "this value never changes after construction." Compose can skip recomposition if @Immutable object reference doesn't change.

Amethyst pattern:

// TextNoteEvent.kt:51-63
@Immutable
class TextNoteEvent(
    id: HexKey,
    pubKey: HexKey,
    createdAt: Long,
    tags: Array<Array<String>>,
    content: String,
    sig: HexKey
) : BaseThreadedEvent(id, pubKey, createdAt, KIND, tags, content, sig) {
    // All properties immutable (val), no mutable state
}

Key principles:

1. All properties immutable: Only val, never var
2. No mutable collections: Use ImmutableList, Array, not MutableList
3. Deep immutability: Nested objects also immutable
4. Compose optimization: Skips recomposition if reference equals

Why it matters:

// Without @Immutable
@Composable
fun NoteCard(note: TextNoteEvent) {  // Recomposes every time parent recomposes
    Text(note.content)
}

// With @Immutable
@Composable
fun NoteCard(note: TextNoteEvent) {  // Only recomposes if note reference changes
    Text(note.content)
}

173+ @Immutable classes in quartz - all events immutable for Compose performance.

Data Classes & Immutability

Pattern:

@Immutable
data class RelayStatus(
    val url: NormalizedRelayUrl,
    val connected: Boolean,
    val error: String? = null
) {
    // Implicit: equals(), hashCode(), copy(), toString()
}

// Usage
val oldStatus = RelayStatus(url, connected = false)
val newStatus = oldStatus.copy(connected = true)  // Immutable update

Key principles:

1. Structural equality: equals() compares properties, not reference
2. copy(): Create modified copies without mutating
3. All properties in constructor: For proper equals()/hashCode()
4. Prefer val: Make properties immutable

kotlinx.collections.immutable

Pattern:

import kotlinx.collections.immutable.ImmutableList
import kotlinx.collections.immutable.persistentListOf
import kotlinx.collections.immutable.toImmutableList

// Instead of List (which could be mutable internally)
val relays: ImmutableList<String> = persistentListOf("wss://relay1.com", "wss://relay2.com")

// Add returns new instance
val updated = relays.add("wss://relay3.com")  // relays unchanged, updated has 3 items

When to use:

• Compose state that needs collection
• Publicly exposed collections
• Shared state across threads

See: references/immutability-patterns.md

---

4. DSL Builders

Type-Safe Fluent APIs

Mental model: DSL (Domain-Specific Language) builders use lambda receivers and method chaining to create readable, type-safe APIs.

Amethyst pattern:

// TagArrayBuilder.kt:23-90
class TagArrayBuilder<T : IEvent> {
    private val tagList = mutableMapOf<String, MutableList<Tag>>()

    fun add(tag: Array<String>): TagArrayBuilder<T> {
        if (tag.isEmpty() || tag[0].isEmpty()) return this
        tagList.getOrPut(tag[0], ::mutableListOf).add(tag)
        return this  // Method chaining
    }

    fun remove(tagName: String): TagArrayBuilder<T> {
        tagList.remove(tagName)
        return this  // Method chaining
    }

    fun build() = tagList.flatMap { it.value }.toTypedArray()
}

// Inline function with lambda receiver (line 90)
inline fun <T : Event> tagArray(initializer: TagArrayBuilder<T>.() -> Unit = {}): TagArray =
    TagArrayBuilder<T>().apply(initializer).build()

Usage:

val tags = tagArray<TextNoteEvent> {
    add(arrayOf("e", eventId, relay, "reply"))
    add(arrayOf("p", pubkey))
    remove("a")  // Remove address tags
}

Key patterns:

1. Method chaining: Return this from mutator methods
2. Lambda receiver: TagArrayBuilder<T>.() -> Unit - lambda has this: TagArrayBuilder<T>
3. inline function: Eliminates lambda overhead
4. apply(): Executes lambda with receiver, returns receiver

DSL Pattern Template

class MyBuilder {
    private val items = mutableListOf<Item>()

    fun add(item: Item): MyBuilder {
        items.add(item)
        return this
    }

    fun build(): Result = Result(items.toList())
}

inline fun myDsl(init: MyBuilder.() -> Unit): Result =
    MyBuilder().apply(init).build()

// Usage
val result = myDsl {
    add(Item("foo"))
    add(Item("bar"))
}

Why inline?

• Eliminates lambda object allocation
• Enables reified type parameters
• Better performance for frequently-called DSLs

See: references/dsl-builder-examples.md for more patterns.

---

5. Inline Functions & reified

inline fun: Eliminate Overhead

Mental model: inline copies function body to call site. No lambda object created, direct code insertion.

Pattern:

// Without inline
fun <T> measureTime(block: () -> T): T {
    val start = System.currentTimeMillis()
    val result = block()  // Lambda object allocated
    println("Time: ${System.currentTimeMillis() - start}ms")
    return result
}

// With inline
inline fun <T> measureTime(block: () -> T): T {
    val start = System.currentTimeMillis()
    val result = block()  // No allocation, code inlined
    println("Time: ${System.currentTimeMillis() - start}ms")
    return result
}

Benefits:

1. Zero overhead: No lambda object allocation
2. Non-local returns: Can return from outer function inside lambda
3. reified enabled: Access to type parameter at runtime

reified: Runtime Type Access

Mental model: reified makes generic type T available at runtime. Only works with inline.

Amethyst pattern:

// OptimizedJsonMapper.kt:48
expect object OptimizedJsonMapper {
    inline fun <reified T : OptimizedSerializable> fromJsonTo(json: String): T
}

// Usage
val event: TextNoteEvent = OptimizedJsonMapper.fromJsonTo(jsonString)
// Compiler inlines and passes TextNoteEvent::class info

Without reified:

// Would need to pass class explicitly
fun <T> fromJson(json: String, clazz: KClass<T>): T {
    return when (clazz) {
        TextNoteEvent::class -> parseTextNote(json) as T
        // ...
    }
}

val event = fromJson(json, TextNoteEvent::class)  // Verbose

With reified:

inline fun <reified T> fromJson(json: String): T {
    return when (T::class) {  // Can access T::class!
        TextNoteEvent::class -> parseTextNote(json) as T
        // ...
    }
}

val event = fromJson<TextNoteEvent>(json)  // Clean

noinline & crossinline

noinline: Prevent specific lambda from being inlined

inline fun foo(
    inlined: () -> Unit,
    noinline notInlined: () -> Unit  // Can be stored, passed around
) {
    inlined()
    someFunction(notInlined)  // Can pass to non-inline function
}

crossinline: Lambda can't do non-local returns

inline fun foo(crossinline block: () -> Unit) {
    launch {
        block()  // OK: crossinline allows lambda in different context
    }
}

---

6. Value Classes (Opportunity)

Mental model: value class is a compile-time wrapper with zero runtime overhead. Single property, no boxing.

Not currently used in Amethyst - potential optimization.

Pattern:

@JvmInline
value class EventId(val hex: String)

@JvmInline
value class PubKey(val hex: String)

// Type safety without runtime cost
fun fetchEvent(eventId: EventId): Event {
    // eventId.hex accessed without wrapper object
}

val id = EventId("abc123")
fetchEvent(id)  // Type safe
// fetchEvent(PubKey("xyz"))  // Compile error!

When to use:

• Type safety for primitives (IDs, hex strings, timestamps)
• High-frequency allocations (event processing)
• Clear domain types without overhead

Restrictions:

• Single property only
• Must be val
• Can't have init block with logic
• Inline at compile-time, may box in some cases

Amethyst opportunity:

// Current (String everywhere, no type safety)
fun fetchEvent(id: String): Event  // Could pass wrong string

// With value class
@JvmInline value class EventId(val hex: String)
@JvmInline value class PubKeyHex(val hex: String)
@JvmInline value class Bech32(val encoded: String)

fun fetchEvent(id: EventId): Event  // Type safe, zero cost

---

Common Patterns

Pattern: StateFlow State Management

class MyViewModel {
    private val _state = MutableStateFlow(State.Initial)
    val state: StateFlow<State> = _state.asStateFlow()

    fun loadData() {
        viewModelScope.launch {
            _state.value = State.Loading
            val result = repository.getData()
            _state.value = when (result) {
                is Success -> State.Success(result.data)
                is Error -> State.Error(result.message)
            }
        }
    }
}

sealed class State {
    data object Initial : State()
    data object Loading : State()
    data class Success(val data: List<Item>) : State()
    data class Error(val message: String) : State()
}

Pattern: Sealed Result with Generics

sealed interface Result<out T> {
    data class Success<T>(val value: T) : Result<T>
    data class Error(val exception: Exception) : Result<Nothing>
    data object Loading : Result<Nothing>
}

// Use with variance
fun <T> fetchData(): Result<T> = ...

val userResult: Result<User> = fetchData()
val itemResult: Result<List<Item>> = fetchData()

Pattern: Immutable Event Builder

@Immutable
data class Event(
    val id: String,
    val kind: Int,
    val content: String,
    val tags: ImmutableList<Tag>
) {
    companion object {
        fun builder() = EventBuilder()
    }
}

class EventBuilder {
    private var id: String = ""
    private var kind: Int = 1
    private var content: String = ""
    private val tags = mutableListOf<Tag>()

    fun id(value: String) = apply { id = value }
    fun kind(value: Int) = apply { kind = value }
    fun content(value: String) = apply { content = value }
    fun tag(tag: Tag) = apply { tags.add(tag) }

    fun build() = Event(id, kind, content, tags.toImmutableList())
}

// Usage
 event = Event.builder()
    .id()
    .kind()
    .content()
    .tag(Tag.P())
    .build()

---

Delegation Guide

When to delegate:

Topic	Delegate To	This Skill Covers
Structured concurrency, channels	kotlin-coroutines agent	Flow state patterns only
expect/actual, source sets	kotlin-multiplatform skill	Platform-agnostic Kotlin
General Compose patterns	compose-expert skill	@Immutable for performance
Build configuration	gradle-expert skill	-

Ask kotlin-coroutines agent for:

• Advanced Flow operators (flatMapLatest, combine, zip)
• Channel patterns
• Structured concurrency (supervisorScope, coroutineScope)
• Error handling in coroutines

This skill teaches:

• StateFlow/SharedFlow state management
• Sealed hierarchies
• @Immutable for Compose
• DSL builders
• Inline/reified patterns

---

Anti-Patterns

❌ Mutable public state:

val accountState: MutableStateFlow<AccountState>  // BAD

✅ Immutable public interface:

val accountState: StateFlow<AccountState> = _accountState.asStateFlow()

---

❌ Sealed class for generic results:

sealed class Result<T> {  // BAD: Can't use variance
    data class Success<T>(val value: T) : Result<T>()
}

✅ Sealed interface for generics:

sealed interface Result<out T> {  // GOOD: Covariance
    data class Success<T>(val value: T) : Result<T>
}

---

❌ Mutable properties in @Immutable class:

@Immutable
data class Event(
    var content: String  // BAD: var breaks immutability
)

✅ All val:

@Immutable
data class Event(
    val content: String
)

---

❌ Passing class explicitly when reified available:

inline fun <T> parse(json: String, clazz: KClass<T>): T  // BAD

✅ Use reified:

inline fun <reified T> parse(json: String): T  // GOOD

---

Quick Reference

Flow Decision Tree

Need to expose state?
    YES → StateFlow (always has value, single latest)
    NO → Need events? → SharedFlow (optional replay, broadcast)

Need to mutate?
    Internal only → MutableStateFlow (private)
    Expose publicly → StateFlow via .asStateFlow()

Sealed Decision Tree

Need common data in base type?
    YES → sealed class
    NO → sealed interface

Need generics with variance?
    YES → sealed interface
    NO → Either works

Need multiple inheritance?
    YES → sealed interface
    NO → Either works

Inline Decision Tree

Passing lambda to function?
    Called frequently? → inline (performance)
    Need reified? → inline (required)
    Need to store/pass lambda? → regular fun (can't inline)

---

Resources

Official Docs

• StateFlow and SharedFlow | Android Developers
• Sealed Classes | Kotlin Docs
• Inline Functions | Kotlin Docs

Bundled References

• references/flow-patterns.md - StateFlow/SharedFlow examples from AccountManager, RelayManager
• references/sealed-class-catalog.md - All sealed types in quartz
• references/dsl-builder-examples.md - TagArrayBuilder, other DSL patterns
• references/immutability-patterns.md - @Immutable usage, data classes, collections
• references/common-utilities.md - Canonical helpers: NumberFormatters, TimeUtils, Hex, PubKeyFormatter, CoroutinesExt.launchIO, OptimizedJsonMapper, etc.

Codebase Examples

• AccountManager.kt:36-50 - sealed class AccountState, StateFlow pattern
• RelayConnectionManager.kt:44-52 - StateFlow state management
• SignerResult.kt:25-46 - sealed interface with generics
• TextNoteEvent.kt:51-63 - @Immutable event class
• TagArrayBuilder.kt:23-90 - DSL builder pattern, inline function
• OptimizedJsonMapper.kt:48 - inline fun with reified

---

Version: 1.0.1 Last Updated: 2026-06-10