amethyst: Skill for Claude Code

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

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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(...)
    }
}