Kotlin 2.0+ Knowledge Patch Skill

Kotlin 2.0+ Knowledge Patch

Claude's baseline knowledge covers Kotlin through 1.9.x and Compose Multiplatform 1.6. This skill provides changes from Kotlin 2.0 (2024) onwards, plus ecosystem updates for Ktor, Coroutines, Serialization, and Compose Multiplatform.

Quick Reference

Kotlin Language Features

| Feature | Version | Status | Enable Flag | |---------|---------|--------|-------------| | Guard conditions in when | 2.1 | Preview | -Xwhen-guards | | Multi-dollar interpolation | 2.1 | Preview | -Xmulti-dollar-interpolation | | Non-local break/continue | 2.1 | Preview | -Xnon-local-break-continue | | Explicit backing fields | 2.3 | Experimental | -Xexplicit-backing-fields | | Unused return value checker | 2.3 | Experimental | -Xreturn-value-checker=check\|full |

See references/language-features.md for syntax and examples.

Standard Library

| API | Version | Status | |-----|---------|--------| | Uuid.generateV4(), generateV7() | 2.3 | Experimental | | Uuid.parseOrNull() | 2.3 | Experimental | | kotlin.time.Clock, Instant | 2.3 | Stable |

See references/language-features.md for details.

Build & Tooling

| Change | Version | |--------|---------| | Compose compiler merged into Kotlin (kotlin("plugin.compose")) | 2.0 | | Power-assert compiler plugin | 2.0 | | KMP Android: use com.android.kotlin.multiplatform.library, rename androidTarget to android | 2.3 |

See references/build-tooling.md for Gradle configuration.

Ecosystem Libraries

| Library | Version | Key Changes | |---------|---------|-------------| | Ktor | 3.0 | kotlinx-io migration, SSE support, CSRF plugin | | Coroutines | 1.9–1.10 | chunked, any/all/none Flow operators | | Serialization | 1.10 | Stabilized JSON APIs, subclassesOfSealed | | Compose Multiplatform | 1.8–1.10 | Navigation 3, unified @Preview, web Beta |

See references/ecosystem-updates.md and references/compose-multiplatform.md.

Reference Files

| File | Contents | |------|----------| | language-features.md | Guard conditions, multi-dollar interpolation, non-local break/continue, explicit backing fields, return value checker, UUID, Clock/Instant | | build-tooling.md | Compose compiler plugin, power-assert, KMP Android target migration | | ecosystem-updates.md | Ktor 3.0 (kotlinx-io, SSE, CSRF), Coroutines 1.9–1.10, Serialization 1.10 | | compose-multiplatform.md | Navigation SavedState, Navigation 3, unified @Preview, web Beta, deprecated aliases |

Critical Knowledge

Compose Compiler Plugin (2.0)

The Compose compiler is now bundled with Kotlin. Replace separate Compose compiler artifacts:

plugins {
    kotlin("multiplatform") version "2.0.0"
    kotlin("plugin.compose") version "2.0.0"  // replaces old compose-compiler
}

Guard Conditions in `when` (2.1, Preview)

Add if conditions to when branches:

when (animal) {
    is Cat if !animal.mouseHunter -> animal.feedCat()
    is Dog -> animal.feedDog()
    else -> println("Unknown")
}

Explicit Backing Fields (2.3, Experimental)

Eliminates the _backing property pattern:

val city: StateFlow<String>
    field = MutableStateFlow("")

fun updateCity(newCity: String) {
    city.value = newCity  // smart cast to MutableStateFlow works
}

Multi-dollar String Interpolation (2.1, Preview)

Use $$ prefix so single $ is literal:

val json = $$"""
{
  "$schema": "https://json-schema.org/draft/2020-12/schema",
  "title": "$${simpleName}"
}
"""

Non-local Break and Continue (2.1, Preview)

break/continue work inside lambdas passed to inline functions:

for (element in elements) {
    val value = element.nullableMethod() ?: run {
        continue  // works in inline lambda
    }
}

Unused Return Value Checker (2.3, Experimental)

@file:MustUseReturnValues  // or annotate class/function
// Suppress: val _ = computeValue()
// Exclude: @IgnorableReturnValue

Enable: -Xreturn-value-checker=check (annotated scopes) or =full (all project functions).

UUID (2.3, Experimental)

val v4 = Uuid.generateV4()         // explicit v4
val v7 = Uuid.generateV7()         // time-ordered v7
val parsed = Uuid.parseOrNull("…") // null instead of throwing
val atTime = Uuid.generateV7NonMonotonicAt(instant)

`kotlin.time.Clock` and `Instant` are Stable (2.3)

No more @OptIn(ExperimentalTime::class) needed:

val now = Clock.System.now()  // returns Instant

Power-assert Plugin (2.0, Experimental)

plugins {
    kotlin("plugin.power-assert") version "2.0.0"
}
powerAssert {
    functions = listOf("kotlin.assert", "kotlin.test.assertTrue")
}

KMP Android Target Migration (2.3)

Use Google's com.android.kotlin.multiplatform.library plugin. Rename androidTarget to android. Old kotlin-android plugin deprecated with AGP 9.0+.

Ktor 3.0 — kotlinx-io Migration

Input/Output/ByteReadChannel/ByteWriteChannel are deprecated. Use kotlinx-io Source/Sink instead.

Ktor 3.0 — SSE Support

// Server
install(SSE)
routing {
    sse {
        send(ServerSentEvent(data = "Hello"))
        close()
    }
}

// Client
val client = HttpClient(CIO) { install(SSE) }
client.sse(host = "127.0.0.1", port = 8080, path = "/sse") {
    incoming.collect { event -> println(event) }
}

Ktor 3.0 — CSRF Plugin

install(CSRF) {
    allowOrigin("https://example.com")
    originMatchesHost()
    checkHeader("X-CSRF") { csrfHeader -> /* validate */ true }
    onFailure { respondText("Forbidden", status = HttpStatusCode.Forbidden) }
}

Flow Terminal Operators (Coroutines 1.10)

flow.any { it > 0 }   // terminal, returns Boolean
flow.all { it > 0 }   // terminal, returns Boolean
flow.none { it > 0 }  // terminal, returns Boolean
flow.chunked(5)        // Flow<List<T>>, groups emissions (1.9)

Serialization 1.10 — Stabilized APIs

Remove @OptIn for: allowTrailingComma, allowComments, decodeEnumsCaseInsensitive, prettyPrintIndent, @EncodeDefault, JsonUnquotedLiteral, JsonPrimitive with unsigned types.

Serialization 1.10 — `subclassesOfSealed`

val module = SerializersModule {
    polymorphic(Base::class) {
        subclassesOfSealed(MySealedClass::class)
    }
}

Compose Multiplatform 1.10 — Navigation 3

PredictiveBackHandler() deprecated in favor of NavigationBackHandler() from the Navigation Event library.

Compose Multiplatform 1.10 — Unified `@Preview`

Use androidx.compose.ui.tooling.preview.Preview in commonMain. Old platform-specific annotations deprecated.

Compose Multiplatform 1.10 — Deprecated Dependency Aliases

compose.ui, compose.material3 aliases in Gradle plugin are deprecated. Use direct version catalog references instead.

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