Kotlin

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name: Kotlin slug: kotlin version: 1.0.1 description: Build robust Android and multiplatform apps with Kotlin idioms, coroutines, and null safety. metadata: {"clawdbot":{"emoji":"🟠","requires":{"bins":["kotlin"]},"os":["linux","darwin","win32"]}}

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Quick Reference

| Topic | File | |-------|------| | Null safety operators and patterns | nullsafety.md | | Coroutines, flows, structured concurrency | coroutines.md | | Collections, sequences, data classes | collections.md | | Scope functions, extensions, sealed classes | idioms.md | | Java interop and common Kotlin mistakes | interop.md | | Android lifecycle, Compose state | android.md | | Delegation, inline, reified, multiplatform | advanced.md |

Critical Rules

Null Safety

!! asserts non-null — crashes on null, use only when you've already checked

Platform types from Java are risky — add null checks or use @Nullable/@NonNull annotations

Elvis with return/throw for early exit — val name = user?.name ?: return

Coroutines

viewModelScope auto-cancels on ViewModel clear — don't use GlobalScope in Android

Structured concurrency: child coroutine failure cancels parent — use supervisorScope to isolate

StateFlow needs initial value and never completes — SharedFlow for one-shot events

Inject dispatchers for testability — don't hardcode Dispatchers.IO

Collections & Data Classes

first() throws on empty — use firstOrNull() for safe access

Only constructor properties in equals/hashCode — body properties ignored

mutableStateListOf for Compose — wrapping mutableListOf in state won't track changes

Scope Functions & Extensions

Don't nest scope functions — readability drops fast, extract to named functions

Extensions are resolved statically — not polymorphic, receiver type matters at compile time

Android/Compose

repeatOnLifecycle(STARTED) for flow collection — launchWhenStarted is deprecated

remember survives recomposition only — use rememberSaveable for config changes

collectAsStateWithLifecycle is the gold standard — lifecycle-aware + Compose state

Java Interop

== is structural equality in Kotlin — === for reference, opposite of Java

SAM conversion only for Java interfaces — Kotlin interfaces need explicit fun interface

@JvmStatic, @JvmOverloads, @JvmField for Java-friendly APIs