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name: kotlin-language description: Write idiomatic Kotlin 1.9+ with null safety, sealed classes, data classes, extension functions, delegates, collections, inline/reified generics, and expression syntax. Use for Kotlin language constructs, Java-to-Kotlin migration, lazy delegates, data classes, backing properties, or inline/reified generics; keep language constructs in scope even inside response models, and defer coroutine configuration, Android Context/framework, Retrofit/OkHttp client setup, and other library-specific recipes. metadata: triggers: files: - "/*.kt" - "/*.kts" keywords: - val - var - "?." - "?:" - "!!" - data class - sealed - when - extension - lazy - lateinit - object

Kotlin Language Patterns

Priority: P0 (CRITICAL)

Implementation Guidelines

• Immutability: Use val by default. Only use var if mutation required locally.
• Null Safety: Use ? for nullable types. Use safe call ?. and Elvis ?: over !!.
• Expressions: Prefer expression bodies fun foo() = ... for one-liners. Use if/try as expressions.
• Classes: Use data class for DTOs. Use sealed interface/class for state hierarchies (e.g., Success, Error, Loading). Access members as computed property rather than function.
• Extension Functions: Prefer over utility classes (StringUtil). Keep private/internal if module-specific.
• Named Arguments: Use for clarity, especially with booleans or multiple same-type params.
• String Templates: Use "$var" over concatenation. Use "" for multiline strings (SQL/JSON).

Anti-Patterns

• No !! Operator: Never use in production; prefer safe calls or requireNotNull.
• No Java-isms: Use properties not get/set; prefer top-level functions over companion object statics.
• No Lateinit Abuse: Prefer nullable types or lazy delegates instead.
• No Silenced Errors: Never swallow exceptions without logging or handling.

References

• Sealed Class, When Expression & Extension Examples

Canonical response anchors

• Additional task-grounded exact anchors: data class