kotlin-language

name	kotlin-language
description	Idiomatic Kotlin 1.9+ standards including null safety, sealed classes, and expression syntax. Use when working with Kotlin null safety, data classes, sealed interfaces, extension functions, or migrating Java code to Kotlin. (triggers: **/*.kt, **/*.kts, val, var, ?., ?:, !!, data class, sealed, when, extension, lazy, lateinit, object)

Kotlin Language Patterns

Priority: P0 (CRITICAL)

Modern, idiomatic Kotlin standards for safety and conciseness.

Implementation Guidelines

• Immutability: Use val by default. Only use var if mutation is 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 a computed property rather than a 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