kotlin

Kotlin Cursor Rules

You are an expert Kotlin developer. Follow these rules:

Null Safety

• Use non-null types by default. Nullable (?) only when truly optional
• Safe calls (?.) and elvis (?:) over manual null checks
• Never use !! except in tests. If you need it, redesign
• Use requireNotNull/checkNotNull for preconditions with clear messages
• let/run/also for scoped null handling

Coroutines

• suspend functions for all async operations
• Use structured concurrency: coroutineScope, supervisorScope
• Never use GlobalScope — it leaks coroutines
• Flow for reactive streams, StateFlow/SharedFlow for state
• withContext(Dispatchers.IO) for blocking operations
• Handle cancellation: use ensureActive() in long loops

Idiomatic Kotlin

• Data classes for DTOs and value objects
• Sealed classes/interfaces for restricted hierarchies
• Extension functions for utility methods on existing types
• Use scope functions (let, run, with, apply, also) appropriately
• when expression over if-else chains for 3+ branches
• String templates over concatenation

Collections

• Prefer immutable collections (List, Map, Set) over mutable
• Sequence for chained operations on large collections
• Use destructuring: val (name, age) = person
• mapNotNull to filter and transform in one pass
• groupBy, associateBy, partition for common patterns

Architecture

• Interface-based design for testability
• Dependency injection via constructor parameters
• Use object for singletons, companion object for factory methods
• Value classes for type-safe wrappers (inline class replacement)
• Use Result type for operations that can fail