kotlin-rules

Kotlin Rules

These rules come from app/rules/kotlin/ in ai-toolkit. They cover the project's standards for coding style, frameworks, patterns, security, and testing in Kotlin. Apply them when writing or reviewing Kotlin code.

Kotlin Coding Style

Naming

• PascalCase: classes, interfaces, objects, type aliases, enum entries.
• camelCase: functions, properties, local variables, parameters.
• UPPER_SNAKE: compile-time constants (const val), top-level val constants.
• Backing properties: prefix with _ (private val _items, val items: List<T>).
• Package names: lowercase, no underscores (com.company.project.feature).

Null Safety

• Use nullable types only when nullability is semantically meaningful.
• Prefer ?.let { }, ?: (Elvis), and safe calls over !!.
• Never use !! except in tests or when null is truly impossible.
• Use requireNotNull() and require() for preconditions at public API boundaries.
• Use checkNotNull() and check() for state assertions.

Data Classes

• Use data class for DTOs, value objects, and state containers.
• Use copy() for immutable updates. Avoid mutable var in data classes.
• Use sealed class / sealed interface for restricted hierarchies.
• Use value class (inline class) for type-safe wrappers with zero overhead.
• Use object for singletons and namespace-like utility groupings.

Functions

• Use expression body (= expr) for single-expression functions.
• Use named arguments for functions with >2 parameters of the same type.
• Use default parameter values instead of overloaded functions.
• Use extension functions to add behavior without inheritance.
• Use suspend functions for async operations, not callbacks.

Collections

• Prefer listOf, mapOf, setOf (immutable) over mutableListOf.
• Use collection operators: map, filter, groupBy, associate.
• Use sequence {} for lazy evaluation on large collections.
• Prefer firstOrNull() over first() for safe access.
• Use destructuring: val (name, age) = user.