Idiomatic Kotlin patterns, best practices, and conventions for building robust, efficient, and maintainable Kotlin applications with coroutines, null safety, and DSL builders. Use when writing or reviewing Kotlin code and idiomatic structure or null safety is in question.

Kotlin Development Patterns Idiomatic Kotlin patterns and best practices for building robust, efficient, and maintainable applications. When to Use Writing new Kotlin code Reviewing Kotlin code Refactoring existing Kotlin code Designing Kotlin modules or libraries Configuring Gradle Kotlin DSL builds How It Works This skill enforces idiomatic Kotlin conventions across seven key areas: null safety using the type system and safe call operators, immutability via val and copy() on data classes, sealed classes and interfaces for exhaustive type hierarchies, structured concurrency with coroutines and Flow , extension functions for adding behaviour without inheritance, type safe DSL builders using @DslMarker and lambda receivers, and Gradle Kotlin DSL for build configuration. Examples Null safety with Elvis operator: Sealed class for exhaustive results: Structured concurrency with async/await: Core Principles 1. Null Safety Kotlin's type system distinguishes nullable and non nullable types. Leverage it fully. 2. Immutability by Default Prefer val over var , immutable collections over mutable ones. 3. Expression Bodies and Single Expression Functions Use expression bodies for concise, readable functions. 4. Data Classes for Value Objects Use data classes for types that primarily hold data. Sealed Classes and Interfaces Modeling Restricted Hierarchies Sealed Interfaces for API Responses Scope Functions When to Use Each Anti Patterns Extension Functions Adding Functionality Without Inheritance Coroutines Structured Concurrency Flow for Reactive Streams Cancellation and Cleanup Delegation Property Delegation Interface Delegation DSL Builders Type Safe Builders Configuration DSL Sequences for Lazy Evaluation Gradle Kotlin DSL build.gradle.kts Configuration Error Handling Patterns Result Type for Domain Operations require, check, error Collection Operations Idiomatic Collection Processing Quick Reference: Kotlin Idioms Idiom Description val over var Prefer immutable variables data class For value objects with equals/hashCode/copy sealed class/interface For restricted type hierarchies value class For type safe wrappers with zero overhead Expression when Exhaustive pattern matching Safe call ?. Null safe member access Elvis ?: Default value for nullables let / apply / also / run / with Scope functions for clean code Extension functions Add behavior without inheritance copy() Immutable updates on data classes require / check Precondition assertions Coroutine async / await Structured concurrent execution Flow Cold reactive streams sequence Lazy evaluation Delegation by Reuse implementation without inheritance Anti Patterns to Avoid Remember : Kotlin code should be concise but readable. Leverage the type system for safety, prefer immutability, and use coroutines for concurrency. When in doubt, let the compiler help you.

kotlin-patterns