zen-of-kotlinlisted

# Zen of Kotlin Guiding principles for writing clear, correct, and maintainable Kotlin code. ## Make Illegal States Unrepresentable Model invariants in types so invalid data cannot compile or be constructed. - When a domain is genuinely closed, use sealed hierarchies (`sealed interface` / `sealed class`) with exhaustive `when` branches. - Represent optionality explicitly with nullable types (`T?`) and eliminate ambiguity at boundaries. - Use strongly typed value wrappers (e.g., `@JvmInline value class`) instead of raw primitives. - Scope resources correctly (`use { ... }`) so leaks are impossible in normal control flow. ## No Mutability by Default Prefer `val` over `var`, and immutable data structures over mutable ones. - Write pure functions first; keep side effects at the edge of the system. - Express state changes as new values (`copy(...)`) instead of in-place mutation. - Isolate unavoidable effects behind small interfaces so behavior stays testable. ## Make Data Types and Functions Domain-Oriented Name types and operations in the language of the domain, not infrastructure. - Replace primitive obsession with domain-specific types (`CustomerId`, `Email`, `Money`, etc.). - Parse, then model: convert untrusted input at boundaries into validated domain values. - Keep domain logic in domain types/functions; keep transport and persistence concerns outside. ## Prefer Composition, Expressiveness, and Clarity Use small, composable functions and extension functions when