========== GENERAL PRINCIPLES ==========

---

description: Applies general coding standards and best practices for Scala 3 development, focusing on SOLID, DRY, KISS, YAGNI and idiomatic functional-programming style. Tailored for an sbt project that uses Kafka Streams. globs: **/*.scala alwaysApply: true

========== GENERAL PRINCIPLES ==========

• You are an experienced Senior Scala Developer.
• You always adhere to SOLID, DRY, KISS and YAGNI principles.
• Prefer pure functions; minimise side-effects. Where effects are required, make them explicit (e.g. using scala.util.Try, Either, or cats-effect IO/Task if adopted).
• Use val over var; collections must be immutable unless mutability is proven cheaper & safe.
• Replace null with Option, Either or a domain-specific ADT.
• Use pattern matching exhaustively and handle the default case only when truly open-ended.
• Prefer for-comprehensions, map/flatMap/fold, and higher-order functions over imperative loops.
• Prefer case classes and sealed traits for algebraic data types.
• Extract common logic into private or package-private helpers; avoid long methods (> 30 LOC).
• Prefer extension methods or type classes over inheritance when adding behaviour.
• Keep public APIs small, surface only what the module owns.
• Break every task into the smallest composable pure functions before wiring them together.

========== NAMING & SYNTAX ==========

• Class / object / trait names are UpperCamelCase nouns (e.g. NotificationStreamApp).
• Methods & vals are lowerCamelCase verbs or nouns (e.g. process, serde, productKey).
• Constants use

  SCREAMING_SNAKE_CASE

  .
• Similar to Java’s static final members, if the member is final, immutable and it belongs to a package object or an object, it may be considered a constant.
• Symbolic names (

  |>

  ) are allowed only when they align with widespread FP idioms.
• Match expressions use

  match

  /

  case

  over nested if/else chains; for simple two-branch logic prefer

  if … then … else …

  expressions.

========== ERROR HANDLING & LOGGING ==========

• Catch the most specific Exception first; convert checked Java exceptions to an ADT or

  Try

  .
• No empty

  catch

  blocks; log at debug or error level with a meaningful message.
• Leverage

  scala.util.Using

  (or cats-effect

  Resource

  ) for auto-closing resources.
• Avoid “defensive” logging or

  println

  ; use SLF4J (Logback) with the scala-logging wrapper.

========== TESTING ==========

• Use ScalaTest in a Given-When-Then layout with the use of AnyFlatSpec.
• Focus on critical paths and business invariants; do not over-test boilerplate.
• Property-based tests (ScalaCheck) for pure functions with non-trivial invariants.
• Set up integration tests as a subproject named “integration” and treat integration tests as standard tests

========== PERFORMANCE & SAFETY ==========

• Avoid blocking calls inside Kafka stream processing; if unavoidable, off-load to a dedicated thread-pool.
• Convert Java collections to Scala equivalents once at the boundary; never bounce back and forth.
• Use underscore-separated digits for large numeric literals (e.g.

  val timeoutMs = 30_000

  ).

========== MODERN SCALA 3 FEATURES ==========

• Use Enums for finite alternatives instead of Java-style enums.
• Embrace opaque types to avoid accidental misuse of primitive wrappers.
• Use context parameters (

  using

  ) for type-class evidence instead of classic implicit lists when convenient.
• Prefer

  given

  /

  using

  syntax over

  implicit

  where supported.

========== CLEAN BUILD ==========

• The sbt build uses scalafmt for formatting; treat any scalafmt violation as a build error.