general-scala-clean-code.mdc

• Declare vals/vars as close as possible to first use.
• Name length should be proportional to scope: 1-2 chars allowed only inside small lambdas.
• Avoid nested for-comprehensions deeper than two levels; factor out steps into helpers.
• Split a single source file by responsibility.
• Use tail-rec optimisation (@tailrec) where appropriate.
• Prefer immutable collections and avoid mutation during iteration.
• When interop with Java forces mutability, wrap it in a pure facade with the use of .asScala, to retain functional API.
• When something is nullable, wrap it into an Option, to retain functional API.
• Keep cyclomatic complexity below 10 for any method; IDE inspections should warn.

general-scala-development-practices.mdc

========== 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.

kafka-development-practices.mdc

• All topic names config values (Typesafe Config or pure-config).
• Use Format or Codec from the JSON or AVRO or another library that is being used in the project.
• Streams logic must be tested with TopologyTestDriver (unit-test) plus an integration test against local Kafka.

linting-formatting.mdc

• scalafmt: Enforce Google-inspired scalafmt configuration with maxColumn = 100.