Scala

• Functions as first-class values: val f = (x: Int) => x + 1
• Higher-order functions: map, filter, flatMap, fold, reduce, collect
• Option[T]: Some/None, map, flatMap, getOrElse, for-comprehensions
• Either[L, R]: Left/Right, error handling without exceptions
• Try[T]: Success/Failure, wrapping exception-throwing code
• Future[T]: async computation, map, flatMap, recover, for-comprehensions
• For-comprehensions: syntactic sugar for flatMap chains
• Immutable collections by default: List, Vector, Map, Set
• Lazy evaluation: lazy val, LazyList (formerly Stream)
• Tail recursion: @tailrec annotation for stack-safe recursion
• Type classes: trait + given instances + extension methods

Collections

• Immutable: List, Vector (default), Map, Set, LazyList
• Mutable: ArrayBuffer, HashMap, HashSet (import scala.collection.mutable)
• Operations: map, flatMap, filter, fold, groupBy, partition, zip, collect
• Parallel collections: .par for parallel processing
• Views: lazy transformations for efficiency

Akka

• Actor model: ActorRef, Behavior, message passing
• Akka Typed: typed actors with Behavior[Message]
• Supervision: restart, stop, resume strategies
• Akka Streams: Source, Flow, Sink, backpressure
• Akka HTTP: routes, directives, marshalling/unmarshalling
• Akka Cluster: distributed computing, sharding

Build Tools

• sbt: build.sbt, libraryDependencies, plugins, multi-project builds
• sbt commands: compile, run, test, console, reload
• Mill: alternative build tool, simpler configuration
• Scala CLI: for scripts and small projects

Testing

• ScalaTest: FunSuite, FlatSpec, WordSpec, matchers (should/shouldBe)
• MUnit: lightweight testing framework
• ScalaCheck: property-based testing
• Mockito-Scala: mocking in Scala-idiomatic way

Best Practices

• Prefer immutable data and pure functions
• Use case classes for data, regular classes for services
• Pattern matching over instanceof/isInstanceOf
• For-comprehensions for monadic composition
• Avoid null — use Option, Either, Try
• Use Scala 3 syntax (given/using) over Scala 2 implicits in new code
• Leverage the type system: sealed traits for exhaustive matching