/understand

Scala Language Prompt Snippet

Key Concepts

• Case Classes: Immutable data carriers with auto-generated equals, hashCode, copy, and pattern-matching support
• Pattern Matching: match expressions destructure ADTs exhaustively; the compiler warns on missing cases for sealed hierarchies
• Traits: Interface-plus-implementation mixins; stackable behavior via linearization
• Implicits / Given Instances: Scala 2 implicit / Scala 3 given+using provide type-class instances and contextual parameters resolved at compile time
• Type Classes: Ad-hoc polymorphism via implicit/given instances (e.g. Cats' Functor, Monad); look for F[_] type parameters
• Higher-Kinded Types: Abstraction over type constructors (F[_]) — the foundation of tagless-final service definitions
• For-Comprehensions: Sugar over flatMap/map chains; the standard way to sequence effects (IO, Future, Either)
• Effect Systems: Cats Effect (IO, Resource, Fiber), ZIO, and FS2 model side effects as composable values run at the "end of the world"
• Companion Objects: Singleton paired with a class/trait holding factory methods, type-class instances, and ADT constructors
• Sealed Hierarchies (ADTs): sealed trait + case classes/objects model closed sums; enums in Scala 3

Import Patterns

• import package.ClassName — import a specific member
• import package.{A, B} — selector list importing several members
• import package._ (Scala 2) / import package.* (Scala 3) — wildcard import
• import package.{Name => Alias} (Scala 2) / import package.Name as Alias (Scala 3) — rename on import
• import cats.syntax.all._ — syntax-extension imports that enable extension methods (common in Typelevel code)

File Patterns

• build.sbt — sbt build definition; project/ holds build support code
• build.sc / build.mill — Mill build definition
• Main.scala / *App.scala — entry points (object ... extends IOApp for Cats Effect, extends App/@main otherwise)
• package.scala — package object holding package-level members (Scala 2 idiom)
• src/main/scala/ — main source root following sbt conventions
• src/test/scala/ — test source root; specs conventionally end in *Spec.scala / *Suite.scala

Common Frameworks

• Cats Effect — pure functional runtime with IO, Resource, and fiber-based concurrency
• ZIO — effect system with typed errors and environment (ZIO[R, E, A])
• Akka / Pekko — actor-based concurrency, streaming, and clustering
• Play Framework — full-stack MVC web framework
• http4s — pure functional HTTP server/client built on Cats Effect and FS2
• Spark — distributed data processing; look for Dataset/DataFrame transformations

Example Language Notes

Defines the service as a tagless-final trait UserRepo[F[_]] so the same business logic runs against IO in production and a state monad in tests. Given/implicit instances in the companion object wire the production implementation.

Uses a sealed trait Command with case-class variants matched exhaustively in the interpreter — the compiler flags any unhandled command when a new variant is added.