Scala 3.4+ Development Specialist

Functional programming, effect systems, and big data processing for JVM applications.

Quick Reference

Auto-Triggers: Scala files (.scala, .sc), build files (build.sbt, project/build.properties)

Core Capabilities:

• Scala 3.4: Given/using, extension methods, enums, opaque types, match types
• Akka 2.9: Typed actors, streams, clustering, persistence
• Cats Effect 3.5: Pure FP runtime, fibers, concurrent structures
• ZIO 2.1: Effect system, layers, streaming, error handling
• Apache Spark 3.5: DataFrame API, SQL, structured streaming

Key Ecosystem Libraries:

• HTTP: Http4s 0.24, Tapir 1.10
• JSON: Circe 0.15, ZIO JSON 0.6
• Database: Doobie 1.0, Slick 3.5, Quill 4.8
• Streaming: FS2 3.10, ZIO Streams 2.1
• Testing: ScalaTest, Specs2, MUnit, Weaver

Module Index

This skill uses progressive disclosure with specialized modules:

Core Language

• functional-programming.md - Scala 3.4 features: Given/Using, Type Classes, Enums, Opaque Types, Extension Methods

Effect Systems

• cats-effect.md - Cats Effect 3.5: IO monad, Resources, Fibers, FS2 Streaming
• zio-patterns.md - ZIO 2.1: Effects, Layers, ZIO Streams, Error handling

Frameworks

• akka-actors.md - Akka Typed Actors 2.9: Actors, Streams, Clustering patterns
• spark-data.md - Apache Spark 3.5: DataFrame API, SQL, Structured Streaming

Implementation Guide

Project Setup (SBT 1.10)

ThisBuild / scalaVersion := "3.4.2"
ThisBuild / organization := "com.example"

lazy val root = (project in file("."))
  .settings(
    name := "scala-service",
    libraryDependencies ++= Seq(
      "org.typelevel" %% "cats-effect" % "3.5.4",
      "dev.zio" %% "zio" % "2.1.0",
      "com.typesafe.akka" %% "akka-actor-typed" % "2.9.0",
      "org.http4s" %% "http4s-ember-server" % "0.24.0",
      "io.circe" %% "circe-generic" % "0.15.0",
      "org.scalatest" %% "scalatest" % "3.2.18" % Test
    ),
    scalacOptions ++= Seq("-deprecation", "-feature", "-Xfatal-warnings")
  )

Quick Examples

Extension Methods:

extension (s: String)
  def words: List[String] = s.split("\\s+").toList
  def truncate(maxLen: Int): String =
    if s.length <= maxLen then s else s.take(maxLen - 3) + "..."

Given and Using:

trait JsonEncoder[A]:
  def encode(value: A): String

given JsonEncoder[String] with
  def encode(value: String): String = s"\"$value\""

def toJson[A](value: A)(using encoder: JsonEncoder[A]): String =
  encoder.encode(value)

Enum Types:

enum Result[+E, +A]:
  case Success(value: A)
  case Failure(error: E)

  def map[B](f: A => B): Result[E, B] = this match
    case Success(a) => Success(f(a))
    case Failure(e) => Failure(e)

Context7 Integration

Library mappings for latest documentation:

Core Scala:

• /scala/scala3 - Scala 3.4 language reference
• /scala/scala-library - Standard library

Effect Systems:

• /typelevel/cats-effect - Cats Effect 3.5 documentation
• /typelevel/cats - Cats 2.10 functional abstractions
• /zio/zio - ZIO 2.1 documentation
• /zio/zio-streams - ZIO Streams 2.1

Akka Ecosystem:

• /akka/akka - Akka 2.9 typed actors and streams
• /akka/akka-http - Akka HTTP REST APIs
• /akka/alpakka - Akka connectors

HTTP and Web:

• /http4s/http4s - Functional HTTP server/client
• /softwaremill/tapir - API-first design

Big Data:

• /apache/spark - Spark 3.5 DataFrame and SQL
• /apache/flink - Flink 1.19 streaming
• /apache/kafka - Kafka clients 3.7

Testing Quick Reference

ScalaTest:

class UserServiceSpec extends AnyFlatSpec with Matchers:
  "UserService" should "create user successfully" in {
    val result = service.createUser(CreateUserRequest("John", "john@example.com"))
    result.name shouldBe "John"
  }

MUnit with Cats Effect:

class UserServiceSuite extends CatsEffectSuite:
  test("should fetch user") {
    UserService.findById(1L).map { result =>
      assertEquals(result.name, "John")
    }
  }

ZIO Test:

object UserServiceSpec extends ZIOSpecDefault:
  def spec = suite("UserService")(
    test("should find user") {
      for result <- UserService.findById(1L)
      yield assertTrue(result.name == "John")
    }
  )

Troubleshooting

Common Issues:

• Implicit resolution: Use scalac -explain for detailed error messages
• Type inference: Add explicit type annotations when inference fails
• SBT slow compilation: Enable Global / concurrentRestrictions in build.sbt

Effect System Issues:

• Cats Effect: Check for missing import cats.effect.* or import cats.syntax.all.*
• ZIO: Verify layer composition with ZIO.serviceWith and ZIO.serviceWithZIO
• Akka: Review actor hierarchy and supervision strategies

Works Well With

• moai-lang-java - JVM interoperability, Spring Boot integration
• moai-domain-backend - REST API, GraphQL, microservices patterns
• moai-domain-database - Doobie, Slick, database patterns
• moai-workflow-testing - ScalaTest, MUnit, property-based testing

Additional Resources

For comprehensive reference materials:

• reference.md - Complete Scala 3.4 coverage, Context7 mappings, performance
• examples.md - Production-ready code: Http4s, Akka, Spark patterns

Last Updated: 2026-01-06 Status: Production Ready (v2.0.0)