--- name: scala-26ab10b1407b description: Expert Scala (Scala 3) assistance covering functional programming, Akka/Pekko, ZIO, Cats Effect, and Spark. Use when developing distributed systems, high-concurrency microservices, or big data pipelines. --- # Scala Scalable Language. It interops seamlessly with Java. ## When to Use - **Big Data Distributed Processing**: Apache Spark, Apache Flink, and large-scale data engineering pipelines. - **High-Concurrency Reactive Systems**: Building resilient distributed services with Akka/Pekko actors or Cats Effect/ZIO. - **Type-Level & Purely Functional Programming**: Enforcing compile-time domain guarantees with Scala 3 given/using and opaque types. - **Enterprise JVM Microservices**: Leveraging the entire Java ecosystem while utilizing concise, expressive FP idioms. ## Quick Start ```scala object Hello extends App { println("Hello, World!") val list = List(1, 2, 3) val doubled = list.map(_ * 2) println(doubled) } ``` ## Core Concepts ### Scala 3 Contextual Abstractions: Given & Using Type classes and dependency injection unified cleanly without implicit conversions: ```scala // Define type class trait JsonEncoder[A]: def encode(a: A): String // Given instances (type class implementations) given JsonEncoder[String] with def encode(a: String): String = s"\"${a.replace(""", "\\"")}\"" given JsonEncoder[Int] with def encode(a: Int): String = a.toString // Contextual function with 'using' clause def toJson[T](value: T)(using encoder: JsonEncoder[T]): String = encoder.encode(value) // Extension methods for clean syntax extension [T](value: T)(using encoder: JsonEncoder[T]) def asJson: String = encoder.encode(value) ``` ### Algebraic Data Types & Exhaustive Pattern Matching Modeling domain states with Scala 3 sealed traits and enums: ```scala enum PaymentStatus: case Pending case Authorized(transactionId: String, amountCents: Long) case Failed(reason: String, retryable: Boolean) def evaluatePayment(status: PaymentStatus): String = status match case PaymentStatus.Pending => "Waiting for gateway response..." case PaymentStatus.Authorized(txId, amt) => s"Payment $txId confirmed for \$${amt / 100.0}" case PaymentStatus.Failed(reason, true) => s"Transient error: $reason. Retrying..." case PaymentStatus.Failed(reason, false) => s"Permanent failure: $reason. Contacting customer." ``` ### Asynchronous Effect Management with Cats Effect Pure functional concurrency with resource safety and fiber cancellation: ```scala import cats.effect.{IO, IOApp, Resource} import scala.concurrent.duration.* object ServerApp extends IOApp.Simple: def acquireDbConnection: IO[String] = IO.println("DB Connected") *> IO.pure("conn-123") def releaseDbConnection(conn: String): IO[Unit] = IO.println(s"DB Disconnected: $conn") val dbResource: Resource[IO, String] = Resource.make(acquireDbConnection)(releaseDbConnection) def run: IO[Unit] = dbResource.use { conn => for fiber <- IO.sleep(100.millis).as("Query result").start res <- fiber.joinWithNever _ <- IO.println(s"Executed on $conn: $res") yield () } ``` ## Common Patterns ### Enum and Pattern Matching in Scala 3 **Problem**: Verbose sealed trait hierarchies for algebraic data types in legacy Scala 2. **Solution**: Use Scala 3 concise `enum` definitions: ```scala enum TransactionStatus: case Pending case Approved(txId: String, amount: BigDecimal) case Rejected(reason: String) def describe(status: TransactionStatus): String = status match case TransactionStatus.Pending => "Transaction is being processed" case TransactionStatus.Approved(id, amount) => s"Approved $id: $$$amount" case TransactionStatus.Rejected(reason) => s"Declined: $reason" ``` ## Best Practices **Do**: - Embrace Scala 3 syntax: use `enum`, `given`/`using`, and top-level definitions instead of Scala 2 package objects. - Use `opaque type` aliases for type-safe domain identifiers (e.g. `UserId`, `OrderId`) without heap allocation overhead. - Structure async applications around functional effect systems (`Cats Effect 3` or `ZIO 2`) rather than raw `scala.concurrent.Future`. - Enable compiler strictness (`-Xfatal-warnings`, `-Wunused:all`) to maintain clean codebases. **Don't**: - Use `var` or mutable collections in domain logic; use immutable collections and `case class .copy()`. - Catch raw `Throwable` without re-throwing non-fatal exceptions; use `scala.util.control.NonFatal`. - Use `Option.get` or `Try.get`; use `getOrElse`, pattern matching, or for-comprehensions. ## Troubleshooting | Error | Cause | Solution | | :---------------------------------------- | :--------------------------------------------------------------------- | :--------------------------------------------------------------------------------------------- | | `type mismatch: found ... required ...` | Incompatible types in expression or missing implicit conversion. | Verify types and import appropriate typeclass instances. | | `No implicit arguments found of type ...` | Missing `using` / implicit parameter in scope (e.g. ExecutionContext). | Import default execution context: `import scala.concurrent.ExecutionContext.Implicits.global`. | | `OutOfMemoryError during sbt compile` | JVM heap limit too low for Scala compiler. | Increase heap in `.sbtopts`: `-J-Xmx4G`. | ## References - [Scala Documentation](https://docs.scala-lang.org/) --- Source attribution: G1Joshi Original project: https://github.com/G1Joshi/Agent-Skills/blob/6a74cf03a8a86dc7d993ead3091b41123dbd0181/skills/languages/scala/SKILL.md License: MIT License information: https://opensource.org/license/mit Original upstream instructions; downloaded from Mega AI Hub. ## Upstream license notice MIT License Copyright (c) 2026 G1Joshi Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions: The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software. THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE. # Mega AI Hub resource metadata Creator / publisher: G1Joshi Original project: https://github.com/G1Joshi/Agent-Skills/blob/6a74cf03a8a86dc7d993ead3091b41123dbd0181/skills/languages/scala/SKILL.md Attribution basis: Source repository record Fields of work: Development & Engineering Roles: Backend Developer, Data Engineer, ML / AI Engineer Specialties: Backend Tools: Akka Country applicability: Country not specified Classification basis: Source catalog labels Instruction SHA-256: 8ee388748b980a7b37186ec5be2127222e67b90b5d9106469f88205f01c210bc