Scala: Idiomatic Efficiency Reference

When to Use

• Use this skill when the task matches this description: Language-specific super-code guidelines for scala.

Table of Contents

1. Collections & Functional Transforms
2. Pattern Matching
3. Case Classes & ADTs
4. Option & Error Handling
5. Implicits & Given/Using
6. Concurrency
7. Anti-patterns specific to Scala

1. Collections & Functional Transforms {#collections}

// ❌ Imperative accumulation
val result = new ArrayBuffer[String]()
for (item <- items) {
  if (item.isActive) result += item.name.toUpperCase
}

// ✅
val result = items.filter(_.isActive).map(_.name.toUpperCase)

// ❌ Manual grouping
val grouped = mutable.Map[String, List[Item]]()
for (item <- items) {
  grouped(item.category) = grouped.getOrElse(item.category, Nil) :+ item
}

// ✅
val grouped = items.groupBy(_.category)

// ❌ Manual fold when sum/product works
var total = 0
for (o <- orders) total += o.amount

// ✅
val total = orders.map(_.amount).sum

// ❌ Using head on potentially empty collection
val first = items.head // throws on empty

// ✅
val first = items.headOption // returns Option[T]

// ❌ Chaining filter + head for find
val found = items.filter(_.id == targetId).head

// ✅
val found = items.find(_.id == targetId) // returns Option[T]

Use view for lazy evaluation on large collections to avoid intermediate allocations.

2. Pattern Matching {#patterns}

// ❌ if-else chain for type dispatch
if (shape.isInstanceOf[Circle]) {
  val c = shape.asInstanceOf[Circle]
  c.radius * c.radius * Math.PI
} else if (shape.isInstanceOf[Rect]) { ... }

// ✅
shape match {
  case Circle(r) => r * r * Math.PI
  case Rect(w, h) => w * h
}

// ❌ Nested match with identical fallthrough
x match {
  case 1 => "low"
  case 2 => "low"
  case 3 => "mid"
  case _ => "high"
}

// ✅
x match {
  case 1 | 2 => "low"
  case 3     => "mid"
  case _     => "high"
}

// ❌ Match to extract then use
val result = opt match {
  case Some(x) => x.toString
  case None    => "N/A"
}

// ✅
val result = opt.map(_.toString).getOrElse("N/A")
// or:
val result = opt.fold("N/A")(_.toString)

3. Case Classes & ADTs {#case-classes}

// ❌ Regular class for data
class User(val name: String, val age: Int) {
  override def equals(obj: Any): Boolean = ...
  override def hashCode(): Int = ...
  override def toString: String = ...
}

// ✅
case class User(name: String, age: Int)

// ❌ Sealed trait with unrelated case objects
sealed trait Result
case class Success(value: Int) extends Result
case class Failure(error: String) extends Result
case object Unknown extends Result // what does "Unknown" mean?

// ✅ — each variant should carry the data it represents
sealed trait Result[+A]
case class Success[A] (value: A) extends Result[A]
case class Failure(error: Throwable) extends Result[Nothing]

// ❌ (Scala 3) Verbose enum
sealed trait Color
object Color {
  case object Red extends Color
  case object Green extends Color
  case object Blue extends Color
}

// ✅ (Scala 3)
enum Color { case Red, Green, Blue }

4. Option & Error Handling {#option}

// ❌ Null checks
val name: String = if (user != null) user.name else "Unknown"

// ✅
val name = Option(user).map(_.name).getOrElse("Unknown")

// ❌ .get on Option (defeats the purpose)
val name = userOpt.get // throws if None

// ✅
val name = userOpt.getOrElse("default")
// or: userOpt.map(process).getOrElse(fallback)
// or: userOpt match { case Some(u) => ... case None => ... }

// ❌ Try with .get
val result = Try(parse(input)).get // throws on failure

// ✅
val result = Try(parse(input)) match {
  case Success(v) => v
  case Failure(e) => handleError(e)
}
// or: Try(parse(input)).getOrElse(default)
// or: Try(parse(input)).toEither

// ❌ Using exceptions for expected failures
def findUser(id: String): User = {
  val user = db.query(id)
  if (user == null) throw new NotFoundException(id)
  user
}

// ✅ — Option for absence, Either for expected errors
def findUser(id: String): Option[User] = db.query(id)
// or:
def findUser(id: String): Either[AppError, User]

5. Implicits & Given/Using {#implicits}

// ❌ (Scala 2) Implicit conversion that hides bugs

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