security-hooks/.crosslink/rules/scala.md

Scala Best Practices

Code Style

• Follow Scala Style Guide
• Prefer immutability (val over var)
• Use case classes for data
• Leverage pattern matching

// GOOD: Idiomatic Scala
case class User(id: String, name: String)

class UserService(repository: UserRepository) {
  def findUser(id: String): Option[User] =
    repository.find(id)

  def processUser(id: String): Either[Error, Result] =
    findUser(id) match {
      case Some(user) => Right(process(user))
      case None       => Left(UserNotFound(id))
    }
}

Error Handling

• Use Option for missing values
• Use Either or Try for operations that can fail
• Avoid throwing exceptions in pure code

// GOOD: Using Either for errors
def parseConfig(json: String): Either[ParseError, Config] =
  decode[Config](json).left.map(e => ParseError(e.getMessage))

// Pattern match on result
parseConfig(input) match {
  case Right(config) => useConfig(config)
  case Left(error)   => logger.error(s"Parse failed: $error")
}

Security

• Use prepared statements for database queries
• Validate input with refined types when possible
• Never interpolate user input into queries