Generated Scala Code Usage

This section explains how to work with the Scala code that Scrooge generates.

Unions

Scrooge generates Scala case classes that can be used to construct union instances. Consider the following union which may have a string value or a User struct value:

struct User {
    1: i64 id
    2: string name
}

union Visitor {
  1: string guestName
  2: User user
}

To create an instance of Visitor, use the case classes named for each field:

Visitor.GuestName("Alice")
Visitor.User(User(123L, "Bob"))

The case classes for union fields can be used to access values in a typesafe way via pattern matching:

val v: Visitor = Visitor.GuestName("Erica")
val displayName = v match {
  case Visitor.GuestName(name) => "guest " + name
  case Visitor.User(u) => u.name
}

Because new fields can be added to unions, Scrooge provides UnknownUnionField which should be used for compatibility with newer versions:

val displayName = v match {
  case Visitor.GuestName(name) => "guest " + name
  case Visitor.User(u) => u.name
  case Visitor.UnknownUnionField(_) => "unknown visitor"
}

Note that you must use the provided extractors when pattern matching against unions; you cannot use the constructors for structs themselves. The following will fail with a MatchError:

userVisitor match {
  case User(id, name) => name
  case s: String => s
}

Optional functionality

For structs, you can opt into generating proxy classes. The generated code delegates to another instance. These may be useful for tests and otherwise.

Given the IDL:

struct User {
  1: i64 id
  2: string name
}

Add the annotation.

struct User {
  1: i64 id
  2: string name
} (com.twitter.scrooge.scala.generateStructProxy = "true")

Which generates the following code on the companion User object,

trait Proxy extends User {
  protected def _underlying_User: User
  override def id: Long = _underlying_User.id
  override def name: String = _underlying_User.name
  override def _passthroughFields: immutable$Map[Short, TFieldBlob] = _underlying_User._passthroughFields
}

Then, for example, Proxy can then be used in the following manner,

val alwaysFifty: User = new User.Proxy {
  protected def _underlying_User: User = aUserInstance
  override def id: Long = 50
}