Deferring

Consider an example of a User case class:

case class User(name: String, avatar: File)

Deserialization logic would look like this (needAvatar downloads the avatar from somewhere):

implicit val userRule = Resolvable.rule[JsValue, User] { __ ⇒
  (__ \ "name").read[String] and
  (__ \ "avatarUrl").read[String].fmap(needAvatar)
}

The problem here is that our entire data tree will wait until the pictures are downloaded. This may take time, and we could spend it more efficiently, for example, by showing at least user names in advance. Therefore a more suitable data structure would be

case class User(name: String, avatar: Future[File])

With this we can manipulate the data and handle the avatar asynchronously. The missing piece is the deserialization, and this is where we are going to use a special trick:

implicit val userRule = Resolvable.rule[JsValue, User] { __ ⇒
  (__ \ "name").read[String] and
  // notice the Resolvable.defer combinator
  (__ \ "avatarUrl").read[String].fmap(needAvatar).fmap(Resolvable.defer)
}

And here is the typical use-case:

async {
  val user = await(needUser("123").go)
  // show the name
  val avatar = await(user.avatar)
  // show the avatar later
}