sjrd / scala-js-pickling   0.4.0


Pickling for Scala.js

Scala versions: 2.11 2.10
Scala.js versions: 0.6

Scala.js Pickling 0.4.0 -- obsolete

This project is obsolete and is not maintained anymore. There are many other serialization libraries for Scala.js that exist now, which do not suffer from this library's limitations.

Scala.js Pickling is a small serialization (aka pickling) library for Scala.js. It also cross-compiles on the JVM so that client and server can exchange pickled data transparently. It was inspired by Scala Pickling, but a major requirement is that everything happens at compile-time, and that pickling and unpickling never falls back on runtime reflection.

Therefore, types that have to be pickled and unpickled must be registered explicitly, (at least) once, in a PicklerRegistry beforehand:

import be.doeraene.spickling._

// custom data types
case class Person(name: String, age: Int)
case object TrivialCaseObject

// register the data types once

register expects implicit picklers and unpicklers for the type. This works out of the box for case classes and case objects. See the reference below for more details and how to create custom picklers.

After that, the case class Person and the case object TrivialCaseObject can be pickled an unpickled using the following in Scala.js:

import scala.scalajs.js

// import the implicits for the js.Any pickle format (~JSON objects)
import be.doeraene.spickling.jsany._

// pickle and unpickle
val pickle: js.Any = PicklerRegistry.pickle(Person("John", 24))
val value: Any = PicklerRegistry.unpickle(pickle)

The js.Any pickle format consists of typical JSON-serializable JavaScript values (primitives, arrays and dictionaries).

On the JVM, a pickle format for the Play! JSON library is provided by default, and is used similarly:

import play.api.libs.json._

// import the implicits for the Play! JSON pickle format
import be.doeraene.spickling.playjson._

// pickle and unpickle
val pickle: JsValue = PicklerRegistry.pickle(Person("John", 24))
val value: Any = PicklerRegistry.unpickle(pickle)

Important caveat with Byte, Short, Float and Double

Serializing a Byte, Short, Float or Double on the JS side and deserializing it on the JVM side gives unexpected results.

Instead of receiving a value of the type you began with, you will systematically receive an Int if the value fits in a Int, and a Double otherwise. In particular, even when the original value is typed as a Double, you can receive an Int.

This will cause the deserialization to go berskerk if the destination type cannot handle Int!

Work around: never use these 4 numeric types in the data structures you want to pickle and unpickle. Use java.lang.Number instead, which can accommodate both Ints and Doubles, then use its doubleValue() method (or another).


Before getting started, you should consider alternative serialization frameworks for Scala.js. Scala.js Pickling is rarely the best one. All the alternatives do not require classes to be registered in advance, for example. They also do more at compile-time (and are therefore faster), and do not have the caveat about numeric types documented hereabove.

Here are the alternatives known at the time of this writing:

The main advantage of Scala.js Pickling with respect to these alternatives is that it is able to serialize and deserialize data whose statically known type is not sealed, in particular, Any.

Getting Started

Scala.js Pickling is published on Maven Central.

On the JS side, all you need to do is to add the following to your build.sbt:

libraryDependencies += "be.doeraene" %%% "scalajs-pickling" % "0.4.0"

On the JVM side, with Play!, use:

libraryDependencies += "be.doeraene" %% "scalajs-pickling-play-json" % "0.4.0"

If you want to depend on the cross-compiling core, use:

libraryDependencies += "be.doeraene" %%% "scalajs-pickling-core" % "0.4.0"

scalajs-pickling 0.4.0 is built and published for Scala.js 0.6.x, with both Scala 2.10 and 2.11.


The basic snippets hereabove should get you started quickly. But it is probably worth reading this section to understand how the public API is articulated.


The main entry point to the library is the PicklerRegistry. At its core, a pickler registry is a thing that can pickle and unpickle values:

trait PicklerRegistry {
  def pickle[P](value: Any)(implicit builder: PBuilder[P],
      registry: PicklerRegistry = this): P
  def unpickle[P](pickle: P)(implicit reader: PReader[P],
      registry: PicklerRegistry = this): Any

Throughout the codebase and the API, P is the type of a pickle in a given format. P can be anything, really. The way the API interacts with generic P is through implicit pickle builders (PBuilder[P]) and pickle readers (PReader[P]). More on this later.

The basic implementation of PicklerRegistry is BasePicklerRegistry, and, for convenience, the top-level object PicklerRegistry is an instance of BasePicklerRegistry.

A BasePicklerRegistry, by default, can only pickle primitive types and strings. Other data types must be registered explicitly using one of its register methods. There are two main register methods: one for types, and one for values (typically singleton objects):

class BasePicklerRegistry extends PicklerRegistry {
  def register[A : ClassTag](implicit pickler: Pickler[A],
      unpickler: Unpickler[A]): Unit = { ... }

  def register[A <: Singleton](obj: A)(
      implicit name: SingletonFullName[A]): Unit = { ... }


object PicklerRegistry extends BasePicklerRegistry

Throughout the codebase and the API, A is the type of a value being pickled or unpickled.

Note that registering a type requires implicit pickler and unpickler for that type. Implicit picklers and unpicklers can be materialized automatically via macros for case classes. More on this later.

For values, a SingletonFullName[A] is required. Singleton full names can be materialized automatically via macros for case objects.

Pickle builders and readers

Pickle builders and readers abstract away the actual format of pickles. Not too much, though: they assume some kind of JSON-ish structure. Their definitions are:

trait PBuilder[P] {
  def makeNull(): P
  def makeBoolean(b: Boolean): P
  def makeNumber(x: Double): P
  def makeString(s: String): P
  def makeArray(elems: P*): P
  def makeObject(fields: (String, P)*): P

trait PReader[P] {
  def isUndefined(x: P): Boolean
  def isNull(x: P): Boolean
  def readBoolean(x: P): Boolean
  def readNumber(x: P): Double
  def readString(x: P): String
  def readArrayLength(x: P): Int
  def readArrayElem(x: P, index: Int): P
  def readObjectField(x: P, field: String): P

The packages be.doeraene.spickling.jsany and be.doeraene.spickling.playjson provide implicit pickle builders and readers for js.Any in Scala.js, and JsValue in Play! JSON, respectively. You may define your own if you want to work with a different implementation of JSON.

Picklers and unpicklers

The actual pickling work is done by type-aware picklers an unpicklers. Their definitions are simple:

trait Pickler[A] {
  def pickle[P](obj: A)(implicit registry: PicklerRegistry,
      builder: PBuilder[P]): P

trait Unpickler[A] {
  def unpickle[P](pickle: P)(implicit registry: PicklerRegistry,
      reader: PReader[P]): A

The library provides implicit picklers and unpicklers for all primitive types of Scala, as well as String. There are also implicit macros that can materialize picklers and unpicklers automatically for case classes. These will pickle recursively the parameters of the case class (in the first parameter list of the constructor). Other fields will not be pickled by the automatic picklers.

You can define custom picklers for your data types simply by implementing these two interfaces, and registering them to a pickler registry.


Scala.js Pickling is distributed under the Scala License.