a typeclass based JSON codec that's backend independent
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sjson-new

sjson-new is a typeclass based JSON codec, or wit for that Jawn.

today's random Philadelphia word

The term wit comes from the Philadelphia area. It's a quick way of ordering a cheesesteak "with" sautéed onions.

overview

sjson-new consists of two parts:

  1. A typeclass-based JSON codec toolkit
  2. Support packages which serialize to third-party ASTs

installation

sjson-new 0.5.x will be cross published for Scala 2.10, 2.11, and 2.12.

Here's how to use sjson-new with Spray:

libraryDependencies += "com.eed3si9n" %%  "sjson-new-spray" % "0.5.1"

Here's how to use sjson-new with Scala JSON:

libraryDependencies += "com.eed3si9n" %%  "sjson-new-scalajson" % "0.5.1"

Here's how to use sjson-new with MessagePack:

libraryDependencies += "com.eed3si9n" %%  "sjson-new-msgpack" % "0.5.1"

converting

sjson-new's converter let's you convert from an object of type A to and an AST of type J, given that you provide JsonFormat[A].

This is an example of how you might use the converter into your code:

scala> import sjsonnew.support.XYZ.Converter
import sjsonnew.support.XYZ.Converter

scala> import sjsonnew.BasicJsonProtocol._
import sjsonnew.BasicJsonProtocol._

scala> Converter.toJson[Int](42)
res0: scala.util.Try[XYZ.JsValue] = Success(42)

scala> Converter.fromJson[Int](res0.get)
res1: scala.util.Try[Int] = Success(42)

In the above substitute XYZ with (spray | scalajson.unsafe | msgpack).

A Converter object provides the following functions for conversion:

def toJson[A](obj: A)(implicit writer: JsonWriter[A]): Try[J]
def toJsonUnsafe[A](obj: A)(implicit writer: JsonWriter[A]): J
def fromJson[A](js: J)(implicit reader: JsonReader[A]): Try[A]
def fromJsonUnsafe[A](js: J)(implicit reader: JsonReader[A]): A

dependencies

  • sjson-new-core has not dependencies other than Scala.
  • The support libraries (e.g. sjon-new-spray) depend on the corresponding ASTs they are supporting.

JsonProtocol

sjson-new uses sjson's Scala-idiomatic typeclass-based approach to connect an existing type A with the logic how to (de)serialize its instances to and from JSON. This notion was further extended by spray-json, and sjson-new reuses some of both sjson and spray-json's code, see the 'Credits' section below.

While the typeclass in the original sjson directly manipulates the JSON AST, sjon-new adds an indirection mechanism, which allows it to be backend-independent. This machinary is inspired both by Jawn and Scala Pickling. This wire protocol indirection is called façade in Jawn, and "format" in Pickling. sjson-new will also call this façade to avoid the mixup.

The typeclass approach has the advantage of not requiring any change (or even access) to As source code. All (de)serialization logic is attached from the outside. There is no reflection involved, so the resulting conversions are fast. Scalas excellent type inference reduces verbosity and boilerplate to a minimum, while the Scala compiler will make sure at compile time that you provided all required (de)serialization logic.

In sjson-new's terminology a JSON protocol is nothing but a bunch of implicit values of type JsonFormat[A], whereby each JsonFormat[A] contains the logic of how to convert instance of A to and from JSON. All JsonFormat[A]s of a protocol need to be "mece" (mutually exclusive, collectively exhaustive), i.e. they are not allowed to overlap and together need to span all types required by the application.

This may sound more complicated than it is. sjon-new comes with a BasicJsonProtocol, which already covers all of Scala's value types as well as the most important reference and collection types. As long as your code uses nothing more than these you only need the BasicJsonProtocol. Here are the types already taken care of by the BasicJsonProtocol:

  • Byte, Short, Int, Long, Float, Double, Char, Unit, Boolean
  • String, Symbol
  • BigInt, BigDecimal
  • Option, Either, Tuple1 - Tuple22
  • List, Array
  • immutable.{Map, Iterable, Seq, IndexedSeq, LinearSeq, Set, Vector}
  • collection.{Iterable, Seq, IndexedSeq, LinearSeq, Set}
  • LList

LList

sjson-new comes with a datatype called LList, which stands for labelled heterogeneous list. List[A] that comes with the Standard Library can only store values of one type A. Unlike the standard List[A], LList can store values of different types per cell, and it can also store a label per cell. Because of this reason, each LList has its own type. Here's how it looks in the REPL:

scala> import sjsonnew._, LList.:*:
import sjsonnew._
import LList.$colon$plus$colon

scala> import BasicJsonProtocol._
import BasicJsonProtocol._

scala> val x = ("name", "A") :*: ("value", 1) :*: LNil
x: sjsonnew.LList.:*:[String,sjsonnew.LList.:*:[Int,sjsonnew.LNil]] = (name, A) :*: (value, 1) :*: LNil

scala> val y: String :*: Int :*: LNil = x
y: sjsonnew.LList.:*:[String,sjsonnew.LList.:*:[Int,sjsonnew.LNil]] = (name, A) :*: (value, 1) :*: LNil

BasicJsonProtocol is able to convert all LList values into a JSON object.

Custom types

In most cases however you'll also want to convert types not covered by the BasicJsonProtocol. In these cases you need to provide JsonFormat[A]s for your custom types

All you have to do is provide an isomorphism between your types and an LList using LList.iso function.

scala> import sjsonnew._, LList.:*:
import sjsonnew._
import LList.$colon$plus$colon

scala> import BasicJsonProtocol._
import BasicJsonProtocol._

scala> case class Person(name: String, value: Int)
defined class Person

scala> implicit val personIso = LList.iso(
         { p: Person => ("name", p.name) :*: ("value", p.value) :*: LNil },
         { in: String :*: Int :*: LNil => Person(in.head, in.tail.head) })
personIso: sjsonnew.IsoLList.Aux[Person,sjsonnew.LList.:*:[String,sjsonnew.LList.:*:[Int,sjsonnew.LNil]]] = sjsonnew.IsoLList$$anon$1@4140e9d0

scala> import sjsonnew.support.spray.Converter
import sjsonnew.support.spray.Converter

scala> Converter.toJson[Person](Person("A", 1))
res0: scala.util.Try[spray.json.JsValue] = Success({"name":"A","value":1})

Using personIso, sjson-new derived the JsonFomrat for Person.

Suppose now that we have an algebraic datatype (ADT) represented by a sealed trait. There's a function to compose the JsonFormat called unionFormat2, unionFormat3, ...

scala> import sjsonnew._, LList.:*:
import sjsonnew._
import LList.$colon$plus$colon

scala> import BasicJsonProtocol._
import BasicJsonProtocol._

scala> :paste
// Entering paste mode (ctrl-D to finish)

sealed trait Contact
case class Person(name: String, value: Int) extends Contact
case class Organization(name: String, value: Int) extends Contact

implicit val personIso = LList.iso(
  { p: Person => ("name", p.name) :*: ("value", p.value) :*: LNil },
  { in: String :*: Int :*: LNil => Person(in.head, in.tail.head) })
implicit val organizationIso = LList.iso(
  { o: Organization => ("name", o.name) :*: ("value", o.value) :*: LNil },
  { in: String :*: Int :*: LNil => Organization(in.head, in.tail.head) })
implicit val ContactFormat = flatUnionFormat2[Contact, Person, Organization]("type")

// Exiting paste mode, now interpreting.

scala> import sjsonnew.support.spray.Converter
import sjsonnew.support.spray.Converter

scala> Converter.toJson[Contact](Organization("Company", 2))
res0: scala.util.Try[spray.json.JsValue] = Success({"type":"Organization","name":"Company","value":2})

The flatUnionFormatN[U, A1, A2, ...]("type") functions assume that type U is the sealed parent trait of the type parameters A1, A2, and encodes them by putting the simple type name (just the class name portion) into the specifiedtype` field along side the value fields as:

{"type":"Organization","name":"Company","value":2}

On the other hand, the unionFormatN[U, A1, A2, ...] functions encodes them by putting the simple type name into type field, and the value in the value field:

{"value":{"name":"Company","value":2},"type":"Organization"}

BEWARE: Both unionFormatN and flatUnionFormatN functions only support leaf types in A1, A2, etc, so if your ADT consists of multiple levels of traits, you need to expand them yourself to a flat list of concrete types.

Low-level API: Builder and Unbuilder

If you want to drop down to a more lower level JSON writing, for example, to encode something as JString, sjon-new offers Builder and Unbuilder. This is a procedural style API, and it's closer to the AST. For instance, IntJsonFormat is defined as follows:

implicit object IntJsonFormat extends JsonFormat[Int] {
  def write[J](x: Int, builder: Builder[J]): Unit =
    builder.writeInt(x)
  def read[J](jsOpt: Option[J], unbuilder: Unbuilder[J]): Int =
    jsOpt match {
      case Some(js) => unbuilder.readInt(js)
      case None     => 0
    }
}

Builder provides other writeX methods to write primitive values. Unbuilder on the other hand provides readX methods.

BasicJsonProtocol already provides encoding for standard collections like List[A], but you might want to encode your own type using JSON array. To write a JSON array, use beginArray(), writeX methods, and endArray(). The builder internally tracks the states, so it won't let you end an array if you haven't started one.

To write a JSON object, you can use the LList isomorphism as described above, or use beginObject(), addField("name", x) method, and endObject(). Here's an example codec of the same case class Person using Builder/Unbuilder:

implicit object PersonFormat extends JsonFormat[Person] {
  def write[J](x: Person, builder: Builder[J]): Unit = {
    builder.beginObject()
    builder.addField("name", x.name)
    builder.addField("value", x.value)
    builder.endObject()
  }
  def read[J](jsOpt: Option[J], unbuilder: Unbuilder[J]): Person =
    jsOpt match {
      case Some(js) =>
        unbuilder.beginObject(js)
        val name = unbuilder.readField[String]("name")
        val value = unbuilder.readField[Int]("value")
        unbuilder.endObject()
        Person(name, value)
      case None =>
        deserializationError("Expected JsObject but found None")
    }
}

The other one was three lines of iso, but this is 25 lines of code. Since it doesn't create LList, it might run faster.

Credits

License

sjson-new is licensed under APL 2.0.

Patch Policy

Feedback and contributions to the project, no matter what kind, are always very welcome. However, patches can only be accepted from their original author. Along with any patches, please state that the patch is your original work and that you license the work to the sjon-new project under the project’s open source license.