Musings about FP and CS

A log of my journey through FP and CS

ADT serialization with Play JSON

by Clement Delafargue on April 13, 2013

Tagged as: playframework, fp, json.

I’m currently working on a HTTP API with playframework and part of my data is modeled using Algebraic Data Types (ADTs).

An ADT is roughly a combination of product types (multiple values together) and sum types (alternatives).

In scala, a product type can be modeled by a case class and sum types by a sealed trait. The sealed keyword ensures the closing of alternatives.

case class Point(x: Float, y: Float)

abstract sealed trait Shape
case class Circle(origin: Point, radius: Float) extends Shape
case class Polygon(points: List[Point]) extends Shape

Play 2.1 provides a great combinator-based API, as well as type-classes (Reads and Writes) to go back and forth between scala values and Json values.

For a quick refresher on typeclasses: http://blog.eklaweb.com/2012/05/23/champs-de-formulaire-custom-avec-play-2/#les_typeclass_en_scala (sorry, it’s in French)

Since there is an isomorphism between case classes and a subset of json (homogeneous lists, and a few more restrictions), Play-JSON is able to derive Reads and Writes instances from case classes with some macro-fu (props to @mandubian).

Mandubian helped me refactor my instances to a pointfree-er style which stresses better the combinatorial nature of the JSON API.

implicit val pointReads = Json.reads[Point]
implicit val pointWrites = Json.writes[Point]

However sum types are not supported by this automatic derivation, so you have to do it by hand:

import play.api.libs.json._
import play.api.libs.functional.syntax._

implicit val shapeReads = {
  val cr = Json.reads[Circle]
  val pr = Json.reads[Polygon]
  __.read[Circle](cr).map(x => x: Shape) |
  __.read[Polygon](pr).map(x => x: Shape)
}

implicit val shapeWrites = Writes[Shape] {
 case circle: Circle => Json.writes[Circle].writes(circle)
 case poly: Polygon =>  Json.writes[Polygon].writes(poly)
}

Observe that we don’t need to create implicit Reads and Writes instances for Polygon and Circle, since everything will be handled by shapeReads and shapeWrites. We directly use the generated instances inside reads and writes. We could have exposed these instances to the outside world but they’re not needed elsewhere and it would create overlapping instances.

Enjoy :)

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