Parsing a recursive trait playsafe json scala

Question

I am using play-json_2.11, and I am trying to parse recursively some case classes

sealed trait Tree

case class Node(value: Float, child: Seq[Tree]) extends Tree

case class Leaf(leaf: Float) extends Tree

So basically, each Node contains a value and a list of Trees (which can be a Node or a Leaf).

So I am defining implicit readers in the companion objects of the case classes. and one in the object called Tree

object Node {
  implicit val reader = Json.reads[Node]
}

object Leaf {
  implicit val reader = Json.reads[Leaf]
}

object Tree {
  implicit val treeReads = 
    __.read[Node].map(x => x:Tree) orElse __.read[Leaf].map(x => x:Tree)
}

As the parsers are referencing to each other, I cannot define them and get the following error:

ScalaFiddle.scala:9: error: No instance of play.api.libs.json.Reads is available for scala.Seq[ScalaFiddle.Tree] in the implicit scope (Hint: if declared in the same file, make sure it's declared before)
implicit val reader = Json.reads[Node]

How can I parse a Tree in this case? (I do not need it to be specifically a Trait)

Here is the fiddle I tried https://scalafiddle.io/sf/sX8OkWI/3

My input is a json like this one

{
    "value": 1.0,
    "child": {
        "leaf": 2.0
    }
}

And I would like to parse it to have

Node(1.0, Leaf(2.0))

Recent version of play json directly support sealed family in Json.format macro — cchantep
– cchantep, Commented Jan 3, 2019 at 16:24

Ivan Stanislavciuc · Accepted Answer · 2019-01-03 15:53:39Z

2

This is what you need

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

sealed trait Tree


case class Node(value: Float, child: Tree) extends Tree
object Node {
  implicit lazy val reader = Json.reads[Node]
}

case class Leaf(leaf: Float) extends Tree
object Leaf {
  implicit lazy val reader = Json.reads[Leaf]
}

object Tree {
  implicit lazy val treeReads: Reads[Tree] = 
    __.lazyRead(Node.reader).map(x => x:Tree) orElse __.lazyRead(Leaf.reader).map(x => x:Tree)
}


val json: JsValue = Json.parse("""
{
    "value": 5.0,
    "child": {
      "leaf": 7
    }
}
""")


println(json)

json.validate[Tree] match {
  case s: JsSuccess[Tree] => {
    val place: Tree = s.get
    println(place)
  }
  case e: JsError => {
    println(e)
  }
}

answered Jan 3, 2019 at 15:53

Ivan Stanislavciuc

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3 Comments

hasumedic Over a year ago

Not sure you need lazyness for the Node and Leaf Reads

Ivan Stanislavciuc Over a year ago

Yeah. Probably not

Sameer Mirji Over a year ago

What would be the format to add implicit writes for Tree? I have an usecase where I need both reads and writes.

airudah · Accepted Answer · 2019-01-03 16:40:41Z

You don't need the implicits in the companion objects.. or objects for that matter:

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

sealed trait Tree

case class Node(value: Double, child: Tree) extends Tree
case class Leaf(leaf: Double) extends Tree

val json: JsValue = Json.parse("""
{
    "value": 1.0,
    "child": {
      "leaf": 2.0
    }
}
""")

implicit val nReader = Json.reads[Node]
implicit val lReader = Json.reads[Leaf]
implicit lazy val treeReads: Reads[Tree] = 
    __.lazyRead(nReader).map(x => x:Tree) orElse __.lazyRead(lReader).map(x => x:Tree)

json.validate[Tree] match {
  case s: JsSuccess[Tree] => {
    val place: Tree = s.get
    println(place)
  }
  case e: JsError => {
    println(e)
  }
}

https://scalafiddle.io/sf/sX8OkWI/13

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