Question

我正在使用play-json_2.11，并且尝试递归解析某些案例类

sealed trait Tree

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

case class Leaf(leaf: Float) extends Tree

因此，基本上，每个节点都包含一个值和一列树（可以是节点或叶子）的列表。

因此，我在案例类的伴随对象中定义了隐式读者。而对象Tree

中的一个

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)
}

由于解析器相互引用，因此我无法定义它们并得到以下错误：

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]

在这种情况下如何解析树？（我不需要是特质）

这是我尝试过的小提琴https://scalafiddle.io/sf/sX8OkWI/3

我的输入是一个像这样的json

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

我想解析它

Node(1.0, Leaf(2.0))

Answer 1

这就是您需要的

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)
  }
}

Answer 2

您不需要伴随对象中的隐式对象..或与此相关的对象：

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

解析递归特征playsafe json scala

2 个答案: