在 http4s 上运行时使用 Circe 将递归数据结构编码为 Json

Question

我正在构建一个非常简单的服务，它应该 return 一个通过递归案例定义的树状结构 class:

case class Node(id: Int, name: String, children: Seq[Node] = Seq())

但由于某种原因，我不断收到以下编译错误：

Error:(24, 70) could not find implicit value for parameter encoder: io.circe.Encoder[Seq[com.ansarada.ds.docviewer.server.Main.Node]]
implicit val nodesEncoder: EntityEncoder[Seq[Node]] = jsonEncoderOf[Seq[Node]]

Error:(24, 70) not enough arguments for method jsonEncoderOf: (implicit encoder: io.circe.Encoder[Seq[com.ansarada.ds.docviewer.server.Main.Node]])org.http4s.EntityEncoder[Seq[com.ansarada.ds.docviewer.server.Main.Node]]. Unspecified value parameter encoder. implicit val nodesEncoder: EntityEncoder[Seq[Node]] = jsonEncoderOf[Seq[Node]]

一旦我删除了一个子元素定义并将一个节点变成一个平面对象，代码就编译好了：

case class Node(id: Int, name: String)

任何人都可以帮我定义正确的 Json 编码器来处理嵌套子项的情况吗？

完整代码：

import org.http4s.circe._
import org.http4s.dsl._
import org.http4s.server.blaze.BlazeBuilder
import org.http4s.server.{Server, ServerApp}
import org.http4s.{EntityEncoder, HttpService}

import scalaz.concurrent.Task

object Main extends ServerApp {
  import io.circe.generic.auto._
  import io.circe.syntax._

  case class Node(id: Int, name: String, children: Seq[Node] = Seq())

  def getNodes: Seq[Node] = Seq(
    Node(0, "#One"),
    Node(1, "#Two"),
    Node(2, "#Three")
  )

  implicit val nodeEncoder: EntityEncoder[Node] = jsonEncoderOf[Node]
  implicit val nodesEncoder: EntityEncoder[Seq[Node]] = jsonEncoderOf[Seq[Node]]

  override def server(args: List[String]): Task[Server] = {
    val nodesService = HttpService {
      case _ @ GET -> Root / "nodes" =>
        Ok(getNodes.asJson)
    }

    BlazeBuilder
      .bindHttp(8080, "localhost")
      .mountService(nodesService, "/api")
      .start
  }
}

Answer 1

好的。使用 circe json 注释让它工作 - @JsonCodec。解决上述问题的分步指南：

先决条件 - 导入 circe 库：

"io.circe" %% "circe-core" % "0.8.0",
"io.circe" %% "circe-generic" % "0.8.0",
"io.circe" %% "circe-literal" % "0.8.0",
"io.circe" %% "circe-parser" % "0.8.0",

在你的 build.sbt:
中打开自动编译器插件
```
autoCompilerPlugins := true
```

添加scalamacros天堂编译器插件：

lazy val root = Project("root", file("."))
    .settings(
        addCompilerPlugin("org.scalamacros" % "paradise" % "2.1.0" cross CrossVersion.patch)
)

用@JsonCodec注释案例类:

@JsonCodec case class Node(id: Int, children: Seq[Node] = Seq())

投射对json的回应：
```
Ok(getNodes.asJson)
```

完整代码片段：

import org.http4s.circe._
import org.http4s.dsl._
import org.http4s.server.blaze.BlazeBuilder
import org.http4s.server.{Server, ServerApp}
import org.http4s.{EntityEncoder, HttpService}

import scalaz.concurrent.Task
object Main extends ServerApp {
  import io.circe.generic.auto._
  import io.circe.syntax._

  @JsonCodec case class Node(id: Int, name: String, children: Seq[Node] = Seq())

  def getNodes: Seq[Node] = Seq(
    Node(0, "#One"),
    Node(1, "#Two"),
    Node(2, "#Three")
  )

  override def server(args: List[String]): Task[Server] = {
    val nodesService = HttpService {
      case _ @ GET -> Root / "nodes" =>
        Ok(getNodes.asJson)
    }

    BlazeBuilder
      .bindHttp(8080, "localhost")
      .mountService(nodesService, "/api")
      .start
  }
}

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