使用 Http4s 的 Circe 编码器和解码器答案

【问题标题】：Circe Encoders and Decoders with Http4s使用 Http4s 的 Circe 编码器和解码器
【发布时间】：2017-06-12 15:31:43
【问题描述】：

我正在尝试使用 http4s、circe 和 http4s-circe。

下面我尝试使用circe的自动推导功能。

import org.http4s.client.blaze.SimpleHttp1Client
import org.http4s.Status.ResponseClass.Successful
import io.circe.syntax._
import org.http4s._
import org.http4s.headers._
import org.http4s.circe._
import scalaz.concurrent.Task
import io.circe._

final case class Login(username: String, password: String)
final case class Token(token: String)

object JsonHelpers {
   import io.circe.generic.auto._
   implicit val loginEntityEncoder : EntityEncoder[Login] = jsonEncoderOf[Login]
   implicit val loginEntityDecoder : EntityDecoder[Login] = jsonOf[Login]
   implicit val tokenEntityEncoder: EntityEncoder[Token] = jsonEncoderOf[Token]
   implicit val tokenEntityDecoder : EntityDecoder[Token] = jsonOf[Token]
}

object Http4sTest2 extends App {
   import JsonHelpers._
   val url = "http://"
   val uri = Uri.fromString(url).valueOr(throw _)
   val list = List[Header](`Content-Type`(MediaType.`application/json`), `Accept`(MediaType.`application/json`))
   val request = Request(uri = uri, method = Method.POST)
      .withBody(Login("foo", "bar").asJson)
      .map{r => r.replaceAllHeaders(list :_*)}.run
   val client = SimpleHttp1Client()
   val result = client.fetch[Option[Token]](request){
      case Successful(response) => response.as[Token].map(Some(_))
      case _ => Task(Option.empty[Token])
   }.run
   println(result)
}

我得到这两个编译器错误的多个实例

Error:scalac: missing or invalid dependency detected while loading class file 'GenericInstances.class'.
Could not access type Secondary in object io.circe.Encoder,
because it (or its dependencies) are missing. Check your build definition for
missing or conflicting dependencies. (Re-run with `-Ylog-classpath` to see the problematic classpath.)
A full rebuild may help if 'GenericInstances.class' was compiled against an incompatible version of io.circe.Encoder.


Error:(25, 74) could not find implicit value for parameter encoder: io.circe.Encoder[Login]
   implicit val loginEntityEncoder : EntityEncoder[Login] = jsonEncoderOf[Login]

【问题讨论】：

我对 http4s 中的设计决策不是很熟悉，但我认为它只为io.circe.Json 提供EntityEncoder 实例是有意的，而不是为io.circe.Encoder 实例提供任何东西。在您的情况下，您可以通过使用 circe 的通用派生并在 Login 中的 Login 上调用 asJson 来跳过所有八个显式定义的实例@。

标签： scala circe http4s http4s-circe

【解决方案1】：

我能够解决这个问题。我在 google 上搜索了 sbt circe 依赖项，然后复制粘贴了第一个搜索结果。那是大约 0.1，这就是为什么事情对我不起作用的原因。

我将依赖项更改为

libraryDependencies ++= Seq(
  "org.http4s" %% "http4s-core" % http4sVersion,
  "org.http4s" %% "http4s-dsl" % http4sVersion,
  "org.http4s" %% "http4s-blaze-client" % http4sVersion,
  "org.http4s" %% "http4s-circe" % http4sVersion,
  "io.circe" %% "circe-core" % "0.7.0",
  "io.circe" %% "circe-generic" % "0.7.0"
)

现在自动推导工作正常，我可以编译下面的代码

import org.http4s.client.blaze.SimpleHttp1Client
import org.http4s._
import org.http4s.headers._
import org.http4s.circe._

import scalaz.concurrent.Task
import io.circe.syntax._
import io.circe.generic.auto._
import org.http4s.Status.ResponseClass.Successful

case class Login(username: String, password: String)
case class Token(token: String)

object JsonHelpers {
   implicit val loginEntityEncoder : EntityEncoder[Login] = jsonEncoderOf[Login]
   implicit val loginEntityDecoder : EntityDecoder[Login] = jsonOf[Login]
   implicit val tokenEntityEncoder: EntityEncoder[Token] = jsonEncoderOf[Token]
   implicit val tokenEntityDecoder : EntityDecoder[Token] = jsonOf[Token]
}

object Http4sTest2 extends App {
   import JsonHelpers._
   val url = "http://"
   val uri = Uri.fromString(url).valueOr(throw _)
   val list = List[Header](`Content-Type`(MediaType.`application/json`), `Accept`(MediaType.`application/json`))
   val request = Request(uri = uri, method = Method.POST)
      .withBody(Login("foo", "bar").asJson)
      .map{r => r.replaceAllHeaders(list :_*)}.run
   val client = SimpleHttp1Client()
   val result = client.fetch[Option[Token]](request){
      case Successful(response) => response.as[Token].map(Some(_))
      case _ => Task(Option.empty[Token])
   }.run
   println(result)
}

【讨论】：