如何将 JSON 转换为 Scala shapeless.hlist？

Question

我得到了 json 像 {"name":"susan","age":25}，还有 json 键集的提示，如 "name:String,age:Int"，如何从 json?

创建一个 HList

Answer 1

基于 code you added and then deleted, it seems, having a runtime-string hint "name:String,age:Int" and runtime-string json {"name":"susan","age":25}, you want to get an HList with runtime reflection。您可以按如下方式进行

import shapeless.HList
import scala.reflect.runtime
import scala.reflect.runtime.universe._
import scala.tools.reflect.ToolBox
val tb = runtime.currentMirror.mkToolBox()

val jsonStr = """{"name":"susan","age":25}"""
val hint = "name:String,age:Int"
val classType = tb.define(tb.parse(s"case class Test($hint)").asInstanceOf[ImplDef]).asClass.toType
val hlist = tb.eval(q"""
  import io.circe.generic.auto._
  import io.circe.parser.decode
  val classInstance = decode[$classType]($jsonStr)

  import shapeless.Generic
  Generic[$classType].to(classInstance.toOption.get)
""").asInstanceOf[HList]
println(hlist) // susan :: 25 :: HNil

请注意，您在运行时执行所有操作，因此您无法在编译时访问类型 String :: Int :: HNil，并且 hlist 仅具有静态类型 HList （不是 String :: Int :: HNil）和 HList 实际上并不比 List[Any].

build.sbt

libraryDependencies ++= Seq(
  scalaOrganization.value % "scala-reflect"  % scalaVersion.value,
  scalaOrganization.value % "scala-compiler" % scalaVersion.value,
  "com.chuusai" %% "shapeless" % "2.4.0-M1",
  "io.circe" %% "circe-core"    % "0.13.0",
  "io.circe" %% "circe-parser"  % "0.13.0",
  "io.circe" %% "circe-generic" % "0.13.0"
)

实际上，我想我们做了一些奇怪的事情。我们使用高度 type-level 旨在静态类型安全的库 (shapeless, circe)，然后运行它们在运行时反射忽略类型安全并实际获得 List[Any] ( HList).

我想如果 List[Any]（字段值列表）对你来说足够了，那么你只需要使用更多运行time 库。例如，使用 json4s

import org.json4s.{JInt, JObject, JString, JValue}
import org.json4s.jackson.JsonMethods._

val jsonStr: String = """{"name":"susan","age":25}"""
val json: JValue = parse(jsonStr) //JObject(List((name,JString(susan)), (age,JInt(25))))
val l: List[JValue] = json.asInstanceOf[JObject].obj.map(_._2) //List(JString(susan), JInt(25))
val res: List[Any] = l.map {
  case JString(s) => s
  case JInt(n)    => n
} //List(susan, 25)

build.sbt

libraryDependencies += "org.json4s" %% "json4s-jackson" % "3.6.9"

其实 Circe 也可以做同样的事情，只是用 parse 而不是 decode[A]

import io.circe.{Json, JsonNumber}
import io.circe.parser.parse

val jsonStr: String = """{"name":"susan","age":25}"""
val json: Json = parse(jsonStr).toOption.get //{"name":"susan","age":25}
val l: List[Json] = json.asObject.get.values.toList //List("susan", 25)
val res: List[Any] = l.map(_.fold[Any](null, null, (_: JsonNumber).toInt.get, identity[String], null, null)) //List(susan, 25)

如果你需要一个 case class 的实例或一个元组而不是 HList 替换

tb.eval(q"""
  import io.circe.generic.auto._
  import io.circe.parser.decode
  val classInstance = decode[$classType]($jsonStr)

  import shapeless.Generic
  Generic[$classType].to(classInstance.toOption.get)
""").asInstanceOf[HList] // susan :: 25 :: HNil

和

tb.eval(q"""
  import io.circe.generic.auto._
  import io.circe.parser.decode
  decode[$classType]($json).toOption.get
""").asInstanceOf[Product] //Test(susan,25)

或

tb.eval(q"""
  import io.circe.generic.auto._
  import io.circe.parser.decode
  val classInstance = decode[$classType]($json)

  import shapeless.Generic
  Generic[$classType].to(classInstance.toOption.get).tupled
""").asInstanceOf[Product] //(susan,25)

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