具有隐式分辨率的无形 pb 的自定义类型类答案

【问题标题】：custom type class with shapless pb with implicit resolution具有隐式分辨率的无形 pb 的自定义类型类
【发布时间】：2017-08-29 17:56:06
【问题描述】：

我正在尝试使用 shapeless 从任意案例类中读取 json。

目前我正在尝试执行以下步骤

从 T，我有一个 FieldType[K1, V1] :: FieldType[K2, V2] :: ... 使用 LabelledGeneric

然后我想构建一个 Reads[V1] :: Reads[V2] 类型的 HList ...

这是我正在使用的代码：

  /*
   * To build the json reads from T 
   */
  trait HReads[PRepr <: HList] {
    type Out
    def reads: Out
  }

  object HReads {
    type Aux[PRepr <: HList, Out1 <: HList] = HReads[PRepr] { type Out = Out1 }

    implicit def readsHNil(): Aux[HNil, HNil] = new HReads[HNil] {
      type Out = HNil
      override def reads: Out = {
        throw new RuntimeException("Oups")
      }
    }

    implicit def readsSingleton[T, K <: Symbol](
       implicit
       kWitness: Witness.Aux[K],
       jsReads: play.api.libs.json.Reads[T]
     ): Aux[FieldType[K, T] :: HNil, Reads[T] :: HNil] = new HReads[FieldType[K, T] :: HNil] {
      type Out = Reads[T] :: HNil
      override def reads: Out = {
        val name: String = kWitness.value.name
        val pathReads: Reads[T] = (__ \ name).read[T](jsReads)
        pathReads :: HNil
      }
    }

    implicit def readsStd[T, K <: Symbol, RestRepr <: HList, Rest <: HList](
     implicit
     kWitness: Witness.Aux[K],
     jsReads: Reads[T],
     hreads: Lazy[HReads.Aux[RestRepr, Rest]]
    ): Aux[FieldType[K, T] :: RestRepr, Reads[T] :: Rest] = new HReads[FieldType[K, T] :: RestRepr] {
      type Out = Reads[T] :: Rest
      override def reads: Out = {
        val name: String = kWitness.value.name
        val pathReads: Reads[T] = (__ \ name).read[T](jsReads)
        val value: Rest = hreads.value.reads
        pathReads :: value
      }
    }

    def jsonReads[P]: JsonReads[P] = new JsonReads[P] {}

    implicit class JsonReadsOps[In](in: JsonReads[In]) {
      def jsonReads[K <: Symbol, T, InRepr <: HList, HR <: HList]()(
          implicit
          gen: LabelledGeneric.Aux[In, FieldType[K, T] :: InRepr],
          hreads: HReads.Aux[FieldType[K, T] :: InRepr, Reads[T] :: HR]
      ): Reads[T] :: HR = {
        hreads.reads
      }
    }
  }

  // And trying to use this like that : 
    import HReads._

  implicit val l = LabelledGeneric[MonPojo]

  private val allReads = jsonReads[MonPojo].jsonReads()
  println(s"All Reads $allReads")
  //[error] validation\validation.scala:428: could not find implicit value for parameter hreads: validation.validations.HReads.Aux[shapeless.labelled.FieldType[K,T] :: InRepr,play.api.libs.json.Reads[T] :: HR]
  //[error]   private val allReads = jsonReads[MonPojo].jsonReads()
  //[error]                                                      ^
  //[error] one error found

有人可以帮助我吗？

谢谢亚历克斯。

【问题讨论】：

标签： scala shapeless play-json

【解决方案1】：

这是 ReadsWithRules 的实现：

  trait ReadsWithRules[T, R <: HList] {
    def withRules(rules: R): Reads[T]
  }

  trait ReadsWithRulesLowerPriority {
    implicit def readsNoRule[T](implicit reads: Reads[T]): ReadsWithRules[T, HNil] = new ReadsWithRules[T, HNil] {
      override def withRules(rules: HNil): Reads[T] = reads
    }

    implicit def readsGeneric[Repr, A, R <: HList](implicit
                                                   gen: LabelledGeneric.Aux[A, Repr],
                                                   readsRepr: Lazy[ReadsWithRules[Repr, R]]
                                                  ): ReadsWithRules[A, R] =
      new ReadsWithRules[A, R] {
        override def withRules(rules: R): Reads[A] = {
          readsRepr.value.withRules(rules).map(r => gen.from(r))
        }
      }

  }

  object ReadsWithRules extends ReadsWithRulesLowerPriority {
    implicit def readHNil[R <: HList]: ReadsWithRules[HNil, R] = new ReadsWithRules[HNil, R] {
      override def withRules(rules: R): Reads[HNil] = implicitly[Reads[HNil]]
    }

    implicit def readNoRuleForHead[K <: Symbol, H, T <: HList, R <: HList](implicit
                                                                           witness: Witness.Aux[K],
                                                                           noRule: LacksKey[R, K],
                                                                           readsH: Reads[H],
                                                                           readsT: ReadsWithRules[T, R]
                                                                          ): ReadsWithRules[FieldType[K, H] :: T, R] =
      new ReadsWithRules[FieldType[K, H] :: T, R] {
        override def withRules(rules: R): Reads[FieldType[K, H] :: T] = new Reads[FieldType[K, H] :: T] {
          override def reads(json: JsValue): JsResult[FieldType[K, H] :: T] = {
            val name = witness.value
            val rH = (__ \ name).read(readsH)
            (rH and readsT.withRules(rules)) ((a, b) => (name ->> a :: b).asInstanceOf[FieldType[K, H] :: T]).reads(json)
          }
        }
      }

    implicit def readRuleForHead[K <: Symbol, H, T <: HList, R <: HList](implicit
                                                                         witness: Witness.Aux[K],
                                                                         at: shapeless.ops.record.Selector.Aux[R, K, Reads[H]],
                                                                         readsH: Reads[H],
                                                                         readsT: ReadsWithRules[T, R]
                                                                        ): ReadsWithRules[FieldType[K, H] :: T, R] =
      new ReadsWithRules[FieldType[K, H] :: T, R] {
        override def withRules(rules: R): Reads[FieldType[K, H] :: T] = new Reads[FieldType[K, H] :: T] {
          override def reads(json: JsValue): JsResult[FieldType[K, H] :: T] = {
            val name = witness.value
            val additionalRule: Reads[H] = at(rules)
            val rH = (__ \ name).read(readsH) andKeep (__ \ name).read(additionalRule)
            (rH and readsT.withRules(rules)) ((a, b) => (name ->> a :: b).asInstanceOf[FieldType[K, H] :: T]).reads(json)
          }
        }
      }

  }

  def readsWithRules[T, R <: HList](rules: R)(implicit readWithRule: ReadsWithRules[T, R]): Reads[T] =
    readWithRule.withRules(rules)

  case class MonPojo(numericField: Int)

  val r: Reads[MonPojo] = 
      readsWithRules[MonPojo, FieldType[Symbol with Tagged["numericField"], Reads[Int]] :: HNil](
        ('numericField ->> (min(0) keepAnd max(150))) :: HNil
      )
  println(
    r.reads(Json.obj(
      "stringField" -> "Tata",
      "numericField" -> 42
    ))
  )

  //JsSuccess(MonPojo(42),)

【讨论】：

【解决方案2】：

然后我想构建一个 Reads[V1] :: Reads[V2] 类型的 HList ...

不清楚为什么需要读取的 HList 而不是 HList 的读取（所以我猜您不需要另一个类型类 HRead，读取应该就足够了）。我猜你需要实现隐式：

implicit val readsHNil: Reads[HNil] = ???

implicit def readHCons[K <: Symbol, H, T <: HList](implicit
                                                     witness: Witness.Aux[K],
                                                     readsH: Reads[H],
                                                     readsT: Reads[T]): Reads[FieldType[K, H] :: T] = ???

implicit def readsGeneric[Repr, A](implicit
                                     gen: LabelledGeneric.Aux[A, Repr],
                                     readsRepr: Lazy[Reads[Repr]]): Reads[A] = ???

如果您需要，还有类似的两个用于副产品。

我写了一些实现

    import shapeless.{:+:, ::, CNil, Coproduct, HList, HNil, Inl, LabelledGeneric, Lazy, Witness}
    import shapeless.labelled.FieldType
    import play.api.libs.json._
    import shapeless.syntax.singleton._

  implicit val readsHNil: Reads[HNil] = Reads {
    case JsArray(values) if values.isEmpty   => JsSuccess(HNil)
    case JsObject(values) if values.isEmpty  => JsSuccess(HNil)
    case _ => JsError()
  }

  private def listToJsResult[K <: Symbol, H, T <: HList](l: List[JsValue])(implicit
                                                                           witness: Witness.Aux[K],
                                                                           readsH: Reads[H],
                                                                           readsT: Reads[T]): JsResult[FieldType[K, H] :: T] = {
    val name = witness.value
    l match {
      case Nil => JsError()
      case scala.::(head, tail) => for {
        h <- readsH.reads(head)
        t <- /*listToJsResult[K1, H1, T1](tail)*/ readsT.reads(JsArray(tail))
      } yield (name ->> h).asInstanceOf[FieldType[K, H]] :: t
    }
  }

  implicit val readsCNil: Reads[CNil] = Reads(_ => throw new Exception)

  implicit def readHCons[K <: Symbol, H, T <: HList](implicit
                                                     witness: Witness.Aux[K],
                                                     readsH: Reads[H],
                                                     readsT: Reads[T]): Reads[FieldType[K, H] :: T] =
    Reads {
      case arr: JsArray => listToJsResult[K, H, T](arr.value.toList)
      case obj: JsObject => listToJsResult[K, H, T](obj.values.toList)
      case js => listToJsResult[K, H, T](List(js))
    }


  implicit def readCCons[K <: Symbol, H, T <: Coproduct](implicit
                                                         witness: Witness.Aux[K],
                                                         readsH: Reads[H],
                                                         readsT: Reads[T]): Reads[FieldType[K, H] :+: T] = {
    val name = witness.value
    Reads { json =>
      (for {
        h <- readsH.reads(json)
      } yield Inl(name ->> h).asInstanceOf[FieldType[K, H] :+: T]) orElse {
        for {
          t <- readsT.reads(json)
        } yield Inr(name ->> t).asInstanceOf[FieldType[K, H] :+: T]
      }
    }
  }


  implicit def readsGeneric[Repr, A](implicit
                                     gen: LabelledGeneric.Aux[A, Repr],
                                     readsRepr: Lazy[Reads[Repr]]): Reads[A] =
    Reads(json => readsRepr.value.reads(json).map(gen.from))

  def reads[A](json: JsValue)(implicit readsInst: Reads[A]): JsResult[A] = readsInst.reads(json)

~~但它们似乎工作不正常~~ 他们似乎工作正常：

  sealed trait MyTrait
  case class MyClass1(x: Int, y: Int, z: Int) extends MyTrait
  case class MyClass2(x: Int, y: Int) extends MyTrait

  reads[MyClass1](JsObject(Seq("x" -> JsNumber(1), "y" -> JsNumber(2), "z" -> JsNumber(3))))

//JsSuccess(MyClass1(1,2,3),)

  reads[MyTrait](JsObject(Seq("x" -> JsNumber(1), "y" -> JsNumber(2), "z" -> JsNumber(3))))

//JsSuccess(MyClass1(1,2,3),)

  reads[MyTrait](JsObject(Seq("x" -> JsNumber(1), "y" -> JsNumber(2))))

//JsSuccess(MyClass2(1,2),)

答案部分基于库shapelaysson。

【讨论】：

我刚刚发布了答案，请参阅上面的回复

【解决方案3】：

我可以让你的代码像这样工作

implicit def readHCons[K <: Symbol, H, T <: HList, R <: HList](implicit
                                                               witness: Witness.Aux[K],
                                                               readsH: Reads[H],
                                                               readsT: Reads[T]): Reads[FieldType[K, H] :: T] =
  new Reads[FieldType[K, H] :: T] {
      override def reads(json: JsValue): JsResult[FieldType[K, H] :: T] = {
        val name = witness.value
        val jsonH = (__ \ name).read(readsH).reads(json)
        val jsonT = readsT.reads(json)
        (jsonH and jsonT) ((a, b) => (name ->> a :: b).asInstanceOf[FieldType[K, H] :: T])
      }
    }

但我的最终目标是能够添加额外的规则：像

  import validation2.ReadsWithRules._
  import play.api.libs.json.Reads._
  import play.api.libs.functional.syntax._

  val r: Reads[MonPojo] = LabelledGeneric[MonPojo].readsWithRules(('numericField ->> (min(0) keepAnd max(150))) :: HNil)
  r.reads(Json.obj(
    "stringField" -> "Tata",
    "numericField" -> 42
  ))
  println(s"All Reads $r")

我试图用这个来调整你的代码

  trait ReadsWithRules[T, R <: HList] {
    def withRules(rules: R): Reads[T]
  }

  trait ReadsWithRulesLowerPriority {

    implicit def readsHNil[R <: HNil]: ReadsWithRules[HNil, R] = new ReadsWithRules[HNil, R] {
      def withRules(rules: R) =
        new Reads[HNil] {
          override def reads(json: JsValue): JsResult[HNil] = JsSuccess(HNil)
        }
    }

    implicit def readHCons[K <: Symbol, H, T <: HList, R <: HList](implicit
                                                                   witness: Witness.Aux[K],
                                                                   readsH: Reads[H],
                                                                   readsT: ReadsWithRules[T, R]): ReadsWithRules[FieldType[K, H] :: T, R] =
      new ReadsWithRules[FieldType[K, H] :: T, R] {
        override def withRules(rules: R) = new Reads[FieldType[K, H] :: T] {
          override def reads(json: JsValue): JsResult[FieldType[K, H] :: T] = {
            val name = witness.value
            val jsonH = (__ \ name).read(readsH)
            val jsonT = readsT.withRules(rules)
            (jsonH and jsonT) ((a, b) => (name ->> a :: b).asInstanceOf[FieldType[K, H] :: T]).reads(json)
          }
        }
      }

  }

  object ReadsWithRules extends ReadsWithRulesLowerPriority {


    implicit def readHConsWithRule[K <: Symbol, H, T <: HList, R <: HList](implicit
                                                                   witness: Witness.Aux[K],
                                                                   at: shapeless.ops.record.Selector.Aux[R, K, Reads[H]],
                                                                   w: <:<[H, JsValue],
                                                                   readsH: Reads[H],
                                                                   readsT: ReadsWithRules[T, R]): ReadsWithRules[FieldType[K, H] :: T, R] =
      new ReadsWithRules[FieldType[K, H] :: T, R] {
        override def withRules(rules: R) = new Reads[FieldType[K, H] :: T] {
          override def reads(json: JsValue): JsResult[FieldType[K, H] :: T] = {
            val name = witness.value
            val additionnalRule: Reads[H] = at(rules)
            val jsonH = (__ \ name).read(readsH).andThen(additionnalRule)
            val jsonT = readsT
            (jsonH and jsonT.withRules(rules)) ((a, b) => (name ->> a :: b).asInstanceOf[FieldType[K, H] :: T]).reads(json)
          }
        }
      }

    implicit def readsGeneric[Repr, A, R <: HList](implicit
                                                   gen: LabelledGeneric.Aux[A, Repr],
                                                   readsRepr: Lazy[ReadsWithRules[Repr, R]]): ReadsWithRules[A, R] =
      new ReadsWithRules[A, R] {
        override def withRules(rules: R) : Reads[A] = {
          readsRepr.value.withRules(rules).map(r => gen.from(r))
        }
      }

    implicit class WithRules[T](gen: LabelledGeneric[T]) {
      def readsWithRules[R <: HList](rules: R)(implicit readWithRule: ReadsWithRules[T, R]): Reads[T] = {
        readWithRule.withRules(rules)
      }
    }
  }

但我有一个隐含的解析错误。

我之前的想法是分解子步骤中的问题：

第一步 T -> Reads[T1] :: Reads[T2] ...
在 Reads[T1] :: Reads[T2] 中合并附加规则 ...
序列读取[T1] ::读取[T2] ... =>读取[T1 :: T2 ...]

但我在第 1 步失败了......

【讨论】：

【解决方案4】：

我终于以另一种方式成功了：

object rules {

  import play.api.libs.json._
  import play.api.libs.functional.syntax._
  import scala.annotation.implicitNotFound
  import shapeless.labelled._
  import shapeless.syntax.singleton._
  import shapeless.{::, HList, HNil, LabelledGeneric, Lazy, Witness}
  import shapeless.ops.record.Selector

  trait SequenceReads[In <: HList] {
    type Out

    def apply(in: In): Reads[Out]
  }

  object SequenceReads {

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

    type Aux[A <: HList, B <: HList] = SequenceReads[A] {type Out = B}

    implicit def sequenceHnil[T, R <: HList, TR <: HList](): Aux[HNil, HNil] = new SequenceReads[HNil] {
      type Out = HNil

      override def apply(in: HNil): Reads[Out] = {
        throw new RuntimeException("Oups")
      }
    }

    implicit def sequenceSingleton[T, K <: Symbol](
                                                    implicit witness: Witness.Aux[K]
                                                  ): Aux[FieldType[K, Reads[T]] :: HNil, T :: HNil] = new SequenceReads[FieldType[K, Reads[T]] :: HNil] {
      type Out = T :: HNil

      override def apply(in: FieldType[K, Reads[T]] :: HNil): Reads[T :: HNil] = {
        val name = witness.value.name
        (__ \ name).read(in.head).map(_ :: HNil)
      }
    }

    implicit def sequence[T, K <: Symbol, R <: HList, TR <: HList](
                                                                    implicit
                                                                    witness: Witness.Aux[K],
                                                                    req: Lazy[SequenceReads.Aux[R, TR]]
                                                                  ): Aux[FieldType[K, Reads[T]] :: R, T :: TR] = new SequenceReads[FieldType[K, Reads[T]] :: R] {

      type Out = T :: TR

      override def apply(in: FieldType[K, Reads[T]] :: R): Reads[Out] = {
        val name = witness.value.name
        val head: Reads[T] = (__ \ name).read(in.head)
        val value: Reads[TR] = req.value.apply(in.tail)
        (head and value) {
          _ :: _
        }
      }
    }

    implicit class SequenceReadsOps[In <: HList](in: In) {

      class Builder[Out <: HList] {
        def apply(
                   implicit
                   sequence: SequenceReads.Aux[In, Out]
                 ): Reads[Out] = {
          sequence(in)
        }
      }

      def sequence[R <: HList](implicit s: SequenceReads.Aux[In, R]) = new Builder[R].apply(s)
    }

  }

  @implicitNotFound("Implicit not found: Rules type or fields are not valid")
  trait RuleValidation[Repr <: HList, Rules <: HList]

  object RuleValidation {

    implicit def validateHNil[Repr <: HList] : RuleValidation[Repr, HNil] =
      new RuleValidation[Repr, HNil] {}

    implicit def validateSingleton[Repr <: HList, K <: Symbol, V] (
       implicit
       sel: Selector.Aux[Repr, K, V]
     ): RuleValidation[Repr, FieldType[K, Reads[V]] :: HNil] =
      new RuleValidation[Repr, FieldType[K, Reads[V]] :: HNil] {}

    implicit def validateHCons[Repr <: HList, H, R <: HList, K <: Symbol, V] (
      implicit
      sel: Selector.Aux[Repr, K, V],
      validation: RuleValidation[Repr, R]
    ): RuleValidation[Repr, FieldType[K, Reads[V]] :: R] =
      new RuleValidation[Repr, FieldType[K, Reads[V]] :: R] {}
  }

  object ReadsWithRules {



    implicit def readsHNil: Reads[HNil] = new Reads[HNil] {
      override def reads(json: JsValue): JsResult[HNil] = JsSuccess(HNil)
    }


    implicit def readHCons[K <: Symbol, H, T <: HList, R <: HList](implicit
                                                                   witness: Witness.Aux[K],
                                                                   readsH: Reads[H],
                                                                   readsT: Reads[T]): Reads[FieldType[K, H] :: T] =
      new Reads[FieldType[K, H] :: T] {
        override def reads(json: JsValue): JsResult[FieldType[K, H] :: T] = {
          val name = witness.value
          val jsonH = (__ \ name).read(readsH)
          val jsonT = readsT
          (jsonH and jsonT) ((a, b) => (name ->> a :: b).asInstanceOf[FieldType[K, H] :: T]).reads(json)
        }
      }

    implicit def readsGeneric[Repr, A, R <: HList](implicit
                                                   gen: LabelledGeneric.Aux[A, Repr],
                                                   readsRepr: Lazy[Reads[Repr]]): Reads[A] = {
      readsRepr.value.map(r => gen.from(r))
    }
  }

  trait JsonRead[T]

  def jsonRead[T]: JsonRead[T] = new JsonRead[T] {}

  implicit class WithRules[A](gen: JsonRead[A]) {
    def readsWithRules[R <: HList, K <: Symbol, V, T0 <: HList, ARepr <: HList, AKeys <: HList, RKeys <: HList, Validation <: HList](rules: FieldType[K, V] :: T0)(
      implicit
      genA: LabelledGeneric.Aux[A, ARepr],
      readsInst: Reads[A],
      sequenceReads: SequenceReads[FieldType[K, V] :: T0],
      validation: RuleValidation[ARepr, FieldType[K, V] :: T0]
    ): Reads[A] = Reads[A] { json =>
      val valueA: JsResult[A] = readsInst.reads(json)
      val valueR: JsResult[sequenceReads.Out] = sequenceReads(rules).reads(json)
      (valueA, valueR) match {
        case (err1: JsError, err2: JsError) => err1 ++ err2
        case (err1: JsError, JsSuccess(_, _)) => err1
        case (JsSuccess(_, _), err2: JsError) => err2
        case (JsSuccess(v, p), _) => JsSuccess(v, p)
      }
    }
  }

}

还有测试

object Test extends App {

  import play.api.libs.json._
  import play.api.libs.json.Reads._
  import play.api.libs.functional.syntax._
  import shapeless._
  import syntax.singleton._
  import rules._

  case class Other(name: String)

  case class MonPojo(toto: String, tata: Int, other: Other)

  object MonPojo {
    implicit val readsOther = Json.reads[Other]
    implicit val reads: Reads[MonPojo] = Json.reads[MonPojo]
  }


  val strReads = pattern(".*".r)

  private val value: Reads[MonPojo] = jsonRead[MonPojo].readsWithRules(
    ('tata ->> (min(0) keepAnd max(150))) ::
      HNil
  )
  println(s"!!! ${
    value.reads(Json.obj(
      "toto" -> "test",
      "other" -> Json.obj("name" -> "test"),
      "tata" -> 25
    ))
  }")

}

感谢您的帮助，我会尝试您的解决方案。

【讨论】：

亲爱的@Larousso，太好了，您终于通过排序实现了您最初的想法。不客气，和你交流很愉快。感谢您接受我的回答。