scala – 找不到参数lgen的隐式值：shapeless.LabelledGeneric.A

发布时间：2020-12-16 18:31:22 所属栏目：安全来源：网络整理

导读：我是Shapeless的新手.我有一个帮助类,它利用无形的“自动类型类派生”( https://gist.github.com/negator/fbbcd1b2ce9a85b762b7)来帮助我填充json的读写. import play.api.libs._import json._import shapeless.{ `::` = :#:,_ }import poly._object SReads

我是Shapeless的新手.我有一个帮助类,它利用无形的“自动类型类派生”( https://gist.github.com/negator/fbbcd1b2ce9a85b762b7)来帮助我填充json的读写.

import play.api.libs._
import json._

import shapeless.{ `::` => :#:,_ }
import poly._

object SReads extends LabelledTypeClassCompanion[Reads] {
  object typeClass extends LabelledTypeClass[Reads] {

    def emptyProduct: Reads[HNil] = Reads(_ => JsSuccess(HNil))

    def product[F,T <: HList](name: String,FHead: Reads[F],FTail: Reads[T]) = Reads[F :#: T] {
      case obj @ JsObject(fields) =>
      for {
        head <- FHead.reads(obj  name)
        tail <- FTail.reads(obj - name)
        } yield head :: tail

        case _ => JsError("Json object required")
      }

    def project[F,G](instance: => Reads[G],to: F => G,from: G => F) = Reads[F](instance.map(from).reads)

    def emptyCoproduct: Reads[CNil] = Reads[CNil](_ => JsError("CNil object not available"))

    def coproduct[L,R <: Coproduct](
      name: String,cl: => Reads[L],cr: => Reads[R]) = Reads[L :+: R]{ js =>

      js match {
        case js @ JsString(n) if n == name => cl.reads(js).map(Inl.apply)
        case js @ _                        => cr.reads(js).map(Inr.apply)
      }
    }
  }
}

这就是我使用它的方式：

case class TrialMember(
  @Key("_id") var id: String,var weeks: String,var `type`: Option[String] = Some("email"),var updatedDate: Date = DateTime.now.toDate
)

object TrialMemberDao extends ModelCompanion[TrialMember,String] {
  def collection = mongoCollection("trial_member")
  val dao = new SalatDAO[TrialMember,String](collection) {}
  def emails() = dao.find(MongoDBObject("type" -> "email")).toList.map(_.id)
  def domains() = dao.find(MongoDBObject("type" -> "domain")).toList.map(_.id)
  def isTrialMember(userEmail: String): Boolean = {
    val trialMembers = emails() // whitelisted emails
    val trialDomains = domains() // whitelisted domains
    trialMembers.contains(userEmail) ||
      trialDomains.filter(userEmail.contains(_)).headOption.isDefined
  }

}

object TrialMember {
  implicit val jsonWrites: Writes[TrialMember] = SWrites.deriveInstance
  implicit val jsonReads: Reads[TrialMember] = SReads.deriveInstance
}

但升级到sbt 0.13.8和Play 2.4之后,现在它给了我这个错误：

could not find implicit value for parameter lgen: shapeless.LabelledGeneric.Aux[T,LKV]
[error]   implicit val reads: Reads[TrialMember] = SReads.deriveInstance

解决方法

麻烦有点搞笑,你的类型类派生完美实现,而编译器找不到应该用于类型字段的Option [String]的Reads实例.只需为您的代码提供以下定义：

object optionFormats {
  def noneReads[T]: Reads[Option[T]] = Reads(Function.const(JsSuccess(None)))

  implicit def optFormat[T](implicit w: Writes[T],r: Reads[T]) =
    Format[Option[T]](
      r.map[Option[T]](Some(_)).orElse(noneReads),Writes(_.fold[JsValue](JsNull)(w.writes)))
}

然后在任何代码中

import optionFormats._

你的实例应该按照需要构建.

（编辑：李大同）

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