Could not find implicit lgen parameter value: shapeless.LabelledGeneric.Aux

Question

I am new to Shapeless. I have a helper class that uses the formless "Automatically generate shadow code" ( https://gist.github.com/negator/fbbcd1b2ce9a85b762b7 ) to help me fill in json, read and write.

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) } } } } 

Here is how I used it:

 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 } 

But after upgrading to sbt 0.13.8 and Play 2.4, now it gives me this error:

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

Answer 1

The problem is that your typeclass developer is perfectly implemented, and the compiler could not find the Reads instance for Option[String] to be used for the type field. Just put your code with something like the following definition:

 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))) } 

And then in any code just

 import optionFormats._ 

And your copy should be designed as desired.