Creating a function with generic type with ScalaPB

Question

I have the following function (which works on a protobuf object MyRequest

  def createRequestFromJson(requestJson: String): MyRequest = {
    val protoJson = getResource(requestJson)
    JsonFormat.fromJsonString[MyRequest](protoJson)
  }

I want to reuse this function with a different object so I added a type

  def createRequestFromJson[A](requestJson: String): A = {
    val protoJson = getResource(requestJson)
    JsonFormat.fromJsonString[A](protoJson)
  }

but then I get an error

Error:(68, 30) type arguments [A] do not conform to method fromJsonString's type parameter bounds [A <: scalapb.GeneratedMessage with scalapb.Message[A]]
JsonFormat.fromJsonString[A](protoJson)

I tried changing the definition to

  def createResponseFromJson[A <: scalapb.GeneratedMessage with scalapb.Message[A]](protoJsonFile: String): A = {

but still gives more errors

What am I doing wrong?

score 0 · Answer 1 · answered Jun 04 '18 at 00:05

JsonFormat.fromJsonString needs an implicit GeneratedMessageCompanion. It would work if you change the signature to:

def createResponseFromJson[A <: scalapb.GeneratedMessage with scalapb.Message[A]
     : GeneratedMessageCompanion](protoJsonFile: String): A

Creating a function with generic type with ScalaPB

1 Answers1