diverging implicit expansion errors with Spray, Autowire and uPickle

[ozeebee]

Hi there, I'm facing compile errors on the server-side while trying to integrate Autowire within my existing Spray code that uses uPickle for serialization.
Here's my UPickleJsonSupport trait mixed into my Spray HttpService:

trait UPickleJsonSupport {
    implicit def genericMarshaller[T: Writer]: Marshaller[T] =
        Marshaller.delegate(ContentTypes.`application/json`)(writeJs(_))

    implicit def jsValueMarshaller: Marshaller[Js.Value] =
        Marshaller.of[Js.Value](ContentTypes.`application/json`) {
            (value, contentType, context) => {
                val entity = HttpEntity(contentType, json.write(value))

                context.marshalTo(entity)
            }
        }

    implicit def entityUnmarshaller[T: Reader]: Unmarshaller[T] =
        Unmarshaller[T](ContentTypeRange(MediaTypes.`application/json`)) {
            case entity => read[T](entity.asString(HttpCharsets.`UTF-8`))
        }
}

Ok, this is for the integration with Spray marshalling and this works fine until I throw Autowire into the mix:
Here's my AutowireServer:

object AutowireServer extends autowire.Server[Js.Value, Reader, Writer] {
    def read[Result: Reader](p: Js.Value) = upickle.default.readJs[Result](p)
    def write[Result: Writer](r: Result) = upickle.default.writeJs(r)
}

And my Spray route definition (rest of the code omitted for brevity):

    def apiRoute = post {
        path("api" / Segments) { s =>
            extract(_.request.entity.asString) { e =>
                complete {
                    AutowireServer.route[Api](ApiImpl)(
                        autowire.Core.Request(
                            s,
                            upickle.json.read(e).asInstanceOf[Js.Obj].value.toMap
                        )
                    )
                }
            }
        }
    }

With this code, I've got a sh*tload of compile errors (all related to this problem), ending with these:

...
[error]  both value derive$macro$173 of type => upickle.default.Writer[upickle.Js.Value]
[error]  and value derive$macro$175 of type => upickle.default.Writer[upickle.Js.Value]
[error]  match expected type upickle.default.Writer[upickle.Js.Value]
[error] /Users/toto/someproject/jvm/src/main/scala/some/pkg/RouteProvider.scala:38: diverging implicit expansion for type upickle.default.Writer[T1]
[error] starting with macro method macroW in trait LowPriX
[error]           AutowireServer.route[Api](ApiImpl)(
[error]                                             ^
[error] 13 errors found

My current knowledge of scala (which is rather poor) does not allow me to find the root cause of the problem. Looks like a clash between implicits generated by the macros ???
Now, the problem may come from uPickle rather than Autowire but since uPickle was working before I added Autowire, I create the ticket here ;)

In the meantime, I will simply generate Spray's HttpEntity myself (thus skipping the Marshaller) by adding this transformation on the router result .map(v => HttpEntity(jsonContentType, upickle.json.write(v))) but I guess (and hope) it should be possible to combine both libs.

Thanks for your help.

[alexmojaki]

I'm getting a somewhat similar error in my Play application:

ambiguous implicit values:
 both value derive$macro$30 of type => upickle.default.Writer[shared.IssueState]
 and value derive$macro$37 of type => upickle.default.Writer[shared.SideState]
 match expected type upickle.default.Writer[T1]

In /Users/alexhall/Dropbox/scala/debate/server/app/controllers/Application.scala:22
19    val requestText = request.body.asText.get
20    val requestJson = upickle.json.read(requestText).asInstanceOf[Js.Obj].value.toMap
21    val coreRequest: Core.Request[Value] = autowire.Core.Request(parts.split("/"), requestJson).asInstanceOf[Core.Request[Value]]
22    val future: Future[String] = AutowireServer.route[SharedApi](ApiImpl)(coreRequest).map(upickle.json.write(_)) 
23    val resultString: String = future.value.get.get
24    Ok(resultString)
25  }

[alexmojaki]

OK, I fixed my error by making sure that all classes to be pickled/unpickled had an unapply method. A more helpful error message would be nice.