How to debug akka http's route dsl - scala

Question

I am attempting to create a unmarshaller for akka http, going from avro to a custom case class. But it gives me a very vague error: "could not find implicit value". How can I debug this or make scala give me hint where the problem is?
I set up the route as such:
class MetricsRoute(implicit val system: ActorSystem, implicit val materializer: ActorMaterializer) {
import system.dispatcher
def getRoute() = {
path("metrics") {
put {
decodeRequest {
entity(as[Metrics]) { metrics: Metrics =>
println(metrics.time)
complete(HttpEntity(ContentTypes.`text/html(UTF-8)`, "<h1>hi!</h1>"))
}
}
}
}
}
In the same class I also created the unmarshaller like this:
implicit def avroUnmarshaller(): FromRequestUnmarshaller[Metrics] =
Unmarshaller.withMaterializer {
implicit ex: ExecutionContext =>
implicit mat: Materializer =>
request: HttpRequest => {
val inputStream = request.entity.dataBytes.runWith(
StreamConverters.asInputStream(FiniteDuration(3, TimeUnit.SECONDS))
)
val reader = new SpecificDatumReader[AvroMetrics](classOf[AvroMetrics])
val decoder:BinaryDecoder = DecoderFactory.get().binaryDecoder(inputStream, null)
//AvroMetrics is a case class generated from the avro schema
val avroMetrics:AvroMetrics = AvroMetrics(0, 0, List())
reader.read(avroMetrics, decoder)
Future {
//converts the avro case class to the case class specific for my application
convertMetrics(avroMetrics)
}
}
}
But this gives me the very vague 'could not find implicit value' error:
[error] /mypath/MetricsRoute.scala:34: could not find implicit value for parameter um: akka.http.scaladsl.unmarshalling.FromRequestUnmarshaller[my.package.types.Metrics]
[error] entity(as[Metrics]) { metrics: Metrics =>
[error] ^
[error] one error found
[error] (compile:compileIncremental) Compilation failed
How do I go about debugging what is missing or what I did wrong?
Edit:
Worth noting is when I specify the unmarshaller myself it does work, so changing
entity(as[Metrics]) { metrics: Metrics =>
to
entity(avroUnmarshaller) { metrics: Metrics =>
Which seems to indicate the marshaller code itself isn't wrong, but I've done something wrong with the types?

Answer 1

The compiler is looking for a FromRequestUnmarshaller[Metrics], what you defined is of type () => FromRequestUnmarshaller[Metrics].
Try defining your implicit with no empty braces, e.g.
implicit def avroUnmarshaller: FromRequestUnmarshaller[Metrics] = ???
instead of
implicit def avroUnmarshaller(): FromRequestUnmarshaller[Metrics] = ???
(also, it could be made a val, but that's not relevant to this issue)