Question

Following on from:

Scala classOf for type parameter

I’ve tried implementing it but seem to get some weird generics problem, actually I’ve muddled my way through a couple of them now, but this is as close to correct I can get it…

I’m using the Scala – Jackson JSON bindings (excellent lib by the way, much easier than SJson)

def genparseResult[T: ClassManifest](t: T,s:String):Either[Tuple2[JsonParseException,String],T] = {
  try{
    val res = jsonSerializer.readValue(s,  classManifest[T].erasure)
    Right(res)
  }
  catch{
    case jpe:JsonParseException => Left((jpe,s))
  }
}

Anyhow, the code above is generating the following compile error:

type mismatch; found : res.type (with underlying type Any) required: T

I’m confused as hell. Should the code above be able to work?

Update
following input from tenshi, I post the completed class

import com.fasterxml.jackson.core.JsonParseException
object DatasiftJsonMapper {
  import java.util.Date
  import com.fasterxml.jackson.databind.{ Module, ObjectMapper }
  import com.fasterxml.jackson.module.scala.DefaultScalaModule

  val jsonSerializer = {
    val m = new ObjectMapper()
    m.registerModule(DefaultScalaModule)
    m
  }

  def parseDSResult(s: String): Either[Tuple2[JsonParseException, String], DatasiftResult] = {
    genparseResult(classOf[DatasiftResult], s)
  }

  def parseQRegRequest(s: String): Either[Tuple2[JsonParseException, String], QRegRequest] = {
    genparseResult(classOf[QRegRequest], s)
  }

  def genparseResult[T: ClassManifest](t: Class[T], s: String): Either[Tuple2[JsonParseException, String], T] = {
    try {
      val res = jsonSerializer.readValue(s, classManifest[T].erasure).asInstanceOf[T]
      Right(res)
    } catch {
      case jpe: JsonParseException => Left((jpe, s))
    }
  }
}

Answer 1

As far as I remember, classManifest[T].erasure returns Class[_] instead of Class[T], so the result of jsonSerializer.readValue(...) would be or type Any. You can try to cast parsing result:

val res = jsonSerializer.readValue(s,  classManifest[T].erasure).asInstanceOf[T]