Why does the Scala Macro for case class copy fail?

Question

I have about 24 Case classes that I need to programatically enhance by changing several common elements prior to serialization in a datastore that doesn't support joins. Since case classes don't have a trait defined for the copy(...) constructor, I have been attempting to use Macros - As a base I've looked at this post documenting a macro and come up with this macro:

When I try to compile, I get the following:

import java.util.UUID

import org.joda.time.DateTime
import scala.language.experimental.macros


trait RecordIdentification {
  val receiverId: Option[String]
  val transmitterId: Option[String]
  val patientId: Option[UUID]
  val streamType: Option[String]
  val sequenceNumber: Option[Long]
  val postId: Option[UUID]
  val postedDateTime: Option[DateTime]
}

object WithRecordIdentification {
  import scala.reflect.macros.Context

  def withId[T, I](entity: T, id: I): T = macro withIdImpl[T, I]

  def withIdImpl[T: c.WeakTypeTag, I: c.WeakTypeTag](c: Context)(
    entity: c.Expr[T], id: c.Expr[I]
  ): c.Expr[T] = {
    import c.universe._

    val tree = reify(entity.splice).tree
    val copy = entity.actualType.member(newTermName("copy"))

    val params = copy match {
      case s: MethodSymbol if (s.paramss.nonEmpty) => s.paramss.head
      case _ => c.abort(c.enclosingPosition, "No eligible copy method!")
    }

    c.Expr[T](Apply(
      Select(tree, copy),
      AssignOrNamedArg(Ident("postId"), reify(id.splice).tree) ::
      AssignOrNamedArg(Ident("patientId"), reify(id.splice).tree) ::
      AssignOrNamedArg(Ident("receiverId"), reify(id.splice).tree) ::
      AssignOrNamedArg(Ident("transmitterId"), reify(id.splice).tree) ::
      AssignOrNamedArg(Ident("sequenceNumber"), reify(id.splice).tree) :: Nil
    ))
  }
}

And I invoke it with something like:

class GenericAnonymizer[A <: RecordIdentification]() extends Schema {
  def anonymize(dataPost: A, header: DaoDataPostHeader): A = WithRecordIdentification.withId(dataPost, header)
}

But I get a compile error:

Error:(44, 71) type mismatch;
 found   : com.dexcom.rt.model.DaoDataPostHeader
 required: Option[String]
    val copied = WithRecordIdentification.withId(sampleGlucoseRecord, header)
Error:(44, 71) type mismatch;
 found   : com.dexcom.rt.model.DaoDataPostHeader
 required: Option[java.util.UUID]
    val copied = WithRecordIdentification.withId(sampleGlucoseRecord, header)
Error:(44, 71) type mismatch;
 found   : com.dexcom.rt.model.DaoDataPostHeader
 required: Option[Long]
    val copied = WithRecordIdentification.withId(sampleGlucoseRecord, header)

I'm not quite sure how to change the macro to support multiple parameters... any sage advice?

can you explain more on "I need to programatically enhance by changing several common elements prior to serialization" ? supposing you have 2 case classes: `case class Foo(a: String, b: String) case class Bar(x: String, y: Int)` — phantomastray, Nov 30 '16 at 18:51
So, suppose I have 25 case classes that all have common key elements (see the RecordIdentification case class in the example); all of these classes need to be anonymized which means that I have to replace the elements. For a single case class, it's easiest and makes the most sense to use the copy constructor. To save myself a lot of duplicate code, I'm searching for a way to do this in a more systematic fashion. — Steven Fines, Nov 30 '16 at 19:00
AFAIK `Macros` are used for compile-time reflection. What you need is runtime reflection. Check my answer. — phantomastray, Nov 30 '16 at 20:38
ok, if that's the case. You can use my approach if you choose to use runtime. :) — phantomastray, Nov 30 '16 at 21:10

phantomastray · Accepted Answer · 2016-11-30T21:05:42.377

Assuming you have a set of following case classes, which you wish to anonymize on certain attributes prior to serialization.

case class MyRecordA(var receiverId: String, var y: Int)
case class MyRecordB(var transmitterId: Int, var y: Int)
case class MyRecordC(var patientId: UUID, var y: Int)
case class MyRecordD(var streamType: String, var y: Int)
case class MyRecordE(var sequenceNumber: String, var streamType: String, var y: Int)

You can use scala reflection library to mutate an instance's attributes in runtime. You can implement your custom anonymize/enhancing logic in implicit anonymize method that the Mutator can use to alter a given instance's field selectively if required as per your implementation.

import java.util.UUID
import scala.reflect.runtime.{universe => ru}

implicit def anonymize(field: String /* field name */, value: Any /* use current field value if reqd */): Option[Any] = field match {
    case "receiverId" => Option(value.toString.hashCode)
    case "transmitterId" => Option(22)
    case "patientId" => Option(UUID.randomUUID())
    case _ => None
}

implicit class Mutator[T: ru.TypeTag](i: T)(implicit c: scala.reflect.ClassTag[T], anonymize: (String, Any) => Option[Any]) {

    def mask = {
        val m = ru.runtimeMirror(i.getClass.getClassLoader)
        ru.typeOf[T].members.filter(!_.isMethod).foreach(s => {
            val fVal = m.reflect(i).reflectField(s.asTerm)
            anonymize(s.name.decoded.trim, fVal.get).foreach(fVal.set)
        })

        i
    }
}

Now you can invoke masking on any instance as:

val maskedRecord = MyRecordC(UUID.randomUUID(), 2).mask

yes, `val` prevents it from being mutable. only the fields that you need to anonymize needs to be `vars`. — phantomastray, Dec 01 '16 at 07:38

Why does the Scala Macro for case class copy fail?

1 Answers1