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Mixing in a trait dynamically
Compile as follows:
scalac Common_1.scala Macros_2.scala
scalac Common_1.scala Test_3.scala -cp <path to the result of the previous compilation>
Tested in 2.10.0-M3, will most likely not compile by the time 2.10.0 final is released, because we're actively rehashing the API.
However the principles will remain the same in the final release, so the concept itself is okay.
upd. Code updated for 2.10.0-M7.
upd. Code updated for 2.10.0-RC1.
trait Persisted {
def id: Long
import language.experimental.macros
import scala.reflect.macros.Context
object Macros {
def toPersisted[T](instance: T, id: Long): T with Persisted = macro impl[T]
def impl[T: c.AbsTypeTag](c: Context)(instance: c.Expr[T], id: c.Expr[Long]) = {
import c.universe._
import Flag._
val t = implicitly[c.AbsTypeTag[T]].tpe // in RC1 this will become c.absTypeOf[T]
val u = t.typeSymbol.asClass
if (!u.isCaseClass)
c.abort(c.enclosingPosition, "toPersisted only accepts case classes, you provided %s".format(t))
// use reflection API to get the list of all declared fields
// more info here:
val accessors = t.members.sorted.collect{ case x: TermSymbol if x.isCaseAccessor && x.isMethod => x }
val fieldNames = accessors map (
// some of the missing parts of the reflection API
// we should be able to come up with something better in future point releases
val PARAMACCESSOR = (1L << 29).asInstanceOf[FlagSet]
// how did I know what trees to generate?
// read up the docs at
val instanceParam = ValDef(NoMods, newTermName("instance"), TypeTree(u.toType), EmptyTree)
val idParam = ValDef(Modifiers(PARAMACCESSOR), newTermName("id"), Ident(definitions.LongClass), EmptyTree)
val superArgs = fieldNames map (fieldName => Select(Ident(, fieldName))
val body = Apply(Select(Super(This(newTypeName("")), newTypeName("")), nme.CONSTRUCTOR), superArgs)
val ctor = DefDef(NoMods, nme.CONSTRUCTOR, Nil, List(List(instanceParam, idParam)), TypeTree(), Block(List(body), Literal(Constant(()))))
val idVal = idParam.duplicate
val tmpl = Template(List(Ident(u), Ident(c.mirror.staticClass("Persisted"))), emptyValDef, List(idVal, ctor))
val cdef = ClassDef(NoMods, newTypeName(c.fresh( + "$Persisted")), Nil, tmpl)
val init = New(Ident(, List(List(instance.tree, id.tree)))
c.Expr(Block(cdef, init))
object Test extends App {
import Macros._
case class Person(first: String, last: String)
val p = toPersisted(Person("hello", "world"), 42)
C:\Projects\Kepler\sandbox>scalac Common_1.scala Macros_2.scala
<scalac has exited with code 0>
C:\Projects\Kepler\sandbox>scalac -Ymacro-debug-lite Common_1.scala Test_3.scala -cp .
typechecking macro expansion Macros.toPersisted[Test.Person](Test.this.Person.apply("hello", "world"), 42L)
at source-C:\Projects\Kepler\sandbox\Test_3.scala,line-4,offset=114
class Person$Persisted1 extends Person with Persisted {
val id: Long = _;
def <init>(instance: Test.Person, id: Long) = {
super.<init>(instance.first, instance.last);
new Person$Persisted1(Test.this.Person.apply("hello", "world"), 42L)
<scalac has exited with code 0>
C:\Projects\Kepler\sandbox>scala Test

I followed all steps and double check it but still the Test outputs 0 as id, the debug shows the same output as the one provided here. Any thoughts?
scala 2.10.0-M7


Is this code still valid? I get this error:
type AbsTypeTag is not a member of


"In 2.10.0-RC1 AbsTypeTag has been renamed to WeakTypeTag. Everything else about macros and type tags remains the same."


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