Instantiating a trait with a type member in a macro

type-member-example.scala

/** Instantiating a trait with a type member in a macro.
  *
  * Complete working example by Travis Brown for this Stack Overflow question:
  *     http://stackoverflow.com/q/13795490/334519
  */

import scala.language.existentials
import scala.language.experimental.macros

trait TypeBuilder { type fieldType }

object TypeMemberExample {
  import scala.reflect.macros.Context

  def builderWithType[T]: TypeBuilder = macro builderWithType_impl[T]

  def builderWithType_impl[T: c.WeakTypeTag](c: Context) = {
    import c.universe._

    val fieldMemberType: Type = c.weakTypeOf[T]

    val anon = newTypeName(c.fresh)

    c.Expr(
      Block(
        ClassDef(
          Modifiers(Flag.FINAL),
          anon,
          Nil,
          Template(
            Ident(newTypeName("TypeBuilder")) :: Nil,
            emptyValDef,
            List(
              constructor(c),
              TypeDef(
                Modifiers(),
                newTypeName("fieldType"),
                Nil,
                TypeTree(fieldMemberType)
              )
            )
          )
        ),
        Apply(Select(New(Ident(anon)), nme.CONSTRUCTOR), Nil)
      )
    )
  }

  def constructor(c: Context) = {
    import c.universe._

    DefDef(
      Modifiers(),
      nme.CONSTRUCTOR,
      Nil,
      Nil :: Nil,
      TypeTree(),
      Block(
        Apply(
          Select(Super(This(tpnme.EMPTY), tpnme.EMPTY), nme.CONSTRUCTOR),
          Nil
        )
      )
    )
  }
}