Macros.scala

package rei

import scala.language.experimental.macros
import scala.reflect.macros.blackbox


object Macros {

  def ?[T](s: T): Option[T] = macro opt_impl[T]
  def opt[T](s: T): Option[T] = macro opt_impl[T]


  def opt_impl[T](c: blackbox.Context)(s: c.Expr[T]): c.Expr[Option[T]] = {
    import c.universe._
    def fail(message: String): Nothing = c.abort(c.enclosingPosition, message)
    val usage =
  """

     Hi, this is a macro for accessing protobuf option fields w/o manually writing "enrichers"
     this is how to use it:
        val optUser: Option[User] = opt(offer.getUser())
     it will wrap the dratted thing with an Option using protoc's "has" method
     because defaults are not nulls and we'd like to distinguish 'em
  """.stripMargin

    def check(that: Boolean, msg: String): Unit = if (!that) fail(s"$usage\n\nProblem: $msg")

    check(s.tree.symbol.isMethod, s"${s.tree.symbol} is not a method")
    //somehow calling 'isGetter' returns false, may be because it's java
    check(s.tree.symbol.owner.isClass, s"Are you sure ${s.tree.symbol.owner} is a protobuf generated class?")

    val get = s.tree.symbol.asMethod
    val item = get.name.toString.substring(3)
    val clazz = s.tree.symbol.owner.asClass
    val maybeHas = clazz.toType.members
      .find(s => s.isMethod && s.asMethod.name.toString == s"has$item")
    check(maybeHas.isDefined,
      s"There is no has$item method in class ${clazz.name.toString}, " +
        s"are you sure ${item.toLowerCase} is a protobuf optional field?")

    val has = maybeHas.get.asMethod

    s.tree match {
      case q"$obj.$get()" =>
        c.Expr(
          q"""
          if ($obj.$has())
            Some($s)
          else
            None
       """)
      case otherwise => fail(s"Something went wrong, tree: $otherwise")
    }
  }
}