ScalaJSDefinedApply.scala

import scala.collection.immutable.Seq
import scala.meta._

class ScalaJSDefinedApply extends scala.annotation.StaticAnnotation {
  inline def apply(defn: Any): Any = meta {
    def collectFields(trt: Defn.Trait) = {
      val params = trt.collect {
        case dv @ Defn.Val(_, pats, Some(tpe), q"js.undefined") =>
          pats.map { case Pat.Var.Term(name) =>
            (name, tpe, q"js.undefined")
          }
        case dv @ Defn.Val(_, _, None, q"js.undefined") =>
          abort(s"@ScalaJSDefinedApply: $dv must have explicit type declarations.")
        case dv @ Defn.Val(_, _, _, _) =>
          abort(s"@ScalaJSDefinedApply: $dv must have = js.undefined.")
      }

      params.flatten
    }

    def genApply(tname: Type.Name, fields: Seq[(Term.Name, Type, Term)]): Defn.Def = {

      val params = fields.map { case (name, tpe, default) =>
        param"$name: $tpe = $default"
      }

      val args = fields.map { case (name, _, _) =>
        arg"(${Lit.String(name.value)}, $name)"
      }

      q"def apply(..$params): $tname = js.Dynamic.literal(..$args).asInstanceOf[$tname]"
    }

    defn match {
      case dt @ Defn.Trait(_, tname, _, _, _) =>
        val fields = collectFields(dt)
        val applyMethod = genApply(tname, fields)
        val companion = q"object ${Term.Name(tname.value)} { $applyMethod }"

        // println(companion)
        Term.Block(Seq(dt, companion))

      case _ =>
        abort("@ScalaJSDefinedApply must annotate a trait.")
    }
  }
}