ScalaJSDefinedApply.scala

package com.emg.slidedeck.js.macros


import scala.collection.immutable.Seq
import scala.meta._
import scala.meta.dialects.Paradise211


class ScalaJSDefinedApply extends scala.annotation.StaticAnnotation {
  inline def apply(defn: Any): Any = meta {
    def extract(trt: Defn.Trait): Seq[(Term.Name, Option[String], Type, Term)] = {
      trt.collect {
        case dv @ Defn.Val(mods, Seq(Pat.Var.Term(name)), Some(tpe), rhs @ q"js.undefined") =>

          val jsName = mods.collectFirst {
            case mod"@JSName(${lit: Lit})" =>
              lit.value match {
                case s: String => s
                case _ => abort(s"@ScalaJSDefinedApply: @JSName must be called with a string literal.")
              }
            // todo: add case of js.Symbol
          }

          (name, jsName, tpe, rhs)

        case dv @ Defn.Val(_, Seq(_, _, _*), None, _) =>
          abort(s"@ScalaJSDefinedApply: $dv must only declare one left-hand side.")

        case dv @ Defn.Val(_,             _, None, _) =>
          abort(s"@ScalaJSDefinedApply: $dv must have an explicit type declaration.")

        case dv @ Defn.Val(_,             _,    _, _) =>
          abort(s"@ScalaJSDefinedApply: $dv must be initialized to js.undefined.")
      }
    }

    def generate(tname: Type.Name, schema: Seq[(Term.Name, Option[String], Type, Term)]): Defn.Def = {
      val (params, args) = schema.map {
        case (name, jsName, tpe, default) =>
          val namestr = Lit.String(jsName.getOrElse(name.value))
          param"$name: $tpe = $default" -> arg"($namestr, $name.asInstanceOf[js.Any])"
      }.unzip

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

    defn match {
      // A companion Object already exists in scope. Modify it.
      case Term.Block(Seq(
        dt @ Defn.Trait(_, tname, _, _, _),
        companion: Defn.Object
      )) =>

        val fields = extract(dt)
        val applyMethod = generate(tname, fields)

        Term.Block(Seq(dt, companion.copy(
          templ = companion.templ.copy(
            stats = companion.templ.stats.map { applyMethod +: _ }
          ))
        ))

      // A companion Object does not yet exist.
      case dt @ Defn.Trait(_, tname, _, _, _) =>
        val schema = extract(dt)
        val apply = generate(tname, schema)
        val companion = q"object ${Term.Name(tname.value)} { $apply }"

        Term.Block(Seq(dt, companion))

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