encoder.scala

package xmlformat

import scala.xml._

import export._
import shapeless._
import shapeless.labelled._

trait Encoder[A] {
  def toXml(a: A): NodeSeq
}
object Encoder extends EncoderLowPriority

@imports[Encoder]
trait EncoderLowPriority {
  implicit val EncoderString: Encoder[String] = new Encoder[String] {
    override def toXml(a: String): NodeSeq = ???
  }

  implicit def EncoderOption[A: Encoder]: Encoder[Option[A]] =
    new Encoder[Option[A]] {
      override def toXml(a: Option[A]): NodeSeq = ???
    }
}

trait DerivedEncoder[T] extends Encoder[T]
@exports object DerivedEncoder {
  implicit def gen[T, Repr](
      implicit
      G: LabelledGeneric.Aux[T, Repr],
      LER: Cached[Strict[DerivedEncoder[Repr]]]
    ): DerivedEncoder[T] = ???

  implicit def hnil: DerivedEncoder[HNil] = ???
  implicit def hcons[Key <: Symbol, Value, Remaining <: HList](
      implicit Key: Witness.Aux[Key],
      LEV: Lazy[Encoder[Value]],
      LER: DerivedEncoder[Remaining]
    ): DerivedEncoder[FieldType[Key, Value] :: Remaining] = ???

  implicit def cnil: DerivedEncoder[CNil] = null
  implicit def ccons[Name <: Symbol, Instance, Remaining <: Coproduct](
      implicit
      Name: Witness.Aux[Name],
      LEI: Lazy[Encoder[Instance]],
      LER: DerivedEncoder[Remaining]
    ): DerivedEncoder[FieldType[Name, Instance] :+: Remaining] = ???

}

package examples {
  sealed trait SimpleTrait
  final case class Foo(s: String) extends SimpleTrait
  final case class Bar() extends SimpleTrait
  case object Baz extends SimpleTrait
  final case class Faz(o: Option[String]) extends SimpleTrait
}

object Example {
  import examples._
  import DerivedEncoder.exports._

  implicitly[Encoder[Foo]]
  implicitly[Encoder[Option[String]]]
  // implicitly[Encoder[SimpleTrait]]
}