unfold.scala

import shapeless.CNil
import shapeless.Coproduct
import shapeless.PolyDefns.Case1
import shapeless.ops.coproduct.Basis
import shapeless.ops.coproduct.Remove

trait Unfold[F, A, C <: Coproduct] {
  type Out = Option[C]
  def apply(a: A): Out
}

object Unfold {
  type Aux[F, A, C <: Coproduct, Z] = Unfold[F, A, C] {
    type Out = Z
  }

  implicit def case_1[F, A, H, C1 <: Coproduct, C2 <: Coproduct]
  (implicit rm: Remove.Aux[C1, H, C2], gt: C1 Basis C2, poly: Case1.Aux[F, A, Option[H]], next: Unfold[F, A, C2]) = new Unfold[F, A, C1] {
    override def apply(a: A): Out =
      poly(a).map((rm.inverse _).compose( Left(_))) orElse
      next(a).map((gt.inverse _).compose(Right(_)))
  }

  implicit def case_0[F, A] = new Unfold[F, A, CNil] {
    override def apply(a: A): Out = None
  }
}