Partition.scala

import shapeless._
import shapeless.ops.coproduct.Selector
import shapeless.ops.coproduct.ToHList
import shapeless.ops.hlist.LiftAll
import shapeless.ops.hlist.Mapper
import shapeless.ops.hlist.ZipConst

object Partition extends App {

  trait Dummy[-A]
  object Dummy {
    implicit val instance: Dummy[Any] =
      new Dummy[Any] { }
  }

  object partitioner extends Poly1 {
    implicit def caseSel[Sum <: Coproduct, A](implicit sel: Selector[Sum, A]) =
      at[(Dummy[A], List[Sum])](_._2.flatMap(sel.apply))
  }

  def partition[
    Sum <: Coproduct,
    Product <: HList,
    Dummies <: HList,
    Zipped <: HList
  ](sums: List[Sum])(
    implicit
    SumToProduct: ToHList.Aux[Sum, Product],
    Dummy: LiftAll.Aux[Dummy, Product, Dummies],
    zipWith: ZipConst.Aux[List[Sum], Dummies, Zipped],
    partition: Mapper[partitioner.type, Zipped]
  ) = partition(zipWith(sums, Dummy.instances))

  type Sum = Int :+: String :+: BigInt :+: CNil
  val sums: List[Sum] = List(
    Coproduct[Sum](1),
    Coproduct[Sum](2),
    Coproduct[Sum](3),
    Coproduct[Sum](4),
    Coproduct[Sum](5),
    Coproduct[Sum]("str1"),
    Coproduct[Sum]("str2"),
    Coproduct[Sum]("str3"),
    Coproduct[Sum]("str4"),
    Coproduct[Sum]("str5"))

  println(partition(sums))
  // List(1, 2, 3, 4, 5) :: List(str1, str2, str3, str4, str5) :: List() :: HNil
}