extracting fields from case classes

fields.scala

import shapeless._

import ops.{ hlist => hl, coproduct => cp }
object FlexibleTypes2 {

/*
it extracts the values of a common type from a hierarchy of case classes
*/
  

  object ints extends Poly1 {
    implicit def caseProduct[T, L <: HList, S <: HList, P <: HList](
      implicit
      gen: Generic.Aux[T, L],
      p:   hl.Partition.Aux[L, Int, P, S],
      tr:  hl.ToTraversable.Aux[P, List, Int]
    ) = at[T]( t => gen.to( t ).filter[Int].to[List] )

    implicit def caseCoproduct[T, L <: Coproduct, R <: Coproduct](
      implicit
      gen:     Generic.Aux[T, L],
      mapper:  cp.Mapper.Aux[this.type, L, R],
      unifier: cp.Unifier[R]
    ) = at[T]( t => gen.to( t ).map( ints ).unify )
  }

  sealed trait F
  case class A( i: Int ) extends F
  case class B( s: String ) extends F
  case class C( i: Int, s: String ) extends F

  val fs: List[F] = List( A( 1 ), B( "b" ), C( 3, "c" ) )

  assert( fs.map( f => ints( f ) ).flatten == List( 1, 3 ) )

}