Shapeless Poly with explicit arguments

PolyWithArgs.scala

object Args {
  // it requires the cats, shapeless, and kittens libraries

  import cats._, implicits._
  import cats.sequence._
  import shapeless._, labelled._
  import shapeless.syntax.singleton._

  // Gitter question:
  // it masks fields of a record according to an explicit argument
  // if the contenct of each field is in the ``fields`` set, convert it to `None`
  // if not, wrap them in Some
  object FilterNot extends Poly1 {
    implicit def caseField[K <: Symbol, A](implicit key: Witness.Aux[K]) =
      at[FieldType[K, A]] { x => (fields: Set[String]) =>
        field[K] {
          if (!fields.contains(key.value.name)) Some(x)
          else None
        }
      }
  }

  val hlist = 'id ->> 1 :: 'name ->> "John" :: HNil
  val fields = Set("name")

  val b = hlist.traverse(FilterNot).apply(fields) // Some(1) :: None :: HNil
}