Unwrapping an hlist

extract.scala

package foo
import shapeless._
import shapeless.poly._

trait Extract[F[_]] {
  def extract[A](fa: F[A]): A
  def extractAll[K <: HList](k: K)(implicit m: ops.hlist.Mapper[Extract.extract.type, K]) =
    Extract.extractAll[F, K](k)(this, m)
}

object Extract {

  def apply[F[_]](implicit F: Extract[F]): Extract[F] = F

  def extractAll[F[_], K <: HList](k: K)(implicit F: Extract[F], m: ops.hlist.Mapper[extract.type, K]) = {
    k map extract
  }

  object extract extends Poly1 {
    implicit def default[F[_] : Extract, A] = at[F[A]] { fa => Extract[F].extract(fa) }
  }
}

case class Wrap[A](value: A)
object Wrap {
  implicit val extract: Extract[Wrap] = new Extract[Wrap] {
    def extract[A](w: Wrap[A]) = w.value
  }
}

object Usage extends App {
  val wrapped = Wrap(1) :: Wrap(true) :: HNil
  val unwrapped = Extract[Wrap].extractAll(wrapped)
  println(unwrapped)
}