Scala - shapeless - CovUnliftFilter

CovUnliftFilter.scala

import scala.language.higherKinds

import shapeless._
import shapeless.ops.hlist.IsHCons

/**
 * Type class supporting access to all the elements of this `HList`, that are lifted sub-types of `U`.
 * Selects all {{{ F[A] }}}, contained in the list `L`, such that {{{ A <: U }}}.
 */
trait CovUnliftFilter[L <: HList, F[_], U] extends DepFn1[L] with Serializable { type Out <: HList }
object CovUnliftFilter {
  def apply[L <: HList, F[_], U](implicit filter: CovUnliftFilter[L, F, U]): Aux[L, F, U, filter.Out] = filter

  type Aux[L <: HList, F[_], U, Out0 <: HList] = CovUnliftFilter[L, F, U] { type Out = Out0 }

  implicit def take[L <: HList, F[_], U, FH, H, T <: HList](
    implicit
    cons: IsHCons.Aux[L, FH, T], ev0: FH <:< F[H], ev: H <:< U, next: CovUnliftFilter[T, F, U]
  ): CovUnliftFilter[L, F, U] = new CovUnliftFilter[L, F, U]{
    type Out = FH :: next.Out
    def apply(t: L): FH :: next.Out = cons.head(t) :: next(cons.tail(t))
  }

  implicit def drop[L <: HList, F[_], U, FH, H, T <: HList](
    implicit
    cons: IsHCons.Aux[L, FH, T], ev0: FH <:< F[H], ev: H <:!< U, next: CovUnliftFilter[T, F, U]
  ): CovUnliftFilter[L, F, U] = new CovUnliftFilter[L, F, U]{
    type Out = next.Out
    def apply(t: L): next.Out = next(cons.tail(t))
  }

  implicit def nil[F[_], U]: CovUnliftFilter[HNil, F, U] = Nil.asInstanceOf[CovUnliftFilter[HNil, F, U]]
  private object Nil extends CovUnliftFilter[HNil, Id, Any]{
    type Out = HNil
    def apply(t: HNil): HNil = HNil
  }
}

/**
 * Type class witnessing that `L` contains at least one element, satisfying corresponding filter conditions.
 */
trait CovUnliftFilteredNonempty[L <: HList, F[_], U]
object CovUnliftFilteredNonempty{
  implicit def evidence[L <: HList, F[_], U, L0 <: HList](
    implicit
    filter: CovUnliftFilter.Aux[L, F, U, L0], cons: IsHCons[L0]
  ): CovUnliftFilteredNonempty[L, F, U] = Evidence.asInstanceOf[CovUnliftFilteredNonempty[L, F, U]]

  private object Evidence extends CovUnliftFilteredNonempty[HList, Id, Any]
}

hlist-ext.scala

import scala.language.higherKinds

import shapeless.{HList, Id}
import shapeless.ops.hlist.LiftAll

package object hlist {

  /** Alias for [[CovUnliftFilter]] when `HList`'s elements are not lifted. */
  type CovFilter[L <: HList, U] = CovUnliftFilter[L, Id, U]
  object CovFilter {
    def apply[L <: HList, U](implicit filter: CovFilter[L, U]): Aux[L, U, filter.Out] = filter

    type Aux[L <: HList, U, Out0 <: HList] = CovFilter[L, U] {type Out = Out0}
  }

  /** Alias for [[CovUnliftFilteredNonempty]] when `HList`'s elements are not lifted. */
  type CovFilteredNonempty[L <: HList, U] = CovUnliftFilteredNonempty[L, Id, U]

  /** An alias for [[LiftAll]] `apply`, that preserves type refinement of lifted instances. */
  object LiftAllInstances{
    def apply[F[_], In <: HList](implicit ev: LiftAll[F, In]): ev.Out = ev.instances
  }

}