unwrap.scala

import shapeless._, ops.function._, ops.hlist._

trait Bar {

  trait Wrapper { type T }
  case class A[U]() extends Wrapper { type T = U }

  trait Unwrapper {
    type Args <: HList
    type F[R] = Args => R
  }

  object Unwrapper {
    type Aux[A]  = Unwrapper { type Args = A }
    implicit def mkUnwrapper[P <: Product, L <: HList, UL <: HList](p: P)
      (implicit gen: Generic.Aux[P, L], ul: ReprUnwrapper.Aux[L, UL]): Aux[UL] =
        new Unwrapper {
          type Args = UL
        }
  }

  trait ReprUnwrapper[P] {
    type Args <: HList
    type F[R] = Args => R
  }

  object ReprUnwrapper {
    type Aux[P, A]  = ReprUnwrapper[P] { type Args = A }

    implicit val hnil: Aux[HNil, HNil] =
        new ReprUnwrapper[HNil] {
          type Args = HNil
        }

    implicit def cons[H <: Wrapper, W[_], UH, T <: HList, UT <: HList]
      (implicit uh: Unpack1[H, W, UH], ut: ReprUnwrapper.Aux[T, UT]): Aux[H :: T, UH :: UT] =
        new ReprUnwrapper[H :: T] {
          type Args = UH :: UT
        }
  }

  implicit def conv[F, P](f: F)(implicit fntop: FnToProduct.Aux[F, P]): P = fntop(f)

  def getFn[R](u: Unwrapper)(f: u.F[R]) = f
}

object Bar extends Bar

object Test {
  import Bar._

  getFn(A[Int], A[String], A[Boolean]){ (a: Int, b: String, c: Boolean) => 1.0 }
}