Simple shapeless foldLeft

FoldLeft.scala

import shapeless.{::, HList, HNil, Poly2}
import shapeless.ops.hlist.Prepend
import shapeless.record._
import shapeless.syntax.singleton._

val h = 1 :: "foo" :: true :: HNil

object withoutAnnotation extends Poly2 {
  implicit def id[Acc <: HList, A, Res <: HList](
      implicit pp: Prepend.Aux[Acc, A :: HNil, Res]) =
    at[Acc, A]((acc, a) => pp.apply(acc, a :: HNil))
}

object withCaseAnnotation extends Poly2 {
  implicit def id[Acc <: HList, A, Res <: HList](
      implicit pp: Prepend.Aux[Acc, A :: HNil, Res]): Case[Acc, A] =
    at[Acc, A]((acc, a) => pp.apply(acc, a :: HNil))
}

object withCaseAuxAnnotation extends Poly2 {
  implicit def id[Acc <: HList, A, Res <: HList](
      implicit pp: Prepend.Aux[Acc, A :: HNil, Res]): Case.Aux[Acc, A, Res] =
    at[Acc, A]((acc, a) => pp.apply(acc, a :: HNil))
}

// does not compile
h.foldLeft(HNil)(withoutAnnotation)

// does not compile
h.foldLeft(HNil)(withCaseAnnotation)

// compiles
h.foldLeft(HNil)(withCaseAuxAnnotation)