HList转换出类型

Question

I have created a wrapper on top on HList that can append an HLists . It is defined as follows:

class HListWrapper[L <: HList](val hl: L) {
  def append[V, Out <: HList](k: Witness, v: V)(implicit updater: Updater.Aux[L, FieldType[k.T, V], Out],
                                                lk: LacksKey[L, k.T]): HListWrapper[Out] = {
    new HListWrapper(updater(hl, field[k.T](v)))
  }
}
object HListWrapper {
  def apply[P <: Product, L <: HList](p: P)(implicit gen: LabelledGeneric.Aux[P, L]) =
    new HListWrapper(gen.to(p))
}

It used like this:

case class CC(i: Int, ii: Int)
val cc = CC(100, 1000)
val hl = HListWrapper(cc)
hl.append('iii, 10000)

But when I try to put the HListWrapper inside another function to capture the type of Out , the compiler cannot seem to resolve the final type of the transformation (fails with type mismatch error):

def cctohlist[Out <: HList]: CC => HListWrapper[Out] = {
  m => {
    val hl = HListWrapper(m)
    hl.append('iii, 10000)
  }
}

The primary reason to create the cctohlist method is to get the type of the HList after append. Is this possible to achieve?

Answer 1

The following code works:

def cctohlist: CC => HListWrapper[Record.`'i -> Int, 'ii -> Int, 'iii -> Int`.T] = {
    m => {
      val hl = HListWrapper(m)
      hl.append('iii, 10000)
    }
  }

When you write

def cctohlist[Out <: HList]: CC => HListWrapper[Out] = ???

this means I can apply cctohlist[Int :: String :: Boolean :: HNil] and have CC => HListWrapper[Int :: String :: Boolean :: HNil] or I can apply cctohlist[AnyVal :: AnyRef :: Any :: HNil] and have CC => HListWrapper[AnyVal :: AnyRef :: Any :: HNil] etc. This is obviously not the case.

---

This is generic one:

def cctohlist[P <: Product, L <: HList, Out <: HList](m: P)(implicit
  gen: LabelledGeneric.Aux[P, L],
  updater: Updater.Aux[L, FieldType[Witness.`'iii`.T, Int], Out],
  lk: LacksKey[L, Witness.`'iii`.T]
  ): HListWrapper[Out] = {
    val hl = HListWrapper(m)
    hl.append('iii, 10000)
  }

cctohlist(cc)