foldRight后的无形HList地图

Question

A type level foldRight works fine (getLabelWithValues), and a follow-on type level map (getValues) also works fine. If I combine both in one method (getValuesFull), it doesn't work any more though. What is the missing piece?

The full source (with sbt ready to ~run with implicit debug output) is here: https://github.com/mpollmeier/shapeless-playground/tree/8170a5b

case class Label[A](name: String)
case class LabelWithValue[A](label: Label[A], value: A)

val label1 = Label[Int]("a")
val labels = label1 :: HNil

object combineLabelWithValue extends Poly2 {
  implicit def atLabel[A, B <: HList] = at[Label[A], (B, Map[String, Any])] {
    case (label, (acc, values)) ⇒
      (LabelWithValue(label, values(label.name).asInstanceOf[A]) :: acc, values)
  }
}

object GetLabelValue extends (LabelWithValue ~> Id) {
  def apply[B](labelWithValue: LabelWithValue[B]) = labelWithValue.value
}

val labelsWithValues: LabelWithValue[Int] :: HNil = getLabelWithValues(labels)
// manually mapping it works fine:
val valuesManual: Int :: HNil = labelsWithValues.map(GetLabelValue)

// using a second function with Mapper works fine:
val valuesSecondFn: Int :: HNil = getValues(labelsWithValues)

// error: could not find implicit value for parameter mapper: shapeless.ops.hlist.Mapper.Aux[Main.GetLabelValue.type,WithValues,Values]
// val valuesFull: Int :: HNil = getValuesFull(labels)


def getLabelWithValues[L <: HList, P, WithValues](labels: L)(
  implicit folder: RightFolder.Aux[L, (HNil.type, Map[String, Any]), combineLabelWithValue.type, P],
  ic: IsComposite.Aux[P, WithValues, _]
): WithValues = {
  val state = Map("a" -> 5, "b" -> "five")
  val resultTuple = labels.foldRight((HNil, state))(combineLabelWithValue)
  ic.head(resultTuple)
}

def getValues[WithValues <: HList, Values <: HList](withValues: WithValues)(
  implicit mapper: Mapper.Aux[GetLabelValue.type, WithValues, Values]
): Values = {
  withValues.map(GetLabelValue)
}

def getValuesFull[L <: HList, P, WithValues <: HList, Values <: HList](labels: L)(
  implicit folder: RightFolder.Aux[L, (HNil.type, Map[String, Any]), combineLabelWithValue.type, P],
  ic: IsComposite.Aux[P, WithValues, _],
  mapper: Mapper.Aux[GetLabelValue.type, WithValues, Values]
): Values = {
  val state = Map("a" -> 5, "b" -> "five")
  val resultTuple = labels.foldRight((HNil, state))(combineLabelWithValue)
  val withValues: WithValues = ic.head(resultTuple)
  withValues.map(GetLabelValue)
}

Answer 1

The issue here is that you're ending up trying to map over an HList where the HNil is statically typed as HNil.type . This doesn't work in general—eg in a simplified case like this:

import shapeless._, ops.hlist.Mapper

val mapped1 = Mapper[poly.identity.type, HNil]
val mapped2 = Mapper[poly.identity.type, HNil.type]

mapped1 will compile, but mapped2 won't.

The trick is to change the HNil.type in your RightFolder types to HNil and then to call foldRight with HNil: HNil . This will make everything work just fine.

There are a few other suggestions I'd make (destructure the tuple in place of P instead of using IsComposite , skip the Aux on mapper and return mapper.Out instead of having a Value type parameter, etc.), but they're probably out of the scope of this question.