Shapeless HList map after foldRight - scala

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—e.g. 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.