Build list of typeclass instances for HList type

Question

I'm trying to improve types for one of our APIs. I want to create data fetcher with following signature:

def runIt[T <: HList](id: Int): T = ???

It should fetch call external server for some entity and return HList with required data. The first task is to build list of API params for required features.

Here it what I made:

import shapeless._
import shapeless.ops.hlist._

trait Feature

trait FeatureFetcher[T] {
  def name: String
}

trait F1 extends Feature
trait F2 extends Feature

implicit case object f1Fetcher extends FeatureFetcher[F1] {
  val name = "f1name"
}

implicit case object f2Fetcher extends FeatureFetcher[F2] {
  val name = "f2name"
}

def makeIt[T <: HList](id: Int)
                      (implicit liftAll: LiftAll[FeatureFetcher, T]) = {

  // I need this, but it does not compile here
  // liftAll.instances.toList.map(_.name).mkString(",")

  liftAll.instances
}

makeIt[F1 :: F2 :: HNil](1).toList.map(_.name).mkString(",")

It actually works. But when I move .toList into makeIt function I get error

Error:(25, 21) could not find implicit value for parameter toTraversableAux: shapeless.ops.hlist.ToTraversable.Aux[liftAll.Out,List,Lub]
liftAll.instances.toList.map(_.name).mkString(",")

How to solve this problem?

Answer 1

I think when the code calls implicit instance with implicit liftAll: LiftAll[FeatureFetcher, T] then the dependent type Out of LiftAll is lost, so the compiler doesn't know which type of instances exactly will return and hence the error.

A possible solution could be to use, use its the from shapeless which is supposed to be a better implicitly.

the[liftAll].instances.toList

Let me know if ut helps!!

Answer 2

Based on segeljakt , try passing liftAll.Out to ToTraversable.Aux via LiftAll.Aux like so

def makeIt[In <: HList, Out <: HList](id: Int)(implicit
  liftAll: LiftAll.Aux[FeatureFetcher, In, Out],
  toTraversable: ToTraversable.Aux[Out, List, FeatureFetcher[_]]
): String = {
  liftAll.instances.toList.map(_.name).mkString(",")
}

type In = F1 :: F2 :: HNil
type Out = FeatureFetcher[F1] :: FeatureFetcher[F2] :: HNil
makeIt[In, Out](1)
// res1: String = f1name,f2name

or based on ToElmTypes

case class MakeIt[In <: HList]() {
  def apply[Out <: HList](id: Int)(implicit
    liftAll: LiftAll.Aux[FeatureFetcher, In, Out],
    toTraversable: ToTraversable.Aux[Out, List, FeatureFetcher[_]]
  ): String = {
    liftAll.instances.toList.map(_.name).mkString(",")
  }
}

MakeIt[F1 :: F2 :: HNil].apply(1)