code.scala

package app

import shapeless.ops.hlist.Mapper
import shapeless.{::, HList, HNil, Poly2}

case class Params()
sealed trait EvaluatableGeneratorFilter[+Result] {
  def eval(p: Params): Result
}

case class FilteredValue[A, Filters <: HList, Outputs <: HList](
  filters: Filters
)(private val f: Outputs => A)(
  implicit
  mapper: Mapper.Aux[FilteredValue.evalPoly.type, Filters, Outputs]
) {
  def eval(p: Params): A = {
    val mapped = filters.map(FilteredValue.evalPoly)
    f(mapped)
  }

  def map[B](
    // Needs to have different name than mapper, otherwise implicit resolution fails...
    resultMapper: A => B
  ): FilteredValue[B, Filters, Outputs] =
    FilteredValue[B, Filters, Outputs](filters) { a =>
      resultMapper(f(a))
    }
}
object FilteredValue {
  object evalPoly extends Poly2 {
    implicit def `case`[T]: Case.Aux[EvaluatableGeneratorFilter[T], Params, T] = at(_.eval(_))
  }

  def constant[A](a: A): FilteredValue[A, HNil, HNil] = FilteredValue(HNil: HNil) { _ => a }
}

object Test {
  val test =
    new EvaluatableGeneratorFilter[Int] { def eval(p: Params) = 3 } ::
      new EvaluatableGeneratorFilter[String] { def eval(p: Params) = "3" } :: HNil

  val res = FilteredValue(test)((_ : (Int :: String :: HNil)) => 3).eval(Params())
}

error

[error] D:\work\scala\shapeless-guide\src\main\scala\app\Main.scala:44:32: could not find implicit value for parameter mapper: shapeless.ops.hlist.Mapper.Aux[app.FilteredValue.evalPoly.type,app.EvaluatableGeneratorFilter[Int] :: app.EvaluatableGeneratorFilter[String] :: shapeless.HNil,Int :: String :: shapeless.HNil]
[error]   val res = FilteredValue(test)((_ : (Int :: String :: HNil)) => 3).eval(Params())