ContrivedAdd.scala

object ContrivedAdd {

  import shapeless._
  import record._
  import syntax.singleton._
  import shapeless.ops.record.Updater
  import scalaz._
  import Scalaz._

  case class S[L <: HList](total: Int, scratch: L)

  // addField works w/o any type parameters in contrivedAdd() below
  def addField[K <: Symbol, A, L <: HList](k: Witness, a: A)(f: Int => Int)
  : IndexedState[S[L], S[FieldType[K, A] :: L], Unit] =
    IndexedState(s => (S(f(s.total), field[K](a) :: s.scratch), ()))

  // with updateField() the compiler is either confused about which implicit to use or cannot find an implicit
  def updateField[K <: Symbol, A, L <: HList](k: Witness, a: A)(f: Int => Int)
    (implicit updater: Updater[L, FieldType[k.T, A]])
  : IndexedState[S[L], S[updater.Out], Unit] =
    IndexedState(s => (S(f(s.total), updater(s.scratch, field[k.T](a))), ()))

  def contrivedAdd[L <: HList](n: Int) = for {
    a <- init[S[L]]
    _ <- addField('latestAdded, n + 123)(_ + n)
    _ <- updateField('latestAdded, n)(_ + n)
//  ambiguous implicit values:
//   both method hnilUpdater in object Updater of type [L <: shapeless.HNil, F]=> shapeless.ops.record.Updater.Aux[L,F,shapeless.::[F,shapeless.HNil]]
//   and method hlistUpdater2 in object Updater of type [K, V, T <: shapeless.HList]=> shapeless.ops.record.Updater.Aux[shapeless.::[shapeless.record.FieldType[K,V],T],shapeless.record.FieldType[K,V],shapeless.::[shapeless.record.FieldType[K,V],T]]
//   match expected type shapeless.ops.record.Updater[L,Int with shapeless.record.KeyTag[Symbol with shapeless.tag.Tagged[String("latestAdded")],Int]]
//     _ <- updateField('latestAdded, n)(_ + n)
//                                      ^    
//  If, instead the type parameters are provided, the implicit cannot be found
//  _ <- updateField[Symbol, Int, L]('latestAdded, n)(_ + n)
//  could not find implicit value for parameter updater: shapeless.ops.record.Updater[L,Int with shapeless.record.KeyTag[Symbol with shapeless.tag.Tagged[String("latestAdded")],Int]]
//     _ <- updateField[Symbol, Int, L]('latestAdded, n)(_ + n)
//                                                      ^
    r <- get
  } yield r.total

  def main(args: Array[String] = Array()): Unit = {
    val initialState = S(0, ('a ->> 5) :: HNil)
    val result = contrivedAdd(555).run(initialState)
    println(result)
  }
}