updater.scala

import shapeless.labelled._
import shapeless.ops.hlist
import shapeless.{HList, HNil, LabelledGeneric}

trait Updater[A, B] {
  def apply(a: A, b: B): A
}

object Updater {
  def apply[A, B](implicit updater: Updater[A, B]): Updater[A, B] = updater

  def update[A, B](a: A, b: B)(implicit updater: Updater[A, B]): A = updater(a, b)

  implicit def updater[A, ARepr <: HList, B, BRepr <: HList](
    implicit
    aGen: LabelledGeneric.Aux[A, ARepr],
    bGen: LabelledGeneric.Aux[B, BRepr],
    multiReplacer: MultiReplacer[ARepr, BRepr]
  ): Updater[A, B] = new Updater[A, B] {
    override def apply(a: A, b: B): A = aGen.from(multiReplacer(
      aGen.to(a),
      bGen.to(b)
    ))
  }

  trait MultiReplacer[A <: HList, B <: HList] {
    def apply(a: A, b: B): A
  }

  implicit def multiReplacerHNil[A <: HList, B <: HNil]: MultiReplacer[A, B] = new MultiReplacer[A, B] {
    override def apply(a: A, b: B): A = a
  }

  implicit def multiReplacerHList[A <: HList, B <: HList, OA, K, V, VA, BH, BT <: HList] (
    implicit
    ev1: hlist.IsHCons.Aux[B, BH, BT],
    ev2: BH <:< FieldType[K, V],
    ev3: V <:< Option[VA],
    replacer: hlist.Replacer.Aux[A, FieldType[K, VA], FieldType[K, VA], (FieldType[K, VA], A)],
    multiReplacer: MultiReplacer[A, BT]
  ): MultiReplacer[A, B] = new MultiReplacer[A, B] {
    override def apply(a: A, b: B): A = multiReplacer(
      b.head.asInstanceOf[Option[VA]] match {
        case Some(x) => replacer(a, field[K](x))._2
        case _ => a
      },
      b.tail
    )
  }
}

///
/// It can be used like this:
///

case class User(id: Int, name: String, country: String)
case class UserPatch(name: Option[String] = None, country: Option[String] = None)

val user = User(10, "Artyom", "Russia")
val userPatch1 = UserPatch(name = Some("John"), country = Some("USA"))
val userPatch2 = UserPatch(country = Some("USA"))
val userPatch3 = UserPatch(name = Some("John"))

Updater.update(user, userPatch1)
// User(10,John,USA)

Updater.update(user, userPatch2)
// User(10,Artyom,USA)

Updater.update(user, userPatch3)
// User(10,John,Russia)