ImplicitTest.scala

import shapeless._

case class Location(lat: Double, lon: Double)
case class User(id: Long, username: String, location: Location)
case class Car(id: Long, location: Location)

object ImplicitLenses extends App {
  implicit val userLocationLens: Lens[User, Location] = {
    val ret = lens[User].location
    ret
  }

  implicit val carLocationLens: Lens[Car, Location] = {
    val ret = lens[Car].location
    ret
  }

  // implicit val carLocationLens: Lens[Car, Location] = lens[Car].location
  // produces:
  // [info] Compiling 1 Scala source to /home/blast/.../target/scala-2.11/classes...
  // [error] /home/blast/.../ImplicitTest.scala:66: polymorphic expression cannot be instantiated to expected type;
  // [error]  found   : [B]mkLens.Out
  // [error]  required: shapeless.Lens[Car,Location]
  // [error]   implicit val carLocationLens: Lens[Car, Location] = lens[Car].location
  // [error]                                                           ^
  // [error] one error found
  // [error] (compile:compile) Compilation failed
  // [error] application - 

  def resetLocation[T <: { def location: Location }](o: T)(implicit locationLens: Lens[T, Location]): T = {
    locationLens.set(o)(Location(0, 0))
  }

  resetLocation(User(5, "foo", Location(123, 456)))
  resetLocation(Car(5, Location(234, 567)))
}