Lens inference in shapeless 2.0.0

gistfile1.scala

import shapeless._
import ops.record.{ Selector, Updater }
import record.{ FieldType, field }

trait PathLens[T, P] {
  type Elem
  def get(t : T) : Elem
  def set(t : T)(e : Elem) : T
}

object PathLens {
  type Aux[T, P, E] = PathLens[T, P] { type Elem = E }

  implicit def mkPathLens[T, R <: HList, P, E]
    (implicit
      gen: LabelledGeneric.Aux[T, R],
      sel: Selector.Aux[R, P, E],
      upd: Updater.Aux[R, FieldType[P, E], R]
    ): PathLens.Aux[T, P, E] =
      new PathLens[T, P] {
        type Elem = E
        def get(t : T) : Elem = sel(gen.to(t))
        def set(t : T)(e : E) : T = gen.from(upd(gen.to(t), field[P](e)))
      }
}

object Demo extends App {
  case class Person(name : String, age : Int)
  case class Dog(name : String, age : Int)

  def updateField[T, F, E](t: T, f: Witness)(e: E)(implicit lens: PathLens.Aux[T, f.T, E]): (T, E) = {
    val old = lens.get(t)
    val updated = lens.set(t)(e)
    (updated, old)
  }

  val mary = Person("Mary", 23)
  val boris = Dog("Boris", 5)

  // Lens for Person at Int field `age` is inferred
  val mary1 = updateField(mary, 'age)(24)
  assert(mary1 == (Person("Mary", 24), 23))

  // Lens for Dog at Int field `age` is inferred
  val boris1 = updateField(boris, 'age)(6)
  assert(boris1 == (Dog("Boris", 6), 5))
}