Materialize lenses for all keys in a record.

MkLenses.scala

import shapeless._
import shapeless.labelled._
import shapeless.ops.hlist._
import shapeless.ops.record._
import shapeless.syntax.singleton._

object MkLenses {
  object mkLenses extends Poly1 {
    implicit def mkLens[R <: HList, K](implicit mk: MkRecordSelectLens[R, K]) =
      at[K](_ => mk())
  }

  trait Dummy[-A]
  object Dummy {
    private val instance = new Dummy[Any] { }
    def apply[A]: Dummy[A] = instance
  }

  def lenses[R <: HList, K <: HList](
    dummy: Dummy[R]
  )(implicit
    keys: Keys.Aux[R, K],
    lensesFor: Mapper[mkLenses.type, K]
  ) = lensesFor(keys())

  val key = 'key.witness
  type Rec = FieldType[key.T, Int] :: HNil
  implicitly[MkRecordSelectLens[Rec, key.T]]
  lenses(Dummy[Rec])
}