Automatically derive a Traverse instance for "Generic Lenses"

GenLens.scala

scala> :paste
// Entering paste mode (ctrl-D to finish)

  trait GenLens[T[_]] {
    def get[B](a: T[B]): B
    def set[A, B](a: T[A], b: B): T[B]
  }

  implicit def lensTraversable[T[_]](implicit l: GenLens[T]): Traverse[T] = new Traverse[T] {
    def traverseImpl[G[_], A, B](fa: T[A])(f: (A) => G[B])(implicit G: Applicative[G]) = {
      G.map(f(l.get[A](fa)))(l.set[A, B](fa, _))
    }
  }

  case class Bar[A](a: A, b: Int, c: Int)

  implicit def barLens: GenLens[Bar] = new GenLens[Bar] {
    def set[A, B](a: Bar[A], b: B) = a.copy(a = b)

    def get[B](a: Bar[B]) = a.a
  }

// Exiting paste mode, now interpreting.

warning: there were 3 feature warning(s); re-run with -feature for details
defined trait GenLens
lensTraversable: [T[_]](implicit l: GenLens[T])scalaz.Traverse[T]
defined class Bar
barLens: GenLens[Bar]

scala> Traverse[Bar].sequenceU(Bar(List("foo"), 1,2))
res8: List[Bar[String]] = List(Bar(foo,1,2))