gistfile1.scala

object ET {
  import scalaz.Lens
  case class Foo[A](a: A, i: Int)
  object Foo {
    def a[A]: Lens[Foo[A],A] = Lens.lensg(x => y => x.copy(a=y), _.a)
    def i1[A]: Lens[Foo[A],Int] = Lens.lensg(x => y => x.copy(i=y), _.i)
    def i2: Lens[Foo[A] forSome {type A},Int] = Lens.lensg(x => y => x.copy(i=y), _.i)
    def i3: Lens[Foo[_],Int] = Lens.lensg(x => y => x.copy(i=y), _.i)
  }

  val myFoo = Foo("hello", 3)
  import Foo._
  println(myFoo)
  println(a.set(myFoo, "goodbye"))
  println(i1.set(myFoo, 4))
  println(i2.set(myFoo, 5))
  println(i3.set(myFoo, 6))
}