vl-optics.kk

effect focus<a>
  ctl focus(a : a): a

fun vl-fst(f: a -> focus<r> b): (((a, c)) -> focus<r> (b, c))
  fn(pair)
    match pair
      (a, c) -> (f(a), c)

fun vl-snd(f: a -> focus<r> b): (((c, a)) -> focus<r> (c, b))
  fn(pair)
    match pair
      (c, a) -> (c, f(a))


// Allowing focus in the modifier might be a problem. Figure out how to get rid of it?
fun over(optic: ((a) -> focus<r> b) -> ((s) -> focus<r> t), modifier: (a -> b), target: s): t
  with fun focus(a)
    a
  val large_modifier = optic(modifier)
  large_modifier(target)

fun over-elim(a: a, action: (a -> focus<r> b)): b
  with fun focus(a)
    a
  action(a)


fun get-elim(a: a, action: (a -> focus<r> b)): exn r
  with handler {
    brk focus(a)
      a
    return(a)
      throw("No focus!")
  }
  action(a)

// Can't figure out how to type this!

// fun get(s: s, optic: ((a) -> focus<a> a) -> ((s) -> focus<b> a)): exn b
//   get-elim(s, optic(focus))



fun main() : console ()
  println("Hello, world!")