DCont.fsx

module DCont =
  type DCont<'k, 'a> = ('k -> 'a)

  let inline shift (f: ('b -> 'c) -> 'a): DCont<('b -> 'c), 'a> = f

  type DContBuilder() =
    member inline __.ReturnFrom(m): DCont<'k, 'a> = m
    member inline __.Return(x): DCont<'a -> 'b, 'b> = fun k -> k x
    member inline __.Bind(x: DCont<'b -> 'c, 'a>, f: 'b -> DCont<'k, 'c>): DCont<'k, 'a> =
      (fun k -> x (fun g -> (f g) k))

  type ResetBuilder() =
    inherit DContBuilder()
    member inline __.Run(dcont: DCont<'b -> 'b, 'a>) = dcont id

  let dcont = DContBuilder()
  let reset = ResetBuilder()

open DCont

let inline get() =
  shift(fun k -> fun state -> k state state)

let inline tick() =
  shift(fun k -> fun state -> k () (state + 1))

let inline runState init thunk =
  init |> reset {
    let! result = thunk
    return (fun _ -> result)
  }

dcont {
  do! tick()
  do! tick()
  let! a = get()
  do! tick()
  let! b = get()
  return b - a
}
|> runState 0
|> printfn "%A"