如何在 OCaml 中使协变可观察

我正在尝试为值制作一个包装器，允许调用者自行注册以获取有关它的通知。这是一些（工作）代码：

module Thing :
  sig
    type +'a t
    val make : 'a -> 'a t
    val watch : ('a -> unit) -> 'a t -> unit
    val notify : 'a t -> unit
  end = struct
    type 'a t = {
      obj : 'a;
      watchers : (unit -> unit) Queue.t
    }

    let make x = {
      obj = x;
      watchers = Queue.create ()
    }

    let watch fn x =
      x.watchers |> Queue.add (fun () -> fn x.obj) 

    let notify x =
      x.watchers |> Queue.iter (fun fn -> fn ())
  end

let () =
  let x = Thing.make (`Int 4) in
  Thing.watch (fun (`Int d) -> Printf.printf "Observed %d\n" d) x;
  let x = (x :> [`Int of int | `None] Thing.t) in
  Thing.notify x

然而，这似乎效率很低。每个排队的观察者都是一个新的闭包，具有自己对该事物的引用。仅对用户的回调进行排队并添加x in notify, e.g.

  ... = struct
    type 'a t = {
      obj : 'a;
      watchers : ('a -> unit) Queue.t
    }

    let make x = {
      obj = x;
      watchers = Queue.create ()
    }

    let watch fn x =
      x.watchers |> Queue.add fn

    let notify x =
      x.watchers |> Queue.iter (fun fn -> fn x.obj)
  end

但拥有'a作为队列类型的一部分意味着'a t不再是协变的。我明白为什么会发生这种情况，但是有人有解决办法吗？即我如何向 OCaml 表明在这种情况下它是安全的？

您可以移动捕获位置：

module Thing :
  sig
    type +'a t
    val make : 'a -> 'a t
    val watch : ('a -> unit) -> 'a t -> unit
    val notify : 'a t -> unit
  end = struct
    type 'a t = {
      obj : 'a;
      watch : ('a -> unit) -> unit;
      notify : unit -> unit;
    }

    let make x =
      let queue = Queue.create () in
      let obj = x in
      let watch f = Queue.add f queue in
      let notify () = Queue.iter (fun f -> f x) queue in
      { obj; watch; notify; }

    let watch fn x = x.watch fn
    let notify x = x.notify ()
  end

如果您想真正感受到经济实惠：

    let make x =
      let queue = Queue.create () in
      let obj = x in
      let rec watch f = Queue.add f queue
      and notify () = Queue.iter (fun f -> f x) queue in
      { obj; watch; notify; }