A 'state t keeps track of updates of type 'state -> 'state queued to it and runs them sequentially.

This has the primary feature that if an update itself schedules another update, that other update will be run after the first update has finished.

For example, consider the code:

let x = create ~init:1 () in
    enqueue_update x (fun x ->
    enqueue_update (fun x -> x + 1);
    x + 1)

At the end, x's state will be 2, as you would expect. However, suppose you did this with an int ref:

let x = ref 1 in
    let update_x f = x := f !x in
    update_x (fun x ->
      update_x (fun x -> x + 1);
      x + 1
    )

At the end of this, !x would be 1.

Another feature is that the initial value does not have to be specified at creation. If it is not, enqueued updates will be kept around until an initial value is specified with init.

type ('perm, 'state) t
val create : ?init:'state -> unit -> ('a, 'state) t
val init : (Core.Std.read_write, 'state) t -> 'state -> unit
val enqueue : (Core.Std.read_write, 'state) t -> ('state -> 'state) -> unit
val watch : ('a, 'state) t -> f:('state -> unit) -> unit
watch t f will call f every time that that t's state is updated. f should not call enqueue_update.
val map : ('a, 'state1) t -> f:('state1 -> 'state2) -> (Core.Std.read_only, 'state2) t
val read_only : ('a, 'state) t -> (Core.Std.read_only, 'state) t