Module Observer.Update

on_update_exn t ~f calls f after the current stabilization and after each subsequent stabilization in which t changes, until disallow_future_use t is called. f will be called at most once per stabilization. Here is a state diagram for the allowable sequences of Update.t's that can be supplied to a particular f:

           /-----------------------------------------------------\
           |                 /                                   |
           |                 |                                   v
          Start ------> Initialized ------> Changed ------> Invalidated
                                              ^  |
                                              \--/

on_update_exn raises if disallow_future_use t was previously called.

type 'a t =
| Initialized of 'a
| Changed of 'a * 'a

Changed (old_value, new_value)

| Invalidated
val compare : ('a -> 'a -> int) -> 'a t -> 'a t -> int
val sexp_of_t : ('a -> Ppx_sexp_conv_lib.Sexp.t) -> 'a t -> Ppx_sexp_conv_lib.Sexp.t