Module 1-Incr.Observer

An observer lets one get the value of an incremental, either by asking directly for the value or installing an on-update handler to run when the incremental's value changes.

One first creates an observer using observe. One must then call stabilize before making any observations on that observer.

Doing let o = observe t causes subsequent calls to stabilize to maintain the value of t, until either:

type 'a t
val sexp_of_t : ('a -> Ppx_sexp_conv_lib.Sexp.t) -> 'a t -> Ppx_sexp_conv_lib.Sexp.t
include Core_kernel.Invariant.S1 with type 'a t := 'a t
type 'a t
val invariant : 'a Base__.Invariant_intf.inv -> 'a t Base__.Invariant_intf.inv
val observing : 'a t -> 'a incremental
val use_is_allowed : _ t -> bool
val value : 'a t -> 'a Core_kernel.Or_error.t

value t returns the current value of t, or Error if t does not currently have a stable value. In particular, value t will return Error in the following situations:

  • in the middle of stabilization.
  • if stabilize has not been called since t was created.
  • if disallow_future_use t has been called.
  • if observing t is invalid.

Rather than using value in a function that runs during stabilization, one should use map or bind to express the dependence of an incremental computation on an incremental.

val value_exn : 'a t -> 'a
module Update : sig ... end

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:

val on_update_exn : 'a t -> f:('a Update.t -> unit) -> unit
val disallow_future_use : _ t -> unit

disallow_future_use t causes all future attempts to use t to fail and on_update_exn handlers added to t to never run again. It also causes incremental to treat t as unobserved, and thus stabilize will not maintain the value of t or any of t's descendants that are needed only to maintain t. disallow_future_use raises if called during stabilization.