Module `Lwt_switch`

Lwt switches

Switch has two goals:

being able to free multiple resources at the same time,
offer a better alternative than always returning an id to free some resource.

For example, consider the following interface:

type id

val free : id -> unit Lwt.t

val f : unit -> id Lwt.t
val g : unit -> id Lwt.t
val h : unit -> id Lwt.t

Now you want to call f, g and h in parallel. You can simply do:

lwt idf = f () and idg = g () and idh = h () in
...

However, one may want to handle possible failures of f (), g () and h (), and disable all allocated resources if one of these three threads fails. This may be hard since you have to remember which one failed and which one returned correctly.

Now if we change the interface a little bit:

val f : ?switch : Lwt_switch.t -> unit -> id Lwt.t
val g : ?switch : Lwt_switch.t -> unit -> id Lwt.t
val h : ?switch : Lwt_switch.t -> unit -> id Lwt.t

the code becomes:

Lwt_switch.with_switch (fun switch ->
  lwt idf = f ~switch ()
  and idg = g ~switch ()
  and idh = h ~switch () in
  ...
)

type t: Type of switches.

val create : unit -> t: create () creates a new switch.

val with_switch : (t -> 'a Lwt.t) -> 'a Lwt.t

with_switch fn is fn switch, where switch is a fresh switch that is turned off when the callback thread finishes (whether it succeeds or fails).

since: 2.6.0

val is_on : t -> bool: is_on switch returns true if the switch is currently on, and false otherwise.

val turn_off : t -> unit Lwt.t: turn_off switch turns off the switch. It calls all registered hooks, waits for all of them to terminate, then returns. If one of the hooks failed, it will fail with the exception raised by the hook. If the switch is already off, it does nothing.

exception Off: Exception raised when trying to add a hook to a switch that is already off.

val check : t option -> unit: check switch does nothing if switch is None or contains an switch that is currently on, and raises Off otherwise.

val add_hook : t option -> (unit -> unit Lwt.t) -> unit: add_hook switch f registers f so it will be called when turn_off is invoked. It does nothing if switch is None. If switch contains an switch that is already off then Off is raised.

val add_hook_or_exec : t option -> (unit -> unit Lwt.t) -> unit Lwt.t: add_hook_or_exec switch f is the same as add_hook except that if the switch is already off, f is called immediately.