Module Lwt_switch

module Lwt_switch : sig..end

Lwt switches



Switch have 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, considers 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 calls f, g and h in parallel. You can simply do:

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


However, one may wants 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 we change a little bit the interface:

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


and the code becomes:

let switch = Lwt_switch.create () in
      try_lwt
        lwt idf = f ~switch () and idg = g ~switch () and idh = h ~switch () in
        ...
      with exn ->
        lwt () = Lwt_switch.turn_off switch in
        raise_lwt exn


type t

Type of switches.

val create : unit -> t

create () creates a new switch.

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 terminates, and the returns. If one of the hook failed, then it will fail with one of the exception raised by hooks. If the switch is already off, then 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 raise Lwt_switch.​Off otherwise.

val add_hook : t option -> (unit -> unit Lwt.t) -> unit

add_hook switch f registers f so it will be called when Lwt_switch.​turn_off is invoked. It does nothing if switch is None. If switch contains an switch that is already off then Lwt_switch.​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 Lwt_switch.​add_hook except that if the switch is already off, then f is called immediatly.