Lwt switches
Switch has two goals:
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.tNow 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.tthe 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 exnval create : unit -> tcreate () creates a new switch.
val is_on : t -> boolis_on switch returns true if the switch is currently on, and false otherwise.
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.
val check : t option -> unitcheck switch does nothing if switch is None or contains an switch that is currently on, and raises Off otherwise.