package async_kernel

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package async_kernel
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The part of the Execution_context that determines what to do when there is an unhandled exception.

Every Async computation runs within the context of some monitor, which, when the computation is running, is referred to as the "current" monitor. By default, a monitor is set up to forward exceptions to another monitor, called a parent monitor, which is always the monitor of the current execution context.

The user can "detach" a monitor via one of the detach* functions, causing the exception to no longer propagate to the parent monitor. Then whoever detached it is responsible for dealing with the exception. If the monitor is still attached, then the exception bubbles to monitor's parent. If an exception bubbles to the root of the monitor tree, that prints an unhandled-exception message and calls exit 1.

Note about the toplevel monitor

It is important to note that in the toplevel monitor, exceptions will only be caught in the Async part of a computation. For example, in:

upon (f ()) g

if f raises, the exception will not go to a monitor; it will go to the next caml exception handler on the stack. Any exceptions raised by g will be caught by the scheduler and propagated to the toplevel monitor. Because of this it is advised to always use Scheduler.schedule or Scheduler.within. For example:

Scheduler.within (fun () -> upon (f ()) g)

This code will catch an exception in either f or g, and propagate it to the monitor.

This is only relevant to the toplevel monitor because if you create another monitor and you wish to run code within it you have no choice but to use Scheduler.within. try_with creates its own monitor and uses Scheduler.within, so it does not have this problem.

module Deferred : sig ... end
type t
val sexp_of_t : t -> Sexplib0.Sexp.t
include Core.Invariant.S with type t := t
val invariant : t -> unit
type 'a with_optional_monitor_name = ?here:Core.Source_code_position.t -> ?info:Core.Info.t -> ?name:string -> 'a
val create : (unit -> t) with_optional_monitor_name

create () returns a new monitor whose parent is the current monitor.

val name : t -> Core.Info.t

name t returns the name of the monitor, or a unique id if no name was supplied to create.

val current : unit -> t

current () returns the current monitor.

val detach : t -> unit

detach t detaches t, removing its parent, so that errors raised to t are not passed to its former parent monitor. If those errors aren't handled in some other way (e.g. via get_next_error), then they will be ignored. One should usually use detach_and_iter_errors so that errors are not ignored.

If t is already detached, then detach does nothing. For example, it does not remove existing handlers added with detach_and_iter_errors.

val detach_and_iter_errors : t -> f:(exn -> unit) -> unit

detach_and_iter_errors t ~f detaches t and passes to f all subsequent errors that reach t, stopping iteration if f raises an exception. An exception raised by f is sent to the monitor in effect when detach_and_iter_errors was called.

val detach_and_get_next_error : t -> exn Deferred.t

detach_and_get_next_error t detaches t and returns a deferred that becomes determined with the next error that reaches t (possibly never).

val detach_and_get_error_stream : t -> exn Tail.Stream.t

detach_and_get_error_stream t detaches t and returns a stream of all subsequent errors that reach t.

Stream.iter (detach_and_get_error_stream t) ~f is equivalent to detach_and_iter_errors t ~f.

val get_next_error : t -> exn Deferred.t

get_next_error t returns a deferred that becomes determined the next time t gets an error, if ever. Calling get_next_error t does not detach t, and if no other call has detached t, then errors will still bubble up the monitor tree. That includes the error returned by get_next_error, which will then be handled twice.

module Monitor_exn : sig ... end
type exn += private
  1. | Monitor_exn of Monitor_exn.t

When extract_exn is set to false, exceptions returned by try_with are wrapped into this exception type which also provides backtrace information.

val extract_exn : exn -> exn

extract_exn exn extracts the exn from an error exn that comes from a monitor. If it is not supplied such an error exn, it returns the exn itself. It removes the backtrace from the error (see discussion in try_with).

val has_seen_error : t -> bool

has_seen_error t returns true iff the monitor has ever seen an error.

val send_exn : t -> ?backtrace:[ `Get | `This of Core.Backtrace.t ] -> exn -> unit

send_exn t exn ?backtrace sends the exception exn as an error to be handled by monitor t. If the backtrace is not specified, it defaults to `Get, which means that the backtrace will be collected automatically. Automatic backtrace collection only works if exn is the most recently raised exception.

val try_with : (?extract_exn:bool -> ?run:[ `Now | `Schedule ] -> ?rest:[ `Log | `Raise | `Call of exn -> unit ] -> (unit -> 'a Deferred.t) -> ('a, exn) Core.Result.t Deferred.t) with_optional_monitor_name

try_with f runs f () in a monitor and returns the result as Ok x if f finishes normally, or returns Error e if there is an exception. It either runs f now, if run = `Now, or schedules a job to run f, if run = `Schedule. Once a result is returned, subsequent exceptions raised to the monitor are handled according to rest:

  • `Log: Logged to a global error log (cannot raise).
  • `Raise: Reraised to the monitor of try_with's caller.
  • `Call f: Passed to f within the context of the monitor of try_with's caller.

The name argument is used to give a name to the monitor the computation will be running in. This name will appear when printing errors.

If extract_exn = true, then in an Error exn result, the exn will be the actual exception raised by the computation. If extract_exn = false, then the exn will include additional information, like the monitor and backtrace.

val try_with_or_error : (?extract_exn:bool -> ?rest:[ `Log | `Raise | `Call of exn -> unit ] -> (unit -> 'a Deferred.t) -> 'a Core.Or_error.t Deferred.t) with_optional_monitor_name

try_with_or_error is like try_with but returns 'a Or_error.t Deferred.t instead of ('a,exn) Result.t Deferred.t. More precisely:

try_with_or_error f ?extract_exn ~rest
= try_with f ?extract_exn ~run:`Now ~rest >>| Or_error.of_exn_result
val try_with_join_or_error : (?extract_exn:bool -> ?rest:[ `Log | `Raise | `Call of exn -> unit ] -> (unit -> 'a Core.Or_error.t Deferred.t) -> 'a Core.Or_error.t Deferred.t) with_optional_monitor_name

try_with_join_or_error f = try_with_or_error f >>| Or_error.join.

val handle_errors : ((unit -> 'a Deferred.t) -> (exn -> unit) -> 'a Deferred.t) with_optional_monitor_name

handle_errors ?name f handler runs f () inside a new monitor with the optionally supplied name, and calls handler error on every error raised to that monitor. Any error raised by handler goes to the monitor in effect when handle_errors was called.

Errors that are raised after f () becomes determined will still be sent to handler, i.e., the new monitor lives as long as jobs created by f live.

val catch_stream : ((unit -> unit) -> exn Tail.Stream.t) with_optional_monitor_name

catch_stream ?name f runs f () inside a new monitor m and returns the stream of errors raised to m.

val catch : ((unit -> unit) -> exn Deferred.t) with_optional_monitor_name

catch ?name f runs f () inside a new monitor m and returns the first error raised to m.

val catch_error : ((unit -> unit) -> Core.Error.t Deferred.t) with_optional_monitor_name

catch_error ?name f runs f () inside of a new monitor m and returns the first error raised to m.

val protect : (?extract_exn:bool -> ?run:[ `Now | `Schedule ] -> ?rest:[ `Log | `Raise | `Call of exn -> unit ] -> (unit -> 'a Deferred.t) -> finally:(unit -> unit Deferred.t) -> 'a Deferred.t) with_optional_monitor_name

protect f ~finally runs f () and then finally regardless of the success or failure of f. It re-raises any exception thrown by f or returns whatever f returned.

The name argument is used to give a name to the monitor the computation will be running in. This name will appear when printing the errors.

val main : t

This is the initial monitor and is the root of the monitor tree. Unhandled exceptions are raised to this monitor.

module Expert : sig ... end
module Exported_for_scheduler : sig ... end
module For_tests : sig ... end