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. Monitors are arranged in a tree -- when a new monitor is created, it is a child of the current monitor.
If a computation raises an unhandled exception, the behavior depends on whether the current monitor is "detached" or "attached". If the monitor has been "detached", via one of the
detach* functions, 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 initial monitor, i.e., the root of the monitor tree, that prints an unhandled-exception message and calls exit 1.
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
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.within. For example:
Scheduler.within (fun () -> upon (f ()) g)
This code will catch an exception in either
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
try_with creates its own monitor and uses
Scheduler.within, so it does not have this problem.
module Deferred : sig ... end
create () returns a new monitor whose parent is the current monitor.
name t returns the name of the monitor, or a unique id if no name was supplied to
val depth : t -> int
val current : unit -> t
current () returns the current monitor.
val detach : t -> unit
detach t detaches
t so that errors raised to
t are not passed to
t's parent monitor. If those errors aren't handled in some other way, then they will effectively be ignored. One should usually use
detach_and_iter_errors so that errors are not ignored.
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.
detach_and_get_next_error t detaches
t and returns a deferred that becomes determined with the next error that reaches
t (possibly never).
detach_and_get_error_stream t detaches
t and returns a stream of all subsequent errors that reach
Stream.iter (detach_and_get_error_stream t) ~f is equivalent to
detach_and_iter_errors t ~f.
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.
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
val has_seen_error : t -> bool
has_seen_error t returns true iff the monitor has ever seen an error.
send_exn t exn ?backtrace sends the exception
exn as an error to be handled by monitor
t. By default, the error will not contain a backtrace. However, the caller can supply one using
`This, or use
`Get to request that
send_exn obtain one using
val try_with : (?extract_exn:bool -> ?run:[ `Now | `Schedule ] -> ?rest:[ `Log | `Raise | `Call of exn -> unit ] -> (unit -> 'a Deferred.t) -> ('a, exn) Core_kernel.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
run = `Schedule. Once a result is returned, subsequent exceptions raised to the monitor are handled according to
`Log: Logged to a global error log (cannot raise).
`Raise: Reraised to the monitor of
`Call f: Passed to
fwithin the context of the caller of
name argument is used to give a name to the monitor the computation will be running in. This name will appear when printing errors.
f () in a new monitor
t that has no parent. This works because
detach_and_get_error_stream t and explicitly handles all errors sent to
t. No errors would ever implicitly propagate to
t's parent, although
try_with will explicitly send them to
t's parent with
rest = `Raise.
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 -> (unit -> 'a Deferred.t) -> 'a Core_kernel.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 = try_with f ?extract_exn ~run:`Now ~rest:`Log >>| Or_error.of_exn_result
~run:`Now is different from
~run:`Schedule. Based on experience, we think
~run:`Now is a better behavior.
val try_with_join_or_error : (?extract_exn:bool -> (unit -> 'a Core_kernel.Or_error.t Deferred.t) -> 'a Core_kernel.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
catch_stream ?name f runs
f () inside a new monitor
m and returns the stream of errors raised to
catch ?name f runs
f () inside a new monitor
m and returns the first error raised to
catch_error ?name f runs
f () inside of a new monitor
m and returns the first error raised to
val protect : (?extract_exn:bool -> ?run:[ `Now | `Schedule ] -> (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
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