Exn (eio.Eio.Exn)

type with_bt = exn * Stdlib.Printexc.raw_backtrace

type err = ..

Describes the particular error that occurred.

They are typically nested (e.g. Fs (Permission_denied (Unix_error ...))) so that you can match e.g. all IO errors, all file-system errors, all permission denied errors, etc.

If you extend this, use register_pp to add a printer for the new error.

type context

Extra information attached to an IO error. This provides contextual information about what caused the error.

exception Io of err * context

A general purpose IO exception.

This is used for most errors interacting with the outside world, and is similar to Unix.Unix_error, but more general. An unknown Io error should typically be reported to the user, but does not generally indicate a bug in the program.

type err +=

| Multiple_io of (err * context * Stdlib.Printexc.raw_backtrace) list
(*
Error code used when multiple IO errors occur.
This is useful if you want to catch and report all IO errors.
*)

val create : err -> exn

create err is an Io exception with an empty context.

val add_context : 
  exn ->
  ('a, Stdlib.Format.formatter, unit, exn) Stdlib.format4 ->
  'a

add_context ex msg returns a new exception with msg added to ex's context, if ex is an Io exception.

If ex is not an Io exception, this function just returns the original exception.

val reraise_with_context : 
  exn ->
  Stdlib.Printexc.raw_backtrace ->
  ('a, Stdlib.Format.formatter, unit, 'b) Stdlib.format4 ->
  'a

reraise_with_context ex bt msg raises ex extended with additional information msg.

ex should be an Io exception (if not, is re-raised unmodified).

Example:

try connect addr
with Eio.Io _ as ex ->
  let bt = Printexc.get_raw_backtrace () in
  reraise_with_context ex bt "connecting to %S" addr

You must get the backtrace before calling any other function in the exception handler to prevent corruption of the backtrace.

val register_pp : (Stdlib.Format.formatter -> err -> bool) -> unit

register_pp pp adds pp as a pretty-printer of errors.

pp f err should format err using f, if possible. It should return true on success, or false if it didn't recognise err.

val pp : exn Fmt.t

pp is a formatter for exceptions.

This is similar to Fmt.exn, but can do a better job on Io exceptions because it can format them directly without having to convert to a string first.

module Backend : sig ... end

Extensible backend-specific exceptions.

type err +=

| X of Backend.t
(*
A top-level code for backend errors that don't yet have a cross-platform classification in Eio.
You should avoid matching on these (in portable code). Instead, request a proper Eio code for them.
*)

exception Multiple of with_bt list

Raised if multiple fibers fail, to report all the exceptions.

This usually indicates a bug in the program.

Note: If multiple IO errors occur, then you will get Io (Multiple_io _, _) instead of this.

val combine : with_bt -> with_bt -> with_bt

combine x y returns a single exception and backtrace to use to represent two errors.

The resulting exception is typically just Multiple [y; x], but various heuristics are used to simplify the result:

Combining with a Cancel.Cancelled exception does nothing, as these don't need to be reported. The result is only Cancelled if there is no other exception available.
If both errors are Io errors, then the result is Io (Multiple_io _).