Module Backtrace

* Low-level management of OCaml backtraces.

Currently, OCaml manages its backtraces in a very imperative way. That is to say, it only keeps track of the stack destroyed by the last raised exception. So we have to be very careful when using this module not to do silly things.

Basically, you need to manually handle the reraising of exceptions. In order to do so, each time the backtrace is lost, you must push the stack fragment. This essentially occurs whenever a with handler is crossed.

Backtrace information
type location = {
loc_filename : string;
loc_line : int;
loc_start : int;
loc_end : int;

OCaml debugging information for function calls.

type frame = {
frame_location : location option;
frame_raised : bool;

A frame contains two informations: its optional physical location, and whether it raised the exception or let it pass through.

type t

Type of backtraces. They're essentially stack of frames.

val empty : t

Empty frame stack.

val push : t -> t

Add the current backtrace information to a given backtrace.

val repr : t -> frame list

Represent a backtrace as a list of frames. Leftmost element is the outermost call.

val print_frame : frame -> string

Represent a frame.

Exception handling
val record_backtrace : bool -> unit

Whether to activate the backtrace recording mechanism. Note that it will only work whenever the program was compiled with the debug flag.

val get_backtrace : -> t option

Retrieve the optional backtrace coming with the exception.

val add_backtrace : exn -> Exninfo.iexn

Add the current backtrace information to the given exception.

The intended use case is of the form:

try foo
  | Bar -> bar
  | err -> let err = add_backtrace err in baz

WARNING: any intermediate code between the with and the handler may modify the backtrace. Yes, that includes when clauses. Ideally, what you should do is something like:

try foo
  with err ->
    let err = add_backtrace err in
    match err with
    | Bar -> bar
    | err -> baz

I admit that's a bit heavy, but there is not much to do...

val app_backtrace : -> ->

Append the backtrace from src to dst. The returned exception is dst except for its backtrace information. This is targeted at container exceptions, that is, exceptions that contain exceptions. This way, one can transfer the backtrace from the container to the underlying exception, as if the latter was the one originally raised.