Exception Handling within Gtkada

ldries46

unread,

Sep 20, 2021, 8:06:02 AM9/20/21

to

I want an exception to be seen within an existing window of Gtkada to be able to see details of the error. So I used:

exception
   when no_const =>
      Main_Window.Buffer.Insert_At_Cursor
        ("-------------------------------------------------------------------------"
         & To_String(CRLF));
      Main_Window.Buffer.Insert_At_Cursor("Error : io_const" & to_String(CRLF));
end Test_Exception;

In this case the the program ends and the reason of the exception is lost. I want this only for a selected nr of exceptions. In this case the exception no_const.

Vadim Godunko

unread,

Sep 21, 2021, 2:49:48 AM9/21/21

to

Generally, Ada exceptions must not left scope of callback function. Thus, such code should be added to each callback/event handler/etc. subprogram of your application.

Dmitry A. Kazakov

unread,

Sep 21, 2021, 3:01:36 AM9/21/21

to

Right. Each handler should end like this:

exception
when Error : others =>
Glib.Message.Log
( "My fancy program",
Log_Level_Critical,
( "Fault in On_Button_Click: "
& Exception_Information (Error)
) );
end On_Button_Click;

--
Regards,
Dmitry A. Kazakov
http://www.dmitry-kazakov.de

Emmanuel Briot

unread,

Sep 21, 2021, 3:24:44 AM9/21/21

to

> > Generally, Ada exceptions must not left scope of callback function. Thus, such code should be added to each callback/event handler/etc. subprogram of your application.
> Right. Each handler should end like this:

We were talking the other day of the high-level Connect subprograms generated by GtkAda (`Gtk.Button.On_Clicked` and so on). Those will always catch exceptions and avoid propagating them to the C layer in gtk+ (which as Dmitry mentions is dangerous). They will in effect call `GtkAda.Bindings.Process_Exception`, which in turns calls a user-defined subprogram, see GtkAda.Bindings.Set_On_Exceptions

I think this should be the recommended approach for general exceptions. Of course, your callbacks should directly handle exceptions that they know how to recover from, and deal with that locally.

Dmitry A. Kazakov

unread,

Sep 21, 2021, 3:40:50 AM9/21/21

to

On 2021-09-21 09:24, Emmanuel Briot wrote:
>>> Generally, Ada exceptions must not left scope of callback function. Thus, such code should be added to each callback/event handler/etc. subprogram of your application.
>> Right. Each handler should end like this:
>
> We were talking the other day of the high-level Connect subprograms generated by GtkAda (`Gtk.Button.On_Clicked` and so on). Those will always catch exceptions and avoid propagating them to the C layer in gtk+ (which as Dmitry mentions is dangerous). They will in effect call `GtkAda.Bindings.Process_Exception`, which in turns calls a user-defined subprogram, see GtkAda.Bindings.Set_On_Exceptions

Very cool.

> I think this should be the recommended approach for general exceptions. Of course, your callbacks should directly handle exceptions that they know how to recover from, and deal with that locally.

Yes, but Ada does that much better. I mean rendezvous, which is Ada's
idea of an event handling. An exception in a rendezvous propagates to
the caller. For a GUI it would mean that if a callback fails the emitter
of the signal gets an exception, which would be the right place to
handle the issue rather than sweeping it under the rug.

ldries46

unread,

Sep 22, 2021, 4:42:33 AM9/22/21

to

Op 21-9-2021 om 9:01 schreef Dmitry A. Kazakov:

I tried different approaches but they cannot solve my problem. Part of
the problem is probably that I am developing a package which should be
usable in all different kind of programs maybe even under programs using
all different kind of GUI's. That means that exception handling cannot
always be done in the package but should be done in at least a package
calling that problem. With this approach I tried to solve an earlier
problem I asked about "Is there a way to see if a value is declared as a
constant". I tried to solve that problem in a way that needed Exception
handling during running to solve a design problem that could be made. I
will need another way to go around that problem.

Dmitry A. Kazakov

unread,

Sep 22, 2021, 6:22:51 AM9/22/21

to

You still may not propagate exceptions through C.

> That means that exception handling cannot
> always be done in the package but should be done in at least a package
> calling that problem. With this approach I tried to solve an earlier
> problem I asked about "Is there a way to see if a value is declared as a
> constant". I tried to solve that problem in a way that needed Exception
> handling during running to solve a design problem that could be made. I
> will need another way to go around that problem.

Posing a problem correctly is a half of solution.

Anyway, if you want to propagate exceptions through GTK, that is
possible to do by marshaling and then re-raising occurrence. GtkAda
contributions does this in the package Gtk.Main.Router. It also gives
stack trace of the exception in a popup dialog. And it can show the
location in GPS if that is running in the server mode.