Re: "subprogram must not be deeper than access type"

["andrew...@gmail.com" <andrew.shvets@gmail.com>]

On Saturday, October 3, 2020 at 2:42:01 AM UTC-4, Dmitry A. Kazakov wrote:
> On 03/10/2020 06:39, andrew...@gmail.com wrote: 
> > Hello, 
> > 
> > I'm trying my hand at a small rosettacode.org example: 
> > 
> > https://codeshare.io/an080j 
> > 
> > What I don't understand is when I'm creating the various packages to serve as handlers, I get the following compile-time errors: 
> > 
> > gprbuild : event_handlers_window.adb:13:03: instantiation error at gtk-handlers.ads:1039 
> > event_handlers_window.adb:13:03: subprogram must not be deeper than access type 
> > event_handlers_window.adb:14:03: instantiation error at gtk-handlers.ads:437 
> > event_handlers_window.adb:14:03: subprogram must not be deeper than access type 
> > 
> > How am I making a subprogram deeper than an access type exactly?
> The access type to the handler function is declared at the library 
> level. Your handler subprogram is declared inside the body of 
> Event_Handlers_Window, which is one step deeper than the library level. 
> 
> You can: 
> 
> 1. Move the handler function into a package. A library package would be 
> on the same level with the access type declaration. That is the 
> recommended way. 
> 
> 2. You can circumvent accessibility checks by declaring a local access 
> type to the function in the procedure (which would be same level) and 
> then convert from this type to the handler type using 
> Unchecked_Conversion. This is not recommended, obviously. 
> 
> 3. Specifically for delete and destroy events in GtkAda Contributions 
> the package Gtk.Missed defines ready-to-use handlers as in the solution #1. 
> 
> -- 
> Regards, 
> Dmitry A. Kazakov 
> http://www.dmitry-kazakov.de

Hi Dmitry,

Thanks for writing back.

I guess I'm not sure what you mean by #1.  The handler package contains another package, Callback.  And the compiler never really gets to the methods (Destroy and Delete_Event), it stops when I try to create a new package from an existing one.  Could you please show me an example of what you mean?

Also, I looked through the docs and couldn't find Gtk.Missed... did you mean some other method/package?

Thank you,
--Andrew