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From: "Frank" <franjoe@frisurf.no>
Newsgroups: comp.lang.ada
References: <D3GJb.279$Mb5.5088@news2.e.nsc.no>
 <Z_GJb.739626$Fm2.662614@attbi_s04>
Subject: Re: To raise an exception in task and handle/catch it in outer-block.
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Date: Sun, 4 Jan 2004 12:58:52 -0800
Xref: archiver1.google.com comp.lang.ada:4096
Date: 2004-01-04T12:58:52-08:00
List-Id: <comp.lang.ada>

Hi!

I agree that the exception has nothing to do with the Stop/Final accept.

What I try to achieve is that the exception is propagated to the outer
block. In this case with outer block, I mean the block that controls the
task- in my example "Test_Task_Main"*.
This issue came up because a exception raised in a task like this is not
propagated automatically by Ada to the outer block, but is resolved within
each task.
Another reason why is that the exception is related to the outer-block or
more precisely the developer of the outer block. I wish to propagate the
exception in the task for the outer-block(*) to
handle it there.

In my real-code, the situation is that a library Im working on, requires an
abstract subprogram to be overridden.

This abstract subprogram is called from within the library itself and it is
in that environment that an exception may be detected.
The business-related exception ("test_exception") will be raised if the
developer of the abstract subprogram behaves bad.
If such a exception is detected, I wish to communicate this to the
outer-block(*).

One question I have, is if this kind of construction is allowed in respect
to Ada standard.

This is a pseudo-code based on the example, here I have included the call to
the abstract subprogram:

package body Test_Task_Exception is

   test_exception : exception;

   task body A_Task is
     Run : Boolean := False;

   begin
      accept Init do -- ... as in example

      accept Start do -- ... as in example

      while Run loop

        -- This is an abstract subprogram, overriden by the developer using
this library.
        -- If the return value is bad, we wish to communicate this to the
outer-block
        Retur_Value := Do_Business_Logic(....);
        if Retur_Value = Bad_Value then
            raise test_exception;
        end if;

        -- Return_Value is ok, we are allowed to stop normally.l
            select
               accept Stop do
                  Ada.Text_IO.Put_Line ("Test_Task_Exception.A_Task.Stop -
Body");
                  Run := False;
               end Stop;

            else
                  null;
            end select;



      end loop;

      accept Final do -- ... as in example

   exception
      when others =>
        begin
          accept Stop do -- ... as in example

          accept Final do -- repeated here for clarity
            Ada.Text_IO.Put_Line ("Test_Task_Exception.A_Task.Final -
Exception");

            -- We wish to communicate exception to the outer-block
            raise test_exception;
          end Final;

        end;

   end A_Task;

end Test_Task_Exception;



Frank


<tmoran@acm.org> wrote in message news:Z_GJb.739626$Fm2.662614@attbi_s04...
> Personally, I would find it somewhat clearer to enclose the stuff that
> may have/handle exceptions in a block of its own, rather than the task's
> begin-end block.  That makes it clearer that exception handling has
> nothing to do with whether the task finishes up with an "accept Stop" etc.
>
>    task body A_Task is
>    begin
>      begin
>        do stuff
>      exception
>        when others => do stuff
>      end;
>      accept Stop...
>    end A_Task;