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Newsgroups: comp.lang.ada
Subject: Re: Structured exception information
From: Georg Bauhaus <bauhaus@arcor.de>
In-Reply-To: <wcczm8k8aj5.fsf@shell01.TheWorld.com>
References: <eog0g5$2bn$1@cernne03.cern.ch>
	 <wcczm8k8aj5.fsf@shell01.TheWorld.com>
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Date: Mon, 15 Jan 2007 19:29:31 +0100
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Date: 2007-01-15T19:27:32+01:00
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On Mon, 2007-01-15 at 12:28 -0500, Robert A Duff wrote:
> Maciej Sobczak <no.spam@no.spam.com> writes:
> 
> > If there are problems during the execution of the constructor function,
> > the exception is raised, so that there is no X object in a bad state.
> > How can I pass some error information from the constructor function out,
> > so that it's used when the exception is handled?
> 
> There is no good way to do this in Ada.  You can attach any information
> you like to an exception, if you are willing to encode it as a String --
> but then you lose static type checking.  You can put the info in a
> global variable, but that's bad for several reasons (not task safe,
> can be accessed outside of any handler, ...).  You can put the info in a
> Task_Attribute, but that's rather a pain -- verbose and inefficient.

I'm still wondering whether or not an Ada compiler could provide
a special kind of closure for this case? We do already get stack
traces. (I don't know compilers so this is just a guess...)

package Handling is
   
   --
   -- info to be attached to an exception occurence
   --
   
   type State is tagged record
      info: Integer;
   end record;
   
end Handling;


with Handling;  use Handling;
procedure foo is
   
   x: Integer;
   
   function Env return State'class is
   begin
      return State'(info => x);
   end Env;
   
begin
   if 1 > 1 then
      raise Constraint_Error with Env'access;
   end if;
end foo;