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NNTP-Posting-Date: Tue, 16 Jan 2007 00:03:46 -0600
From: tmoran@acm.org
Newsgroups: comp.lang.ada
Subject: Re: Structured exception information
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Date: Tue, 16 Jan 2007 00:03:46 -0600
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Date: 2007-01-16T00:03:46-06:00
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> You could have:
>
>     procedure My_Own_Raise (E : in Ada.Exceptions.Exception_ID;
>                             Info : Extra_Info);
>
> and the implementation could encode Info as a String, and use that as
> the Message passed to Raise_Excpetion.  The handler then decodes the
> String.  The stream attributes could be used for encoding/decoding.
> Maybe wrapping this mess in some generics could make it more type safe.
>
> Yuck.

   You could make it type safe with an extended tagged record.  Make
   a key handler that saves a pointer to the classwide record, returning
   a "safe deposit key" whose (short) 'image constitutes the message.
   Risking posting untried pseudo code, how about:

package exception_info is

  exception_with_info : exception;

  type base_type is abstract tagged record
    actual_occurrence : ada.exceptions.exception_occurrence_access;
  end record;

  procedure handle(x : in out base_type);
  -- Called by handler for descendant of base type.  Does a Free
  -- on x.actual_occurrence

  type p_exception_info_type is access base_type'class;

  procedure raise_with_info(p : in p_exception_info_type);
  -- Trades the pointer to the information for a unique key from a
  -- (protected) key handler, then raises Exception_With_Info
  -- with key_type'image(key) as the message string.

  function fetch_info(egad : ada.exceptions.exception_occurrence)
  return p_exception_info_type;
  -- Extracts the key_type'value from the message, gives it to
  -- the protected key handler, and returns the saved pointer.


and to use declare
  ...
    type speeding_ticket_type is new exception_info.base_type with record
      time : ada.calendar.time;
      speed : positive;
    end record;

    procedure handle(ticket : in out speeding_ticket_type) is
    begin
      ada.text_io.put_line(integer'image(ticket.speed) & " mph."
        & ada.exceptions.exception_information(ticket.actual_occurrence.all));
      handle(exception_info.base_type(ticket));
    end handle;

    ...

when there's an exception that needs to pass speeding ticket info do
    when oops:constraint_error =>
      declare
        p : exception_info.p_exception_info_type
          := new speeding_ticket_type(
                actual_occurrence=>ada.exceptions.save_occurrence(oops),
                time=>ada.calendar.clock,
                speed=>speedometer_value);
      begin
        exception_info.raise_with_info(p);
      end;

and the catcher is
    when aha:exception_info.exception_with_info =>
      declare
        p : exception_info.p_exception_info_type
          := exception_info.fetch(aha);
      begin
        handle(p.all'class);
        free(p);
      end;