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From: tmoran@bix.com
Subject: Re: Access to procedures
Date: 1999/06/25
Message-ID: <t1Pc3.386$or1.17508@typhoon-sf.snfc21.pbi.net>#1/1
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References: <37737285.5CF9ED5F@Maths.UniNe.CH> article
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 25 Jun 1999 10:31:05 PDT)
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NNTP-Posting-Date: Fri, 25 Jun 1999 10:31:05 PDT
Newsgroups: comp.lang.ada
Date: 1999-06-25T00:00:00+00:00
List-Id: <comp.lang.ada>

>"subprogram must not be deeper than access type"
>even when it doesn't seem depper at all!
  Your "main program" is in fact a procedure, and
procedure My_feedback is inside that (main program) procedure.
  Consider what would happen if you started up another task,
passed an access to My_feedback to it, and then let the "main program"
go to completion.  The task would then be able to call My_feedback,
which might try to use variables declared in "procedure main_program",
but those variables no longer exist.

>So, my question is: how to do it ?
How about adding a dummy tagged type parameter to Extract, where Extract's
"proc" call dispatches on the dummy tagged type.  eg something like:

modify Extract's spec to
package whatever is
...
  type feedback_selector is abstract tagged null record;
  procedure proc ( Which_Feedback : in out feedback_selector;
                   percents_done: natural; user_abort: out boolean)
    is abstract;

  procedure Extract(from : zip_info; what : String;
                    method     : out pkzip_method;
                    feedback   : feedback_selector'class);
 
and inside of Extract, make calls on

    proc(feedback, percents_done=>xxx, user_abort=>yyy);

which will call the "proc" procedure for the particular "feedback".

Then any time you need to make one or more particular feedback procs

  package feedback_handlers is
...
    type My_feedback is new whatever.feedback_selector;
    procedure proc ( Which_Feedback : in out My_feedback;
                     percents_done: natural; user_abort: out boolean);

    type Your_feedback is new whatever.feedback_selector;
    procedure proc ( Which_Feedback : in out Your_feedback;
                     percents_done: natural; user_abort: out boolean);
...
  end feedback_handlers;
  package body feedback_handlers is
    procedure proc ( Which_Feedback : in out My_feedback;
                     percents_done: natural; user_abort: out boolean) is
    new_done_dots: natural:= (dots * percents_done) / 100;
    begin
      for i in done_dots .. new_done_dots loop
        Put('.');
      end loop;
      user_abort:= false;
    end proc;
    procedure proc ( Which_Feedback : in out Your_feedback;
                     percents_done: natural; user_abort: out boolean) is
...
  end feedback_handlers;

and to use them

  procedure Do_Unzip is
      Feedback1 : feedback_handlers.My_feedback;
      Feedback2 : feedback_handlers.Your_feedback;  -- for instance
    begin
...
          whatever.Extract(zi, Argument(i), method, Feedback1 );
...
    end Do_unzip;

(typed but not tested)

>NB: a generic solution doesn't seem a solution as some packages,
>basically non-generic, have to call the feedback procedure; making
  Those packages would just 'with whatever' and take a
whatever.feedback_selector'class parameter.  To call one of those
routines with a specific proc in mind, just pass it an object of
the appropriate kind.  Essentially, instead of passing "proc1'access"
or "proc2'access", you pass in Feedback1 or Feedback2, or ...