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Date: 29 Sep 92 05:22:16 GMT
From: munnari.oz.au!yoyo.aarnet.edu.au!news.adelaide.edu.au!cs.adelaide.edu.au!
andrewd@uunet.uu.net  (Andrew Dunstan)
Subject: Re: Ada's (in)visibility in the engineering community
Message-ID: <1a8p68INN8ke@huon.itd.adelaide.edu.au>
List-Id: <comp.lang.ada>

In article <1750@aviary.Stars.Reston.Unisys.COM>, dmarshal@Stars.Reston.Unisys.
COM (Dave Marshall) writes:
|> In article <1a0ijeINNfei@huon.itd.adelaide.edu.au>, andrewd@cs.adelaide.edu.
au (Andrew Dunstan) writes:

|> > 
|> >   with text_io; use text_io;
|> >   with unchecked_conversion;
|> >   procedure file_conv is
|> > 
|> Ack! A totally UNJUSTIFIED use of UNCHECKED_CONVERSION!  Flagellate yourself
|> thrice daily for a week.
|>

I did say it was a hack. I did ask how to do it otherwise.
 

|> > (1) APART from any righteous feelings about breaking the language's
|> > privacy mechanism, are there any good reasons for not doing something
|> > like this?
|> 
|> Yes.  Don't used UNCHECKED_CONVERSION if you don't have to.
|>

Read the question again.
 
|> > 
|> > (2) Is there a better way to do it, one which does not rely on this
|> > sort of hack?
|> 
|> Yes. Access types.  A simple example follows at the end of this response.
|> 

|> 
|> with TEXT_IO;
|> 
|> procedure FILE_TEST is
|> 
|>   type FILE_POINTER is access TEXT_IO.FILE_TYPE;
|> 
|>   type POINTERS is array ( INTEGER range <>) of FILE_POINTER;
|> 
|>   MY_ARRAY   : POINTERS(1..5);
|> 
|>   SOME_NAMES : constant array (INTEGER range 1..5) of STRING(1..8) :=
|>     ( "myfile.1",
|>       "myfile.2",
|>       "myfile.3",
|>       "myfile.4",
|>       "myfile.5");


This won't work for what I want to do. I want to be able to stack the
current input, whatever that might be (including, possibly, standard_input).

What good to me is a pointer to something that I can't assign in the first 
place?

Arthur Evans (ae@sei.cmu.edu) wrote
|> andrewd@cs.adelaide.edu.au (Andrew Dunstan) asks about a stack of items
|> of type Text_IO.File_Type, which is limited private.
|>
|> It would be hard to do if you are using a stack package.  However, if
|> you implement the stack as an array and an index (probably reasonable in
|> your case), just open the file right into the stack.  Leaving out a lot,
|> like this:
|>
|>      File_Stack: array(0..File_Stack_Max) of Text_IO.File_Type;
|>      Current_File: integer := 0;     -- Current item in File_Stack


This won't work either, for the same reason.



here's what I want to be able to do, whether or not I use a stacks
package or an explicit array:


function include(file_name : string) return boolean is
   ifile : file_type;
begin
   open(ifile,in_file,file_name);
   push(some_expression(current_input));
   set_input(ifile);
   return true;
exception
   when others => return false;
end;

function yywrap return boolean is
begin
   if empty_stack then
      return true;
   else
      close_file(current_input);
      set_input(some_other_expression(top_of_stack));
      pop_stack;
      return false;
   end if;
end;


I suspect it is not possible to do using good coding practices, but I would 
like to know, and not just from idle curiosity.


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#  Department of Computer Science   #   but thinking makes it so.     #
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