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From: davidk@OS7.ifs (David Kristola)
Subject: Re: Syntax for tagged record types (was Re
Date: 1997/05/28
Message-ID: <5mg7d7$6t5@butch.lmms.lmco.com>#1/1
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Distribution: world
References: <mheaney-ya023680002305971754170001@news.ni.net>
Organization: Sun Microsystems, Inc.
Reply-To: davidk@OS7.ifs
Newsgroups: comp.lang.ada
Date: 1997-05-28T00:00:00+00:00
List-Id: <comp.lang.ada>


In article ya023680002305971754170001@news.ni.net, mheaney@ni.net (Matthew Heaney) writes:
>In article <JSA.97May22203202@alexandria>, jsa@alexandria (Jon S Anthony) wrote:
>
>
>>> Here is what I'd like to do:
>>> 
>>> declare
>>>    The_File : File_Type'Open (Name => "my_file.dat", Mode => In_Mode);
>>> begin
>>> 
>>> Renaming a function return as a constant object won't fly, because I have
>>> to modify the object.
[snip]
>What I am proposing is a new syntax, by adding a constructor abstraction;
>File_Type'Open in this example.  (Today, in Ada 95, Text_IO.Open is an
>ordinary procedure.)  It would look something like:
>
>   constructor Open (Name : String; Mode : File_Mode) return File_Type;
>
>or maybe
>
>   constructor File_Type'Open (Name : String; Mode : File_Mode);
[snip]

Or how about a change to the rules that allow a constructor function
(using normal function notation) when a limited object is created:

declare
    -- Fine since this is creation
   The_File : File_Type := Text_IO.Open (Name => "my_file.dat", Mode => In_Mode);
begin
    -- Illegal since The_File already exists.
    The_File := Text_IO.Open (Name => "my_other_file.dat", Mode => In_Mode);


where in Text_IO, there is a function named Open that returns the limited
type Text_IO.File_Type.

The only time an object of a limited type could be set by a function is
on creation.  I would also be allowed to provide my own function, say one
that checked a flag and returned Text_IO.Standard_Input or Text_IO.Open(...).

Hmm, might this cause problems?  Maybe mark the allowable functions with
the word "limited" as in:

limited function Open(...) return File_Type;

Then, limited functions would only be allowed in the package that defines
the limited type.  If the implementation allowed The_File to contain
whatever is returned by Standard_Input, then Standard_Input could also
be marked as limited.

I would hesitate to add a new key word to Ada, especially if existing key
words could be used.

But then, this is all just fancy.  :-)

--david kristola (not speaking for Lockheed Martin or SAMCO)
(My news reader does not understand the firewall, oh well, automatic spam shield)
Home: David95037 at aol dot com
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