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From: Robert A Duff <bobduff@shell01.TheWorld.com>
Newsgroups: comp.lang.ada
Subject: Re: Seeking for papers about tagged types vs access to subprograms
Date: Sun, 12 May 2013 18:15:24 -0400
Organization: The World Public Access UNIX, Brookline, MA
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Xref: number.nntp.dca.giganews.com comp.lang.ada:181609
Date: 2013-05-12T18:15:24-04:00
List-Id: <comp.lang.ada>

Jeffrey Carter <spam.jrcarter.not@spam.not.acm.org> writes:

> I'm not sure I see the point in having an object for a closed file,

Yeah, I was about to post basically the same thing.  A closed file
is pretty useless.  It's like an uninitialized variable -- you can't
do anything with it.

> other than the requirements of low-level languages in which such things
> were 1st implemented.

Low-level languages like Ada 83?  ;-)

>...Why not something like
>
> type File_Info (<>) is tagged limited private;

Yes, but I would have separate types for Input_File and Output_File.
Possibly another type for the rare case when you want to
read and write to/from the same file handle.

> function Open   (Name : ...; ...) return File_Info;
> function Create (Name : ...; ...) return File_Info;

Note that these are build-in-place functions.  You can't call them
as the right-hand side of an assignment statement.

> function Read   (File : in out File_Info) return ...;
> procedure Write (File : in out File_Info; Item : in ...);
>
> declare
>    File : File_Info := Open ("junk", ...);
> begin
>    Data := Read (File);
>    ...
> end;

You could use it as above, or like this:

    Grind_Upon_File(Open("junk", ...));

Or like this:

    X := new File_Info'(Open(...));

But you couldn't use it like this:

    File : File_Info; -- Illegal!
    ... -- some code that computes File_Name
    File := Open (File_Name); -- Illegal!

which is a limitation, compared to the current design of Text_IO
and friends.

> A File_Info must be opened or created when declared, and is closed when
> it's finalized.

Right.

- Bob