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From: "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>
Newsgroups: comp.lang.ada
Subject: Re: Your wish list for Ada 202X
Date: Tue, 22 Apr 2014 11:14:44 +0200
Organization: cbb software GmbH
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Date: 2014-04-22T11:14:44+02:00
List-Id: <comp.lang.ada>

On Tue, 22 Apr 2014 02:30:17 -0600, Shark8 wrote:

> On 22-Apr-14 02:02, Dmitry A. Kazakov wrote:
>> But there should be no Open operation at all.
> 
> Why not?
> It seems to me that Open could be ideal for some object, like say an 
> object representing flood-gates, or an internally used operation for an 
> "e-mail reader", or dozens of other things (perhaps file processing 
> [e.g. Get_File_List, Filter, Open as basic 'operations' of the processor]).

This is a completely irrelevant case. I talked (and I believe Randy did
too) about Open like file open and hence the problem that it cannot be
inherited with consequent maintenance problems for clients.

>> This is another language problem that Ada forces you to introduce Open,
>> which adds unusable object states (not yet opened object), must be
>> overridden (but there is no way to spell that requirement), must call
>> to the parent's Open (but. again, no way to ensure that).
> 
> Not entirely true; with pre/post conditions, and perhaps type-invariants 
> you can prohibit such states from being entered (which will result in an 
> exception).

Which is exactly the problem - this introduces erroneous states, where
there should be none. From the SW design POV there shall be no way to
create a closed file, an uninitialized widget, a float with the FPU turned
off etc.

The elementary principle of SW design is that the program must be
functional in each of its states., in other words be a correct program.

-- 
Regards,
Dmitry A. Kazakov
http://www.dmitry-kazakov.de