From mboxrd@z Thu Jan  1 00:00:00 1970
X-Spam-Checker-Version: SpamAssassin 3.4.4 (2020-01-24) on polar.synack.me
X-Spam-Level: 
X-Spam-Status: No, score=-1.9 required=5.0 tests=BAYES_00
	autolearn=unavailable autolearn_force=no version=3.4.4
Path: 
 eternal-september.org!reader01.eternal-september.org!feeder.eternal-september.org!news.swapon.de!fu-berlin.de!uni-berlin.de!individual.net!not-for-mail
From: Niklas Holsti <niklas.holsti@tidorum.invalid>
Newsgroups: comp.lang.ada
Subject: Re: Problem with Position of the enumeration Type
Date: Thu, 24 Jan 2019 01:20:16 +0200
Organization: Tidorum Ltd
Message-ID: <gasb4hFp1btU1@mid.individual.net>
References: <3d782720-227d-4d86-b403-eacbd1b9d0d2@googlegroups.com>
 <gas5dpFns14U1@mid.individual.net> <lyva2fuhmp.fsf@pushface.org>
Mime-Version: 1.0
Content-Type: text/plain; charset=windows-1252; format=flowed
Content-Transfer-Encoding: 7bit
X-Trace: individual.net VnvY+o7p0rnVE5iVurOTEAwbCiXogtkwkPPgcReXwJHiQokMCT
Cancel-Lock: sha1:yFgQESjFrqM10Zi6QRromDaAgq0=
User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.8; rv:45.0)
 Gecko/20100101 Thunderbird/45.8.0
In-Reply-To: <lyva2fuhmp.fsf@pushface.org>
Xref: reader01.eternal-september.org comp.lang.ada:55349
Date: 2019-01-24T01:20:16+02:00
List-Id: <comp.lang.ada>

On 19-01-24 00:35 , Simon Wright wrote:
> Niklas Holsti <niklas.holsti@tidorum.invalid> writes:
>
>> Such rules also commonly say that such types should not be used as
>> array index types, because of the possible introduction of gaps in the
>> array representations.
>
> Not, it seems, with GNAT:

      [skip example]

Nice to know.

> I'd think any other behaviour would be unreasonable (though I can't see
> any statement about it in the ARM).

Avoiding gaps in such arrays means that GNAT is effectively using the 
position number as the (machine-level) index, which means that indexing 
with an arbitrary enumeration value (not a loop induction variable) 
requires the use of the 'Pos function, with its possible longish 
execution time for a gappy enumeration. So it's a space vs time 
trade-off, and GNAT chooses to reduce space at the cost of time.

I too think that this is the choice less likely to surprise users.

While enumeration representation clauses are a logical and sometimes 
useful feature, they are not very often the best design approach -- or 
not as often as one first thinks, when learning about them. I tend to 
use them only for outputs on the HW level, not so much for inputs 
(because of the need for validity checking that I commented on).

-- 
Niklas Holsti
Tidorum Ltd
niklas holsti tidorum fi
       .      @       .