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!reader02.eternal-september.org!feeder.eternal-september.org!aioe.org!.POSTED!not-for-mail
From: "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>
Newsgroups: comp.lang.ada
Subject: Re: Interesting article on ARG work
Date: Sun, 8 Apr 2018 09:46:13 +0200
Organization: Aioe.org NNTP Server
Message-ID: <pachc4$mkr$1@gioia.aioe.org>
References: <p9s890$mmt$1@franka.jacob-sparre.dk>
 <1b44444f-c1b3-414e-84fb-8798961487c3@googlegroups.com>
 <62ee0aac-49da-4925-b9aa-a16695b3fc45@googlegroups.com>
 <pa0vga$rbe$1@franka.jacob-sparre.dk>
 <9879872e-c18a-4667-afe5-41ce0f54559f@googlegroups.com>
 <pa3jmj$9n3$1@franka.jacob-sparre.dk> <pa4ip3$1ton$1@gioia.aioe.org>
 <pa67ci$t34$1@franka.jacob-sparre.dk> <pa77nc$1pu1$1@gioia.aioe.org>
 <pa9a6q$ii$1@franka.jacob-sparre.dk> <paa5gk$knn$1@gioia.aioe.org>
 <80db2d05-744f-4201-ba1b-4436f8040491@googlegroups.com>
 <paaptu$1pce$1@gioia.aioe.org>
 <59f9ab6d-d6ba-45ff-a6f0-c5699983d9e8@googlegroups.com>
NNTP-Posting-Host: kQkuQcRDy1QFvWpyB1foYw.user.gioia.aioe.org
Mime-Version: 1.0
Content-Type: text/plain; charset=utf-8; format=flowed
Content-Transfer-Encoding: 8bit
X-Complaints-To: abuse@aioe.org
User-Agent: Mozilla/5.0 (Windows NT 10.0; WOW64; rv:52.0) Gecko/20100101
 Thunderbird/52.7.0
Content-Language: en-US
X-Notice: Filtered by postfilter v. 0.8.3
Xref: reader02.eternal-september.org comp.lang.ada:51397
Date: 2018-04-08T09:46:13+02:00
List-Id: <comp.lang.ada>

On 2018-04-08 02:14, Dan'l Miller wrote:
> On Saturday, April 7, 2018 at 11:00:02 AM UTC-5, Dmitry A. Kazakov wrote:
>> On 2018-04-07 17:27, Dan'l Miller wrote:
>>> On Saturday, April 7, 2018 at 5:11:35 AM UTC-5, Dmitry A. Kazakov wrote:
>>>> On 2018-04-07 04:25, Randy Brukardt wrote:
>>>>
>>>>> The biggest problem with a lexical solution (and the reason that Ichbiah
>>>>> hated them) is that it's trivial to accidentally create a program that is
>>>>> only legal in one mode or the other. One has to continually compile
>>>>> everything in each possible mode to ensure that there aren't silly syntax
>>>>> errors involved. A common mistake that I make all of the time is:
>>>>>
>>>>>        if Foo then
>>>>>               Bar;
>>>>>        else
>>>>> @        Internal_Error;
>>>>>        end if;
>>>>>
>>>>> With the conditional compilation off, the "Internal_Error" is commented out
>>>>> and the above isn't syntactically legal.
>>>>
>>>> Yes, this is important but I think this could be resolved, e.g. by
>>>> compiling conditionals into corresponding null-effect constructs.
>>>
>>> Yeah, right.  This is an artificially contrived “problem” that is in fact not extant in reality.  Simply put the @ symbols in front of the •entire• construct to elide:  the entire else branch.
>>>
>>>       if Foo then
>>>              Bar;
>>> @ else
>>> @        Internal_Error;
>>>       endif;
>>>
>>
>> "@ else"
>> "@        Internal_Error;"
>>
>> Is not a valid sequence of statements.
> 
> That is a non sequitur, Dmitry.
> 
>       if Foo then
>             Bar;
> @ else
> @        Internal_Error;
>       endif;
> 
> produces either this legal Ada if-else statement,

[...]

ARM 5.1 (2/3) defines sequence of statements. "else Internal_Error;" is 
not a sequence of statements. The rule is simple, you can insert only 
complete statements.

    if Foo then
@     Bar;
@  end if;
@  if Boo then
       Baz;
    end if;

would not be legal either.

More complicated rules are required for declarations, e.g. that 
conditionally declared entities would not be visible outside conditional 
code:

declare
@  X : Integer;
    Y : Integer;
begin
    Y := X; -- This is illegal
@  Y := X; -- This is OK
end;

or

@ with Text_IO;
use Text_IO; -- No, that does not work

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