Re: Interesting article on ARG work

["Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>]

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