Re: Studying and Maintaining GNAT, Is There Any Interest in a New Group?

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

On 2018-08-29 02:16, Randy Brukardt wrote:
> "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de> wrote in message
> news:pm2tfi$1sv1$1@gioia.aioe.org...
>> On 2018-08-27 23:27, Randy Brukardt wrote:
>>> "Lucretia" <laguest9000@googlemail.com> wrote in message
>>> news:1ceec6d8-c5c4-49b1-9808-a3580bba3f8e@googlegroups.com...
>>> ...
>>>> I never said anything about not learning from DIANA, I said don't
>>>> implement it. Reason is simple, it was designed using Ada83, we have
>>>> OO now and OO fits a compiler perfectly and would be a hell of a lot
>>>> nicer than a bunch of variant records/enums.
>>>
>>> Fitting OO into a compiler design could very easily turn into a nightmare
>>> of required overriding routines, making it very painful to add new kinds of
>>> nodes.
>>
>> 1. How do you extend Choice:
> 
> Why would you want to? In the case of something like a compiler, there is no
> reason to extend anything rather than simply modifying it to add the
> additional capabilities.

Which makes the initial claim void. There is no nightmare and nothing to 
override what is not known in advance. For each "when" of the variant 
case there is an "override" in the inheritance scenario. If you know all 
"whens" you also know all "overrides".

>> Since nothing of that kind is possible, what you propose is monolithic
>> design AKA "god-class", when the variant record must know *in advance* all
>> future "extensions", which are extensions no more.
> 
> Why in advance? One *modifies* code in maintenance; one doesn't tie both
> hands behind your back! It's hard enough to work on a compiler without
> making arbitrary rules preventing the modification of types.

That is the exact meaning of "in advance": before the final version of 
the program unit gets ready all variants must be known.

>> You will need to change the declarations of Choice and Var each time and
>> then revisit all client code they appear.
> 
> Yes, of course. And Ada makes this easy to do safely.

Mostly yes, but the distributed overhead of maintenance is still there 
and it is no so safe as advertised. Consider the client code:

    if Current.Token = Integer_Token then
       -- Do something
    else
       -- Do something else
    end if;

Here Integer_Token was used in the meaning "scalar". Later on somebody 
adds Float_Token and boom, it meant to be "scalar"! Even if replaced by 
case it is still could be unsafe if subtypes used:

    case Current.Token is
       when Scalar_Tokens => -- A subtype with Integer_Token in it
       when others =>
    end case;

You must remember to make the subtype Scalar_Tokens covering the newly 
added Float_Token or else you get an intractable bug.

Note that inheritance would handle this safely. It will be:

    Current.Token.Do_Something; -- A dispatching call override

Do_Something would be abstract must-be-overridden for Float_Token.

>> And if the above were possible, how would it be different from inheritance
>> as we know it?
> 
> Inheritance is nearly useless! It helps a tiny bit in code reuse, but it is
> better still to only write the code once! Having to write dozens of
> overridding subprograms that don't actually add any semantics is just busy
> work of no value.

It helps massively, especially interfaces, of course only when used 
properly during analysis and design phase in order to describe 
abstraction. One should use the strengths of typing instead of working 
around them. Variant records in place of interfaces is a path leading to 
weak typing. I don't say that one should never use variant records, I 
say that one should not misuse them.

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