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

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

On 2018-08-29 23:43, Randy Brukardt wrote:
> "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de> wrote in message
> news:pm5l0r$dek$1@gioia.aioe.org...
>> 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".
> 
> Surely you "know" them; my point is that there is an incredible amount of
> busy-work needed to write them (as opposed to just modifying the handful of
> places that need that).
> 
> For instance, one has to write tree-walking code into every override, as
> there is no good way to do that with inheritance (the override has to make a
> dispatching call on itself on the child mode(s)). The only way to do that
> with though inheritance is to move all of the tree-walking stuff outside of
> the OO operations (as is done in the tree container) -- but then you are no
> longer using strong typing on the children -- the number and kinds of the
> children have to be checked with manual code.

This I don't understand. Why tree operations cannot be primitive and/or 
class-wide?

As a general observation, anything you can do with subtypes of a single 
type you can also do with a class of a types hierarchy. [There could be 
some obstacles regarding indefinite vs definite types, but they usually 
apply to both designs.]

>>>> 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.
> 
> Umm, we're talking about a compiler, and there never is a "final" version of
> a compiler.

Final = deployed, of course.

>>>> 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"!
> 
> Sure, but OO doesn't really help here: most properties aren't strictly
> tree-like. For instance, many type categories don't follow inheritance rules
> (limited being the obvious example).

[Non-]limited should have been an interface. There is nothing in it that 
is not. Non-limited = have copying operations and constructors. 
[Requires MD]

Same applies to by-reference vs. by-copy. By-reference = has identity.

Most Ada bells and whistles could be broken down into well-defined 
interfaces, which should be the goal of the language future changes.

>> 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.
> 
> Sure, but that's what's maddening about using OOP in this way. You have to
> define and implement *every* such operation that way for a new node before
> anything can be done (even compiling the changed program).

Exactly. This is called being typed. Operations do not exist without 
types. The design with case-statements spread all over the client code 
is untyped because chunks of client code under "whens" are not 
attributed to any type, have no contract etc. OO allows us

1. To give a contract to this code
2. To refactor and reuse this code
3. To place it in separate compilation units *related* to the types 
involved.

> There are likely
> to be hundreds of such operations for a compiler (certainly dozens). That is
> very much like monolithic waterfall development -- there is nothing agile
> about it.

Here is where MI comes into play. These hundreds of operations belong to 
different interfaces. Tree manipulation and node semantics must be 
described separately.

> I try to keep the compiler compiled almost all of the time when doing
> development, even when making major changes like new node kinds.
> Compile-time errors tend to get eliminated first. That's in part because
> code that isn't compilable shouldn't be checked in, and moreover we always
> need to be able to make fixes for customers. The time while the compiler
> isn't compilable is a risk period.
> 
> If instead you use some tool to make an empty set of TBDs for all of these
> routines, now you've totally eliminated any compiler-based help (all of the
> needed routines exist, they just don't *work*) and you have exactly the same
> need for testing that you would have had using a variant. So you've gained
> nothing.

Morale, do not use crappy tools. Somebody might someday come with a tool 
that would generate all "whens" in all case statements filled with null- 
or raise-statement. Would that be a failure of Ada case-statement 
design? No. If you do that mess on language level (see dynamic 
predicates), that is the language mess. If you do mess with a tool, 
well, that is no language problem.

>>>> 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.
> 
> I use strong typing to ensure that nodes don't get mixed with symbols or
> with memory allocation record or many other things.

But you allow mixing nodes freely? That is the problem. In a tree all 
nodes are same, but there is semantics of what the node represents and 
you allow mixing that semantics without mapping it onto types. Only one 
aspect of your design is strongly typed: tree manipulations, BTW, the 
aspect more or less marginal to computer design.

> I don't see any point in
> treating nodes as different types. Spending a lot of extra time doing
> analysis to try to bring structure where there is little natural structure
> is wasteful at best.

There might be categories of nodes sharing certain properties and in the 
end some refactored code. You cannot describe such things without some 
form of interface, dynamic (tagged) or static (generics).

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