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From: "Vladimir Olensky" <vladimir_olensky@yahoo.com>
Subject: Re: tagged types extensions - language design question
Date: 2000/02/01
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 <3895EC61.9B2157A8@rational.com>
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Date: 2000-02-01T00:00:00+00:00
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Mark Lundquist wrote in message <3895EC61.9B2157A8@rational.com>...
>Vladimir Olensky wrote:
>
>> The question is :
>>
>>    Why in Ada it is  not allowed to extend both public
>>  and private part of the tagged type at one step
>
>Good question!
>
>None of the heavy hitters (Tucker et al) have weighed in, so let me
>venture my guess... I'd say the bottom line is, there isn't any really
>good reason.  That's the short answer, stay with me if you want the
>details... :-)

First of all, thanks for your very interesting response.
I've got real pleasure to follow your considerations. Sometimes
question  that popped out impulsively because your eyes were
caught  by something that seems unusual leads to very interesting
results.

<...>
>The way of thinking that's "natural" to Ada is to think of the derived
>type as providing an extension to the parent type, which extension
>is either public or private.

I always try not to put myself into the prison of any particular way
of thinking. It helps to avoid becoming slave of some particular "club"
and be open for anything that might be interesting which is outside
the area covered by some particular way of thinking and also to
avoid thinking "inertia".

In this respect any programming language is just some tool
to do some job.  Ada is not exception from that rule.
And as any other tool each one has it's advantages and drawbacks.
Any master should not become the slave of any particular tool.
He should be the master who  knows  what is the best way to use
each of them. Tools should be the slaves of the master. Of course any
master has the tool that he likes most of all and that's fine.

<...>

>So I think the real question is not, why can't you extend publicly and
>privately "all at once", but rather, why not let records and extensions
>have private components?  It seems clear that any compromise of
>3.9.1(1) is a very slippery slope that shoots you straight down to this.

>As I said before, I don't see any compelling reason why this couldn't
>have or shouldn't be done.  Is there a compelling reason to do it (or to
>have done it)?  I don't know.  You could probably make a pretty good
>case for it.

This  twist  is very interesting and exciting and I think it could
be elaborated even further.

We may ask several further questions  that  lead us very close
to some notion that will be discussed a little bit later:

1. What if  in some child  package we will  want to have parent
public fields to be private and some parent private fields (not
all of them) to be public.
  And having  some  declarations in the parent package we stick
to them forever.  There  of course exists  workaround but it is
not very clean.
2. May be it is better to have most of the parent fields to be private
and have the ability make some of them publicly available when
needed ?

These questions  are not new and differently addressed in different
existing languages.
In C++ we have class access modifiers that allows  to make public
fields to be protected or private in the descendant classes. But this
solves only one part of the  above problem.
First of all these modifies are applied to the whole class and not to the
particular field. So if I use private or protected modifier in the derived
class all the parent public members become private and protected
in the child class.
On the other hand there is  no way to make some private parent
components to be public in  derived classes.

C++ notion of friend does not help much in this issue.
So C++ approach is not flexible at all regarding this issue.

Another approach was taken in Modula-3 by introducing opaque
types. They  were discussed several times  here in CLA and several
times I gave a reference to very interesting article which discuss
M3 opaque types.
This is  "Partial Revelation and Modula-3" at:
http://www.research.digital.com/SRC/modula-3/html/partial-rev/index.html

Most interesting thing about M3 opaque type is not the fact that their
properties could be completely hidden in the implementation module.
Some people tend to think that this is the main advantage of the
opaque types but in fact it is not. This kind of property  could be easily
emulated in C++ and Ada.
The main virtue of opaque types lies in their ability to reveal some
of their hidden properties when needed - so called "partial revelation".
This allows to have parent type to be completely hidden from the
client and  any descendant to reveal only those parent properties
that needed for particular interface to the given implementation.

Here again we come up to some more general design issues.
For quite some time there exists the notion of the "Black Box"
design approach that becomes more and  more popular.
Oberon even called their new product as "Black Box" framework.

There are two approaches to the "Black Box" design model.

1. One is that we first create "Black Box"  interface that describes
it's functionality and then we implement that functionality in a way
hidden from the client. In this model "Black Box" is the
implementation of the given interface.
This is approach that followed by the most programming languages
that support the "Black Box" design approach in a way that allows
to hide  "Black Box" functionality implementation from the client.
Ada, Java, C++, Modula-2   fall  more or less in this category.

2. There exists another approach which was used so far more in
hardware design than in software design and which is more close
to real life model.
Using this approach the main focus is the "Black Box" itself and it's
functionality. "Black Box" is something that provides some kind of
universal functionality in some domain which in turn could be
provided by the set of some more small  "Black Boxes".
This functionality could be used by completely different clients
that may need different interfaces to that functionality.
So here we come to the point where  we need to define more
then one interface (for different clients) to  the given "Black Box".
There are not many languages that support directly this concept,
but Modula-3 is among them.

This is the area where Modular-3 opaque types and partial
revelation shine most of all. This is exactly what is needed to
support the second design model.
In Modula-3 we can create "Black Box" component and define
multiple interfaces for different clients  using opaque types
and partial revelation.

I tend to think that Modular-3 designers were at least ten years
ahead of their time regarding this issue. But who knows may be
will see reincarnation of this idea pretty soon in some other
real life language. May be in Ada , who knows :-).
Though Ada does not support directly exporting several
interfaces to the same implementations this could be partially
done via child packages. The only question is how to reveal
hidden/private details. As shown below this could be done using
existing Ada syntax  as well.
The only one thing which could not be expressed using existing
syntax is the partial type revelation.
Namely this is ability to show in one interface that  exported type
is a descendant of some parent type and in another interface to
show that it also has properties of another type.

Some people may argue that the last approach does not differ
much from the first one and even that the first one is more close
to the real life development process.
I can answer to this: Look around. Among all devices and
appliances that are around us we see mostly the second
model and not the first one.
Not some boxes are built around their interfaces. Quite the
contrary - all the interfaces are built to provide access to
box  functionality.

Just look at a simple PC box which implements some general
kind of functionality and provides many different interfaces
to this functionality for different clients.

It is interesting to note that CORBA and COM/DCOM
technologies also tend to support more the second model.
Both of them allow to use  multiple interfaces to the
distributed "Black Box"  component .

And again here I see that  opaque types and partial revelation
perfectly fit to and support   these two modern  technologies.

So elaboration of the answer to my original question leads us
to very interesting area indeed.

Another interesting topic  related to this subject is object/class
properties (design and run time)  that   we want to make available
to the client.
Any "Black Box" component may have some properties the state
of which  could be needed to be available to the client.
There again exists several approaches.

The one of them used in Delphi Object Pascal  (DOP)  is very
close to Ada syntax.
In  DOP we can just declare something

type T = class (some_parent_class)
    private
       function        Get_Property1 () : Integer;
       function        Get_Property2 () : Integer;
       procedure    Set_Property1 (prop :Integer);
       procedure    Set_Property2 (prop: Integer);
    public
         property  prop_1 : Integer read Get_Property1 write Set_Property1;
    published
         property  prop_2 : Integer read Get_Property2 write Set_Property2;
   end;

Then having an instance of type T ,  we can use that attributes directly :

program
Var
   obj      :  T;
   p1, p2  : Integer;
begin
   p1 := obj.Prop_1;
   p2 := obj.Prop_2;
{ or }
   obj.Prop_1 := 2;
   obj.Prop_2 := 10;
end;

In Ada it seems that we can do something like that  by
overloading ":=" operator.
But we can not do that for distinct type but not for particular
property instance/field.
So this is not perfect solution.

On the other hand I see that current Ada rep syntax  perfectly
fits here.

We can just declare:

package P is

  type T is tagged private;

   type T1 is new T with record
        --  public properties A and B;
        --  they are public views of some internal  states
        --  which in turn could be fields of some complex
        --  "Black Box" internal  structures;
        A : [property]  constant Integer;
        B : [property]  Integer;
   end record;

private
     type T is tagged record
       < some fields>
     end record;

       function        Get_Property1 (obj : in out T ) : Integer;
       function        Get_Property2 (obj : in out T ) : Integer;
       procedure    Set_Property2 ( obj : in out T; prop: Integer);

-- and then rep clause:

   for T.A'Get   use Get_Property1 (obj : in out T ) : Integer;

   for T.B'Get   use Get_Property1 (obj : in out T ) : Integer;
   for T.B'Set   use Set_Property1 ( obj : in out T;   prop: Integer);

end P;

No need to introduce any new keywords.
Ada allows rep clauses for fully visible types to be in the
private  area.
May be only  property keyword could be introduced for clarity
but generally there  is no need of it  at all;

Everything fit perfectly into current Ada syntax;

This approach can  also could be used to reveal some private
details of parent type in a new interface using direct mapping:

package N is
    type T is new P with record
        A: some_type;
        B: some_type;
    end T;
private
      for T.A  use  P.something;
      for T.B  use  P.something_else;

-- or using renaming clause:
      T.A  renames P.something;
      T.B  renames use  P.something_else;
end N;


All above is just brief description of what comes to mind when
starting  to elaborate the answer to "innocent" at the fist
glance question.


Regards,
Vladimir Olensky