Re: Real OO (was Choice of OO primitives in Ada95)

Re: Real OO (was Choice of OO primitives in Ada95)

[Matt Kennel]

Jean-Pierre Rosen (rosen@EMAIL.ENST.FR) wrote:
> >Taking each in turn (and not attempting to justify them),
> >
> >1) Best provided by pure OO (eg. Eiffel, Smalltalk) followed by hybrid OO
> >   language with proper classes (extendable types mapped onto modules)
> >   eg. (C++, myriad others) followed by hybrid OO with non-encapsulated classes
> >   (eg. Ada, CLOS?).
> Of course, I do not agree. For example, the presence of ONCE functions in
> Eiffel is a kludge to overcome the absence of package initialization that is
> nicely provided by Ada.

Um maaaaybe, but 'once functions' do what they do with no fuss and with
very clear semantics, without having to define an "elaboration" notion for
packages. 

I'm not sure that's a "kludge" compared to the complexity of package
initialization in Ada. 

>  And as for separating specs and implementations, Ada
> is the only language that does it nicely, without giving up safety.

This isn't true.

You write abstract classes in Eiffel/Sather/C++ which has the spec
(including pre/post/invariant) and write an implementation class which
supports it. 

What is the problem?

> >
> >2) The linchpin of OO - any language claiming to be OO offers this.
> Including Ada 95, although I still think it has been overadvocated.
> >
> >3) The only significant OO language I know of that offers this is Eiffel.
> >   Assertions are executable conditions which must be satisfied to guarantee
> >   that an abstraction has a valid state whenever it is updated. These include:
> > [...]
> It was considered in the early stages of Ada  (83) design, and rejected.
> Note also that assertions in Eiffel are much LESS reliable than one could
> think, since assertions are disabled while evaluation the condition for an
> assertion. This is necessary to prevent semantic dependencies to end up in
> infinite recursion, but limits the trust you can have in assertions.

No.  Meyer figured out, quite successfully IMHO, exactly the kinds of
rules that you need to turn "assertions" from just 'check this predicate
and bomb if false' into a powerful *human* protocol "programming by
contract". 

These special rules are exactly what you need so you CAN trust assertions,
because it means that you can put in truly useful checks that don't barf
because of stupidities like the above. 

> >
> >Note that Ada only fully satisfies 2) and has a sub-optimal mechanism for 1).
> I have a package for 3) if you want. It's less than 50 lines long, including
> comments.

> So what ? Your statements were biased towards the conclusion that Eiffel was
> the only appropriate language. I can  come up with equally biased statements
> that will "naturally" conclude that Ada is the best OOP language. What
> about, for example:

>    OOP is about modelling real-world objects. Since real-world objects evolve
>    concurrently, no language should be called OO unless it provides concurrency.

> OK, this kills C++, Smalltalk, Eiffel, CLOS....

     OOP is about modelling real-world objects. Since real world objects couple
     to the Einstein mass-energy tensor, no langauge should be called OO unless
     its operation can provide sufficient extra energy dissipation to cause 
     similar effects.

:-) 


I think it *is* quite fair to say that Eiffel is more object-oriented---ok
more oriented-around-objects than Ada 95: it's the idee' fixe for Eiffel. 

This doesn't prove 'prima facie' superiority, though I tend to like
Eiffel better.  (I've never been convinced about kludges to handle the 
'multiple withing' problem).


> +------------------------------------o-------------------------------------+
> | P-mail:                            | E-mail: rosen@enst.fr               |
> |   ADALOG - 27 avenue de Verdun     |    Tel: +33 1 46 45 51 12           |
> |   92170 Vanves - FRANCE            |    Fax: +33 1 46 45 52 49           |
> +------------------------------------o-------------------------------------+

Re: Real OO (was Choice of OO primitives in Ada95)

[Jean-Pierre Rosen]

At 03:17 22/02/1996 GMT, Don Harrison wrote:
This time I can't  resist responding.... Here we go:

>The real issue to me is that the Ada 95 OO model falls short of what real
>object-
>orientation is about. The purpose of OO, as I see it is this:
>
>"To enable the cost-effective development of correct, reliable and maintainable
> software".
Agreed.

>The sub-goals required to facilitate this are:
>a) Allow decomposition of the software into intelligible modules each of which
>   represents a clearly defined abstraction and has a clearly defined interface
>   with other abstractions
>
>b) Allow the construction, where possible, of abstractions using abstractions
>   that are already defined, customising them as necessary rather than
>   defining abstractions from sctrach.
>
>c) Allow the valid state of an abstraction to be clearly defined and enforced.
Are you sure these are the best sub-goals ? That there are no other ones ?
And that in  stating the goals, you were not influenced by the conclusion
you wanted to draw ? But let's go with it for the moment.

>In order to acheive those goals optimally, an OO language should offer:
>
>1) encapsulated abstractions (classes)
OK, but of course you mean that there must be a clear separation, at
language level, between specification and implementation

>2) inheritance/polymorphism of those abstractions
How  can you deduce this from b) ? Inheritance is definitely one possible
way, but you are claiming here it is the only one. I think building an
abstraction by putting together standardized, appropriate building blocks
and lower level abstractions is another way of achieving b) that does not
require inheritance.
Both approaches have benefits and drawbacks. I'm just irritated each time I
see that because inheritance is one way of solving a problem, people think
it is the only one.

>3) control over the state of those abstractions (assertions).
I think a typing system that does not allow  adding a length to a velocity
is much more important for c) than assertions that are nothing more that a
user definable function call.
>
>Taking each in turn (and not attempting to justify them),
>
>1) Best provided by pure OO (eg. Eiffel, Smalltalk) followed by hybrid OO
>   language with proper classes (extendable types mapped onto modules)
>   eg. (C++, myriad others) followed by hybrid OO with non-encapsulated classes
>   (eg. Ada, CLOS?).
Of course, I do not agree. For example, the presence of ONCE functions in
Eiffel is a kludge to overcome the absence of package initialization that is
nicely provided by Ada. And as for separating specs and implementations, Ada
is the only language that does it nicely, without giving up safety.
>
>2) The linchpin of OO - any language claiming to be OO offers this.
Including Ada 95, although I still think it has been overadvocated.
>
>3) The only significant OO language I know of that offers this is Eiffel.
>   Assertions are executable conditions which must be satisfied to guarantee
>   that an abstraction has a valid state whenever it is updated. These include:
> [...]
It was considered in the early stages of Ada  (83) design, and rejected.
Note also that assertions in Eiffel are much LESS reliable than one could
think, since assertions are disabled while evaluation the condition for an
assertion. This is necessary to prevent semantic dependencies to end up in
infinite recursion, but limits the trust you can have in assertions.

>
>Note that Ada only fully satisfies 2) and has a sub-optimal mechanism for 1).
I have a package for 3) if you want. It's less than 50 lines long, including
comments.

So what ? Your statements were biased towards the conclusion that Eiffel was
the only appropriate language. I can  come up with equally biased statements
that will "naturally" conclude that Ada is the best OOP language. What
about, for example:

   OOP is about modelling real-world objects. Since real-world objects evolve
   concurrently, no language should be called OO unless it provides concurrency.

OK, this kills C++, Smalltalk, Eiffel, CLOS....
+------------------------------------o-------------------------------------+
| P-mail:                            | E-mail: rosen@enst.fr               |
|   ADALOG - 27 avenue de Verdun     |    Tel: +33 1 46 45 51 12           |
|   92170 Vanves - FRANCE            |    Fax: +33 1 46 45 52 49           |
+------------------------------------o-------------------------------------+

Re: Real OO (was Choice of OO primitives in Ada95)

[Valery Croizier]

In article <199602221711.SAA18350@email.enst.fr> Jean-Pierre Rosen <rosen@EMAIL.ENST.FR> writes:

> >3) The only significant OO language I know of that offers this is Eiffel.
> >   Assertions are executable conditions which must be satisfied to guarantee
> >   that an abstraction has a valid state whenever it is updated. These include:

> I have a package for 3) if you want. It's less than 50 lines long, including
> comments.

If you can simulate Eiffel-like assertions in a trivial way, then
please show it. I know how to check arguments and raise an exception,
but not so that these checkings are inherited by children, like in Eiffel. 

--
Valery CROIZIER
Valery.Croizier@enst-bretagne.fr

Re: Real OO (was Choice of OO primitives in Ada95)

[Jon S Anthony]

In article <dewar.824966245@schonberg> dewar@cs.nyu.edu (Robert Dewar) writes:

> Don says
> 
> "Note that Ada only fully satisfies 2) and has a sub-optimal mechanism for 1)."
> 
> Well that's no more than an opinion, you provide no objective criterion to
> justify this opinion. In fact I think that Ada 95 satisfies 1) more
> convincingly than Eiffel. This has been discussed many times on this news
> group, and I don't particularly have anything to add to those previous
> discussions, I think it has all been said before!
> 
> The value of executable assertions has been hotly debated. To regard them
> as an essential part of OO programming seems odd to me, but of course
> people are free to say "x is essential for OO in my view" for almost
> any X (and all sorts of x's have been suggested). One trouble with
> executable assertions is that they are at the wrong level of abstraction,
> since they are confined to be at the semantic level of the language, and
> generally you want to talk about abstractions at a higher level.

Yes, Don says a lot of things.  He also says the following in
c.l.e:

<<<
Newsgroups: comp.lang.eiffel
Path: alexandria.organon.com!uunet!in2.uu.net!usc!howland.reston.ans.net!nntp.coast.net!harbinger.cc.monash.edu.au!news.mel.connect.com.au!news.syd.connect.com.au!warrane.connect.com.au!assip.csasyd!news
From: donh@syd.csa.com.au (Don Harrison)
Subject: Real OO
Message-ID: <Dn5sLw.A95@assip.csasyd.oz>
Sender: news@assip.csasyd.oz
Reply-To: donh@syd.csa.com.au
Organization: CSC Australia
Date: Thu, 22 Feb 1996 04:03:31 GMT
Lines: 11
 
For anyone interested, I've just stirred up the Ada nest with a posting 
titled 'Real OO'.
 
I didn't cross post it to here, for fear of boring you all with endless
follow-ups.
 
Regards,
Don.
>>>

Basically, he's just trolling.  So, it is best to not waste time
replying to anything he posts.

/Jon
-- 
Jon Anthony
Organon Motives, Inc.
1 Williston Road, Suite 4
Belmont, MA 02178

617.484.3383
jsa@organon.com

Real OO (was Choice of OO primitives in Ada95)

[Don Harrison]

To Norman Cohen:

Thanks for giving the example about Family Trees.
I've gained access to an RM and Rationale. I'll go and check whether my
understanding is correct. (I was asking for trouble debating an issue without
the reference material handy).

To Jon Anthony:

On the necessity or otherwise of "tagged" and "'Class", I acknowledge that with
hybrid OO model adopted in Ada 95, it may be appropriate to require these of
developers - I'll reserve judgement until I've re-read the RM and Rationale.

---------------------------------------------------------------------------------
The real issue to me is that the Ada 95 OO model falls short of what real object-
orientation is about. The purpose of OO, as I see it is this:

"To enable the cost-effective development of correct, reliable and maintainable
 software".

The sub-goals required to facilitate this are:

a) Allow decomposition of the software into intelligible modules each of which
   represents a clearly defined abstraction and has a clearly defined interface
   with other abstractions

b) Allow the construction, where possible, of abstractions using abstractions
   that are already defined, customising them as necessary rather than
   defining abstractions from sctrach.

c) Allow the valid state of an abstraction to be clearly defined and enforced.

In order to acheive those goals optimally, an OO language should offer:

1) encapsulated abstractions (classes)
2) inheritance/polymorphism of those abstractions
3) control over the state of those abstractions (assertions).

Taking each in turn (and not attempting to justify them),

1) Best provided by pure OO (eg. Eiffel, Smalltalk) followed by hybrid OO
   language with proper classes (extendable types mapped onto modules)
   eg. (C++, myriad others) followed by hybrid OO with non-encapsulated classes
   (eg. Ada, CLOS?).

2) The linchpin of OO - any language claiming to be OO offers this.

3) The only significant OO language I know of that offers this is Eiffel.
   Assertions are executable conditions which must be satisfied to guarantee
   that an abstraction has a valid state whenever it is updated. These include:

   i)   pre-conditions on operations that callers are required to satisfy
   ii)  invariant conditions that must always be true when the abstraction is updated
   iii) post-conditions on operations stating what the abstraction guarantees to
        have done when the operation has been executed.

   There are others.

Note that Ada only fully satisfies 2) and has a sub-optimal mechanism for 1).

While Eiffel is strong in the OO stakes (it satifies all 3), it is weak
(unfortunately) in the RT arena. These weaknesses are:

a) poor low level facilities (bit fiddling)
b) GC renders it an inappropriate tool for hard real-time systems (although the
   emergence of mainstream parallel architectures will permit this)
c) no concurrency (an elegant model currently being implemented - not yet part
   of the standard).

I think these will all be addressed in time but if none are a concern to you,
I suggest taking a look at Eiffel.

Don.

Re: Real OO (was Choice of OO primitives in Ada95)

[Robert Dewar]

Don says

"Note that Ada only fully satisfies 2) and has a sub-optimal mechanism for 1)."

Well that's no more than an opinion, you provide no objective criterion to
justify this opinion. In fact I think that Ada 95 satisfies 1) more
convincingly than Eiffel. This has been discussed many times on this news
group, and I don't particularly have anything to add to those previous
discussions, I think it has all been said before!

The value of executable assertions has been hotly debated. To regard them
as an essential part of OO programming seems odd to me, but of course
people are free to say "x is essential for OO in my view" for almost
any X (and all sorts of x's have been suggested). One trouble with
executable assertions is that they are at the wrong level of abstraction,
since they are confined to be at the semantic level of the language, and
generally you want to talk about abstractions at a higher level.

Re: Real OO (was Choice of OO primitives in Ada95)

[Spencer Allain]

In article <199602221711.SAA18350@email.enst.fr> Jean-Pierre Rosen <rosen@EMAIL.ENST.FR> writes:

   >In order to acheive those goals optimally, an OO language should offer:
   >
   >1) encapsulated abstractions (classes)
   OK, but of course you mean that there must be a clear separation, at
   language level, between specification and implementation

<snip>

   >Taking each in turn (and not attempting to justify them),
   >
   >1) Best provided by pure OO (eg. Eiffel, Smalltalk) followed by hybrid OO
   >   language with proper classes (extendable types mapped onto modules)
   >   eg. (C++, myriad others) followed by hybrid OO with non-encapsulated classes
   >   (eg. Ada, CLOS?).
   Of course, I do not agree. For example, the presence of ONCE functions in
   Eiffel is a kludge to overcome the absence of package initialization that is
   nicely provided by Ada. And as for separating specs and implementations, Ada
   is the only language that does it nicely, without giving up safety.


Uhhh... since when has "encapsulated abstractions" been defined as limited 
to classes?  "Classes" are a way to encapsulate all the operations that are
associated with an object type.  "Interfaces" (where classes are really a
more restricted form of these) encapsulate all the operations that are
associated with the types visible to that specification -- there isn't
the single type restriction.

I also obviously do not agree with the explanation for (1), but I also
have to disagree with the statement that Ada is the only language that
separates specifications from their implementations, in a nice and
safe manner.  Modula-3 has a similar interface/implementation
separation, which is just as nice and equally as safe.  (It actually
resolves other safety issues not related to pure OO programming, but
that isn't what is being discussed here)

I think Ada has a powerful encapsulation system, but it is not the
only language with one.

-Spencer

----------------------------------------------------------------------
Spencer Allain                    E-mail:  spencer@era.com
Engineering Research Associates   Phone :  (703) 734-8800 x1414
1595 Spring Hill Road             Fax   :  (703) 827-9411
Vienna, VA  22182-2235

Ask me why _NOT_ to buy anything from Royal Prestige, a Hycite Co.
----------------------------------------------------------------------
<A HREF=http://www.research.digital.com/SRC/modula-3/html/home.html>
             Modula-3 Home Page DEC SRC</A>
<A HREF=http://www.vlsi.polymtl.ca/m3/>Modula-3 FAQ, etc. </A>
----------------------------------------------------------------------

Re: Real OO (was Choice of OO primitives in Ada95)

[Jon S Anthony]

In article <4git9e$cqi@stc06.ctd.ornl.gov> kennel@msr.epm.ornl.gov (Matt Kennel) writes:

> > Of course, I do not agree. For example, the presence of ONCE functions in
> > Eiffel is a kludge to overcome the absence of package initialization that
> > is nicely provided by Ada.
> 
> Um maaaaybe, but 'once functions' do what they do with no fuss and with
> very clear semantics, without having to define an "elaboration" notion for
> packages. 
> 
> I'm not sure that's a "kludge" compared to the complexity of package
> initialization in Ada. 

I would agree that "once functions" make sense in the Eiffel design
pattern and are not really a kludge.  Ada elaboration, of course, fits
its design pattern.  In this particular case, there really isn't much
difference between them in the effects that can be accomplished.  Ada
elaboration provides for a few more things but that is more due to the
fact that packages are not types.


> >  And as for separating specs and implementations, Ada
> > is the only language that does it nicely, without giving up safety.
> 
> This isn't true.

Correct, M3 (and M2) for example provide this as well.


> You write abstract classes in Eiffel/Sather/C++ which has the spec
> (including pre/post/invariant) and write an implementation class which
> supports it. 
> 
> What is the problem?

I don't know.  But it is not the same.  You still have the interface
of the so called "implementation" class which is inextricably hooked
to _its_ implementation.  The fact is that Meyer (for various reasons)
decided _against_ separation of interface and implementation.  So, if
you are going to argue about this, you should be arguing about whether
this choice was "a good idea" (and not pretending that it isn't true).


> No.  Meyer figured out, quite successfully IMHO, exactly the kinds of
> rules that you need to turn "assertions" from just 'check this predicate
> and bomb if false' into a powerful *human* protocol "programming by
> contract". 

Well, more or less.  To me the real important thing about Eiffel
assertions is that they "go in the right place".  For example, it
is up to the client (enforced via assertions) that the pre-conditions
of a "server" are satisfied.


> These special rules are exactly what you need so you CAN trust assertions,
> because it means that you can put in truly useful checks that don't barf
> because of stupidities like the above. 

Well, more or less.


> I think it *is* quite fair to say that Eiffel is more object-oriented---ok
> more oriented-around-objects than Ada 95: it's the idee' fixe for Eiffel. 

From the standpoint of the 'everything is an "object"'
fanat-ahhh-ideology this is clearly true.  Actually, it is even more
specific than this, viz, all the world's a class.  I'm not
particularly enamored of this point of view.  For one thing it is not
obvious what the statement even says, if for no other reason than the
fact the central concept is so ill defined as to be meaningless.  Of
course, this sort of fact has never bothered programmers before - and
probably never will.


> This doesn't prove 'prima facie' superiority, though I tend to like
> Eiffel better.  (I've never been convinced about kludges to handle the 
> 'multiple withing' problem).

Check.  As for the "withing" problem, well it is clearly an easy and
legitimate target.

/Jon
-- 
Jon Anthony
Organon Motives, Inc.
1 Williston Road, Suite 4
Belmont, MA 02178

617.484.3383
jsa@organon.com

Re: Real OO (was Choice of OO primitives in Ada95)

[Robert A Duff]

In article <4git9e$cqi@stc06.ctd.ornl.gov>,
Matt Kennel <kennel@msr.epm.ornl.gov> wrote:
>Um maaaaybe, but 'once functions' do what they do with no fuss and with
>very clear semantics, without having to define an "elaboration" notion for
>packages. 
>
>I'm not sure that's a "kludge" compared to the complexity of package
>initialization in Ada. 

I'd say both mechanisms leave something to be desired.

>I think it *is* quite fair to say that Eiffel is more object-oriented---ok
>more oriented-around-objects than Ada 95: it's the idee' fixe for Eiffel. 

Yes, of course Eiffel is more object oriented than Ada.  (Not sure why
you add "-around-objects".)  This is because Ada has many features that
are not object-oriented at all.  There's no support, for example, for
polymorphic operations on tasks or arrays.  But this has nothing to do
with the package-vs-class issue.  The features of Ada that *do* support
OOP are just as OOP as anything else.

>This doesn't prove 'prima facie' superiority, though I tend to like
>Eiffel better.  (I've never been convinced about kludges to handle the 
>'multiple withing' problem).

What is the "multiple withing" problem?

- Bob

Re: Real OO (was Choice of OO primitives in Ada95)

[Gene Ouye]

You realize, of course, that to the outside world (more 
specifically, to many that I know in the C++ community), 
watching this arguing between Eiffel and Ada advocates is 
like watching two hyenas fighting over the last scraps of 
a carcass after the pride of lions has had its fill...

You also realize, of course, that I'm not trying to say 
that any of this discussion is futile (if I ever find that 
to be true, I will be very depressed!).

Gene Ouye <geneo@rational.com>

Re: Real OO (was Choice of OO primitives in Ada95)

[Robert A Duff]

In article <yrgg2c3dmz1.fsf@borniche.rennes.enst-bretagne.fr>,
Valery Croizier <croizier@rennes.enst-bretagne.fr> wrote:
>If you can simulate Eiffel-like assertions in a trivial way, then
>please show it.

Yes, please do!

>... I know how to check arguments and raise an exception,
>but not so that these checkings are inherited by children, like in Eiffel. 

More importantly, how do you do class invariants in Ada?
The only way I know of, is to put some code in *every*
procedure, which is a pain.

I think that Eiffel's assertions are a pretty nice feature,
especially class invariants, which are hard to do by hand.

- Bob

Re: Real OO (was Choice of OO primitives in Ada95)

[Robert A Duff]

In article <Dn5qGH.9xH@assip.csasyd.oz>,
Don Harrison <donh@syd.csa.com.au> wrote:
>... (I was asking for trouble debating an issue without
>the reference material handy).

:-) :-) :-) 

>Note that Ada only fully satisfies 2) and has a sub-optimal mechanism for 1).

You've said elsewhere that you don't like Ada's support for 1).  I
suppose it's something of a matter of taste.  I happen to think that
Ada's support for 1) is *better* than Eiffel, and the other languages
that combine Ada's notion of package and type into one.

You seemed to object strongly to the idea that one can put two types in
the same package, on the grounds that it decreases encapsulation.  On
the contrary, it *increases* encapsulation.  When two types are closely
related to each other, by their nature, *forcing* them to be in separate
modules means that *more* operations need to be exported from each
module.

Encapsulation isn't some sort of Goodness, such that the more you have
of it, the better.  No, you want to encapsulate things at the "right"
level of granularity for the problem at hand.  Usually, that's a single
type, plus the operations belonging to that type.  However, in some
cases, two types are so closely related that the right granularity is to
put them both in the same package.  (After all, in Fortran 66, the only
module is the subroutine, which is a *smaller* granularity than the
class.  But that doesn't make the encapsulation better.)

An example is a List type, plus a List_Cursor type.  The List_Cursor
points at a certain place in the list, and has operations for moving
forward and/or backward and so forth.  These operations cannot be
written without visibility on implementation details of *both* types.
So if class=module, you have to *export* a lot of those implemenation
details from one of them, so the other one can do what it needs to do.
Putting them both in the same module, however, actually gives you
*better* encapsulation, because fewer (and higher-level) operations get
exported.

Last time I looked at Eiffel, the standard library classes put the
cursor movement stuff inside the list.  That seems like a poor design,
since you can only be looping through a given list once, at any given
time.  It seems to me that you badly want the List_Cursor type to be
separate from the List type, although they *are* part of the same
"module".

>While Eiffel is strong in the OO stakes (it satifies all 3), 

I agree that Eiffel is superior to Ada with respect to your number 3) --
that is, the assertion stuff.  Ada does have *some* support for
assertions.  For example, if I say, "type T is range 1..10;", then
there's an invariant on T -- all objects of type T have to be in the
range 1 to 10.  This is pretty weak, compared to Eiffel, though.

I think it's silly to call that part of OO, though.  It's a completely
orthogonal issue.  Assertions would be useful in a non-OO language, too.

If you define "OO" as being "anything I happen to think is good", then
the term becomes useless, and people end up arguing about how, "My Daddy
is more OO than your Daddy."

Meyer also said that garbage collection is a fundamental part of OO.
Nonsense.  GC existed before OO, and it's not necessary in order to
support the real fundamentals of OO, such as polymorphism.

Why not just say that GC is a Good Thing, or assertions are a Good
Thing, or Eiffel's assertions are better than Ada's assertions, or C++
sucks because it makes it hard to implement GC?  It's silly to use the
term "OO" as a synonym for "the set of all Good Things".

>...it is weak
>(unfortunately) in the RT arena. These weaknesses are:
>
>a) poor low level facilities (bit fiddling)
>b) GC renders it an inappropriate tool for hard real-time systems (although the
>   emergence of mainstream parallel architectures will permit this)

Some people believe quite strongly that GC is appropriate for hard
real-time systems.  But that's another thread.

>c) no concurrency (an elegant model currently being implemented - not yet part
>   of the standard).

So I've heard.  How does this new elegant model deal with the issue I
mentioned above, about putting list-cursor information inside the list
itself?  If you have multiple tasks, you definitely want to have two
tasks iterating through the same shared list (or whatever data
structure).  Has the standard class library been changed to solve this
problem?

>I think these will all be addressed in time but if none are a concern to you,
>I suggest taking a look at Eiffel.

Indeed.  Even if they *are* a concern, it's still instructive to learn
Eiffel, and to read Meyer's classic book on OOP.  Programming by
contract is a useful concept in any language.

- Bob

Re: Real OO (was Choice of OO primitives in Ada95)

[Valery Croizier]

In article <Dn9BzB.IMA@world.std.com> bobduff@world.std.com (Robert A Duff) writes:

> You seemed to object strongly to the idea that one can put two types in
> the same package, on the grounds that it decreases encapsulation.  On
> the contrary, it *increases* encapsulation.  When two types are closely
> related to each other, by their nature, *forcing* them to be in separate
> modules means that *more* operations need to be exported from each
> module.
> [...]
> An example is a List type, plus a List_Cursor type.  The List_Cursor
> points at a certain place in the list, and has operations for moving
> forward and/or backward and so forth.  These operations cannot be
> written without visibility on implementation details of *both* types.
> So if class=module, you have to *export* a lot of those implemenation
> details from one of them, so the other one can do what it needs to do.
> Putting them both in the same module, however, actually gives you
> *better* encapsulation, because fewer (and higher-level) operations get
> exported.

Yes, you have to export internals to the other class, but
encapsulation is not weaker, because you can do a selective export, so
that no one (except List_Cursor) can see List's internals.
Putting two types in the same module weakens encapsulation, because it
exports : 
	o *all* internals 
	o to *each other* types 
in that module, while Eiffel allowes the programmer to export only
useful data to classes that need them.
For example, type Person has a field Disease.  Only a Doctor can see
this field, because only doctors are able to identify diseases.
You can't put Person and Doctor in the same package, because :
	o Doctor would see *all* private data of Person
and, that is worse,
	o Person would see all private data of *Doctor*.
Ada's solution works fine with the List_Iterator issue, because
List and List_Cursor aren't complex structures.

> [...]
> So I've heard.  How does this new elegant model deal with the issue I
> mentioned above, about putting list-cursor information inside the list
> itself?  If you have multiple tasks, you definitely want to have two
> tasks iterating through the same shared list (or whatever data
> structure).  Has the standard class library been changed to solve this
> problem?

The issue have little to do with tasking. Even in a sequential
program, a List can have several clients.
As far as Eiffel Libraries are concerned, I don't know them well, but
I've just ckecked ISE's and Tower's Web site, and there is no list in
the Library Kernel.  Of course, vendors provide their own librairies,
and the most prominent ones have traversable lists (at least Sig and
Tower have).

--
Valery CROIZIER

Re: Real OO (was Choice of OO primitives in Ada95)

[Gene Ouye]

James O'Connor wrote:
> You also realize that to the Smalltalk world, the argument is interesting 
> because, from a Smalltalk perspective, Ada95 and C++ have much more in 
> common than the do differeneces...

True, but most of the arguing in this thread(s) was over Eiffel vs Ada.  
I actually found it refreshing after the endless "C/C++ sucks" threads,
Unfortunately, I've gained most of my Smalltalk knowledge from the 
endless "C++ is not (or, Smalltalk is) a pure OO language" threads on 
comp.object.  Maybe we could use more Smalltalk vs. Ada arguments in 
this group...
                       ;-)

Gene Ouye <geneo@rational.com>

Re: Real OO (was Choice of OO primitives in Ada95)

[Don Harrison]

Thanks to numerous respondents for your input on this thread. Hope to reply within
the next couple of days.

No, it is not a troll. Nor is any other I have contributed to.

Don.

Re: Real OO (was Choice of OO primitives in Ada95)

[James O'Connor]

Gene Ouye wrote:
> 
> You realize, of course, that to the outside world (more
> specifically, to many that I know in the C++ community),
> watching this arguing between Eiffel and Ada advocates is
> like watching two hyenas fighting over the last scraps of
> a carcass after the pride of lions has had its fill...
> 
> You also realize, of course, that I'm not trying to say
> that any of this discussion is futile (if I ever find that
> to be true, I will be very depressed!).
> 
> Gene Ouye <geneo@rational.com>

You also realize that to the Smalltalk world, the argument is interesting 
because, from a Smalltalk perspective, Ada95 and C++ have much more in 
common than the do differeneces...

James O'Connor
oconnor@apci.net

Re: Real OO (was Choice of OO primitives in Ada95)

[Matthew B. Kennel]

Robert A Duff (bobduff@world.std.com) wrote:

: Encapsulation isn't some sort of Goodness, such that the more you have
: of it, the better.  No, you want to encapsulate things at the "right"
: level of granularity for the problem at hand.

I definitely agree with this. 


: An example is a List type, plus a List_Cursor type.  The List_Cursor
: points at a certain place in the list, and has operations for moving
: forward and/or backward and so forth.  These operations cannot be
: written without visibility on implementation details of *both* types.
: So if class=module, you have to *export* a lot of those implemenation
: details from one of them, so the other one can do what it needs to do.
: Putting them both in the same module, however, actually gives you
: *better* encapsulation, because fewer (and higher-level) operations get
: exported.

Of course, Eiffel has selective export, where individual features or sets
of features can be mutually exported and imported.   

Ada gives this set a name.  Better?  Perhaps. 

Eiffel can express notions of "who gets to see what" which are not easily
compatible with strictly hierarchical packages.   (Consider three
classes, A B and C with A sharing some routines with B and some with C, but
B and C sharing nothing. )

: Last time I looked at
Eiffel, the standard library classes put the : cursor movement stuff inside
the list.  That seems like a poor design, : since you can only be looping
through a given list once, at any given : time.  It seems to me that you
badly want the List_Cursor type to be : separate from the List type,
although they *are* part of the same : "module".

I might agree, but there's always the question of efficiency.  Most of the
time you're looping through one list once, needing to allocate extra
objects may be expensive.

What I really want, of course, is Sather's iterators which totally solves
the problem.  :-) 

: >While Eiffel is strong in the OO stakes (it satifies all 3), 

: I agree that Eiffel is superior to Ada with respect to your number 3) --
: that is, the assertion stuff.  Ada does have *some* support for
: assertions.  For example, if I say, "type T is range 1..10;", then
: there's an invariant on T -- all objects of type T have to be in the
: range 1 to 10.  This is pretty weak, compared to Eiffel, though.

: I think it's silly to call that part of OO, though.  It's a completely
: orthogonal issue.  Assertions would be useful in a non-OO language, too.

I wouldn't call it completely orthogonal.  A major point of polymorphic
object oriented programming, and specificially "design by contract" is
validating "who is substitutable for whom here?".  Eiffel expresses this
notion very directly in the combined type AND assertion system, as
descendents must be compatible both in type and assertion set with their
ancestors. 

I'm not a master of Ada, but it seems from reading Ada programs that
this information is not quite as readily obvious. 

: Meyer also said that garbage collection is a fundamental part of OO.
: Nonsense.  GC existed before OO, and it's not necessary in order to
: support the real fundamentals of OO, such as polymorphism.

: Why not just say that GC is a Good Thing, or assertions are a Good
: Thing, or Eiffel's assertions are better than Ada's assertions, or C++
: sucks because it makes it hard to implement GC?  It's silly to use the
: term "OO" as a synonym for "the set of all Good Things".

No argument.  I wait for the day whan "object orientation" is as much
an obvious (and uninteresting) prerequisite as "uses Unicode character set
in parsing". 

: >I think these will all be addressed in time but if none are a concern to you,
: >I suggest taking a look at Eiffel.

: Indeed.  Even if they *are* a concern, it's still instructive to learn
: Eiffel, and to read Meyer's classic book on OOP.  Programming by
: contract is a useful concept in any language.

: - Bob

Re: Real OO (was Choice of OO primitives in Ada95)

[Don Harrison]

Robert A Duff writes:

:In article <DnKnsz.C8C@assip.csasyd.oz>,
:Don Harrison <donh@syd.csa.com.au> wrote:
:>The only 'cost' is the effort of writing them in the first place but the benefit
:>is enormous because that correctness is propagated through reuse. When assertions
:>are used, there is nothing to fear from inheritance and no reason why all
:>abstractions should not be candidates for reuse by inheritance.
:
:Well, assertions are a nice feature of Eiffel, but they don't
:*guarantee* correctness.  For example, there are some things that can't
:be formalized, like "this procedure prints an error message that is
:easily understood by any competent user".  That's why Eiffel allows some
:assertions to be merely comments, and of course if it's just a comment,
:you can't know for sure that it's obeyed, unless you look at every
:overriding.

Granted. Not everything can be formalised. Of course, it's clearly valuable to 
exploit those aspects of a design that may be formalised.

:>In contrast, assertion-less inheritance propagates errors through reuse.
:>
:>'Frozen' is used when the designer knows the implementation will not change.
:>
:>: Of course, the counter-argument is that you don't always know ahead of
:>: time when you might need dispatching.
:>
:>Exactly.
:>
:>: I'm not sure I agree with Ray Toal, though, that requiring "tagged" is a
:>: Good Thing.  Even without that rule, Ada still gives you control over
:>: whether things dispatch, on a call-by-call basis.  So, I view "tagged"
:>: as simply an efficiency hack.  Efficiency is a Good Thing, too, though.
:>
:>Efficiency hack is a good way of describing it. 
:
:Would you say "frozen" is (merely) an efficiency hack?  I view it as
:partly an efficiency hack, and partly an aid to understanding the
:program.

The two are different in a couple of ways:

a) purpose - 'tagged' concerns dispatching whereas 'frozen' concerns redefinition

b) syntactic necessity
   - 'tagged' is not necessary if the language allows dispatching by default. 
   It is necessary, if dispatching is optional, as in Ada for the purpose of 
   efficiency.
   [I happen to think dispatching by default would be better because the human
   benefits of flexibility and simplicity would outweigh the cost of bigger and 
   better hardware].
 
   - 'frozen' is necessary to indicate whether an operation may be redefined 
   (which is not related to efficiency).

:- Bob

Don.

Re: Real OO (was Choice of OO primitives in Ada95)

[Don Harrison]

In article <Do7unJ.1sq@world.std.com>, bobduff@world.std.com (Robert A Duff) writes:
:In article <Dns8pv.BDx@assip.csasyd.oz>,
:Don Harrison <donh@syd.csa.com.au> wrote:
:>Various arguments have been advanced to justify co-encapsulation. Below is a
:>collective response.
:>
:>1) Co-encapsulation preserves the symmetry of operands.
:>
:>As indicated in a previous post, the requirements of polymorphism demand that
:>operands are intrinsically asymmetric which suggests that uni-encapsulation is
:>more appropriate.
:
:Correct when there are two tagged types involved (unless you have
:multi-methods, as in CLOS).
:
:However, the symmetry is *not* lost when you have something like:
:
:    procedure Union(X, Y: Set) return Set;
:
:In other OO languages, the parameter X is the special one, and symmetry
:is lost.  Not a big deal, I admit, but I find it slightly cleaner,
:conceptually, to consider X and Y to be on equal footing.

I agree this visually more symmetrical. Of course, the operation still 
semantically belongs to tagged type Set - it is a primitive operation of it.

:
:- Bob
:

Don.