An extended discussion of extensibility

From: H. v. L. <"H.>
Date: Mon, 11 Oct 2004 12:56:43 +0200

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Hello all participants,

having been off for a week I stumbled over all the interesting arguments
 from various sides.

To take it short, I am in line with Rene's arguing. Handling encapsulation
only on the module level really seems to be a big step back to the times
of Modula-2. And then I don't believe a procedure call in the style of
Module.Init(o) to be the same as o.Init. The former allows any object of
the correct type as argument (and it might well be the wrong object)
whereas the latter initializes exactly the calling object and can't - by
mischief - touch any other. This is a different degree of safety, isn't
it? (see Rene's node example)
To me encapsulation on the module level really is an advantage of
modularized languages over class based languages but it primarily serves
as enclosure for data and actions with a high degree of interdependence so
they can be intimate, a feature for example Java doesn't offer (inner
classes don't do, they are a construct reminding me of the queer inner
modules in Modula-2).
Encapsulation on the level of classes is more and as Rene pointed out
allows independent development, you only have to know the immediate
superclass (and the exporting module of course) for your specialized class.
And this brings me back to the chain of modules A, B, C and their
respective classes inheriting one from another and it brings me back to
EXTENSIBLE procedures.
I guess we agree that - on the pure Component Pascal level - EXTENSIBLE
RECORDs are unneccesary. The question was whether EXTENSIBLE procedures
are neccessary. My arguments on Init procedures were based on the chain of
initializations each base type needs and I didn't see how to avoid super
calls in this case until ...
I confess, my ears blushed!
Marc, many thanks for your marvellous reminder of upcalls (yes, I knew
about them but simply didn't think of it). I believe you are quite right,
it is in the responsibility of the lowest base class to provide for the
Init2 procedure(s) needed by extending classes and it is really bad
programming to export the Init procedure as EXTENSIBLE as I did; instead
implement only export of an otherwise ABSTRACT procedure like in your
example is the way to go. I always felt unhappy with implementation
inheritance which is used when doing supercalls but, as I said, I didn't
see the way to avoid it until now.
So with this the argument for EXTENSIBLE procedures vanished and they are
a matter of personal style as were EXTENSIBLE RECORDs and Fjodor is right,
both might - on the level of Component Pascal at least - be banned.
A final question remains, mainly concerning Oberon Micro Systems. In
Marc's example PROCEDURE (b: B) Init2- is exported (implement only) again,
the compiler enforces this. Nevertheless it seems silly because on one
hand the export mark signals it as implement only and it actually is the
implementation, it can't be anything else. Thus I would prefer not to be
forced to export it again, I should prefer to keep it private, it
unnessecarily burdens the interface (yes, I know, I can attribute it as
EXTENSIBLE - there it is again: EXTENSIBLE - and then export it again with
all consequences, but I just want it private).

Regards

Harro von Lavergne
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Received on Mon Oct 11 2004 - 12:56:43 UTC