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From: bertrand@eiffel.UUCP (Bertrand Meyer)
Newsgroups: comp.lang.eiffel,comp.object
Subject: Re: Inheritance and Information Hiding
Message-ID: <498@eiffel.UUCP>
Date: 9 Feb 91 23:39:29 GMT
References: <1991Feb8.171341.19413@cs.tcd.ie> <2140@cs.rit.edu>
Organization: Interactive Software Engineering, Santa Barbara CA
Lines: 54

** Note: As Markku Sakkinen requested before, it would be nice
if everyone left the original groups in the Newsgroups line.
Someone, for some reason, had removed comp.lang.eiffel. 
I have put it back. ** end of note.

This is simply to point out that the issue of ``implementation
reuse'' versus ``specification reuse'' has nothing to do
with this whole discussion. The debate (e.g. my original
questions regarding polygons and rectangles) can be
entirely confined, if need be, to the purely mathematical
notion of abstract data types.  We can talk about inheritance
relations as being morphisms in categories, and we don't even
need to refer to computers.

I should add that ``implementation reuse'' is, for
me, a term devoid both of meaning and of interest.
The separation between concept and implementation in software is
an historical evil, not a desirable situation. With a good notation we
can remove the difference. For one thing, this is clearly the
goal of Eiffel. But of course the human tendency to make
things difficult when they should be simple is eternal.

I also know that there is very little chance that anyone
beyond Eiffel users will accept this today. It will
take several years; too many people have been
taught that implementation is dirty and ``specification''
or ``design'' is good. (See the lengthy letter in the January
Communications of the ACM, by a gentleman from the US Coast
Guard, claiming that he represents the trenches of the computing
profession and, among other things, that software engineering has
nothing to do with implementation.)

Of course if one's world is an environment where implementation
is done in Cobol or C, the need for something at a higher level
is obvious. But this is irrelevant to object-oriented
technology, or software engineering for that matter. 

Somewhere in his essays on linguistics, Roman Jakobson
illustrates a point by telling the following story.

A missionary was blaming natives because they always
went everywhere naked. ``Cover your body!'', he would scream.
Once they asked back: ``But you too have an area
of uncovered flesh, right there, haven't you?''.
``Of course'', said the Father, ``but that's different! It's my face''. 
``Well then'', retorted the natives, ``with us, everything is the face''.

I like to think of this as a guide for software construction.
One day we will stop being ashamed of implementation,
not because we will have removed the need for implementation
but for exactly the opposite reason: with us, everything
is the implementation.

-- Bertrand Meyer
bertrand@eiffel.uucp