Dr. Alan Kay on the Meaning of “Object-Oriented Programming”
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E-Mail of 2003-07-23
Dr. Alan Kay was so kind as to answer my questions about the term “object-oriented programming”.
Clarification of "object-oriented" [E-Mail]
Date: Wed, 23 Jul 2003 09:33:31 -0800
To: Stefan Ram [removed for privacy]
From: Alan Kay [removed for privacy]
Subject: Re: Clarification of "object-oriented"
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Hi Stefan --
Sorry for the delay but I was on vacation.
At 6:27 PM +0200 7/17/03, Stefan Ram wrote:
> Dear Dr. Kay,
>
> I would like to have some authoritative word on the term
> "object-oriented programming" for my tutorial page on the
> subject. The only two sources I consider to be "authoritative"
> are the International Standards Organization, which defines
> "object-oriented" in "ISO/IEC 2382-15", and you, because,
> as they say, you have coined that term.
I'm pretty sure I did.
>
> Unfortunately, it is difficult to find a web page or source
> with your definition or description of that term. There are
> several reports about what you might have said in this regard
> (like "inheritance, polymorphism and encapsulation"), but
> these are not first-hand sources. I am also aware that later
> you put more emphasis on "messaging" - but I still would like
> to know about "object oriented".
>
> For the records, my tutorial page, and further distribution
> and publication could you please explain:
>
> When and where was the term "object-oriented" used first?
At Utah sometime after Nov 66 when, influenced by Sketchpad, Simula,
the design for the ARPAnet, the Burroughs B5000, and my background in
Biology and Mathematics, I thought of an architecture for
programming. It was probably in 1967 when someone asked me what I was
doing, and I said: "It's object-oriented programming".
The original conception of it had the following parts.
- I thought of objects being like biological cells and/or individual
computers on a network, only able to communicate with messages (so
messaging came at the very beginning -- it took a while to see how to
do messaging in a programming language efficiently enough to be
useful).
- I wanted to get rid of data. The B5000 almost did this via its
almost unbelievable HW architecture. I realized that the
cell/whole-computer metaphor would get rid of data, and that "<-"
would be just another message token (it took me quite a while to
think this out because I really thought of all these symbols as names
for functions and procedures.
- My math background made me realize that each object could have
several algebras associated with it, and there could be families of
these, and that these would be very very useful. The term
"polymorphism" was imposed much later (I think by Peter Wegner) and
it isn't quite valid, since it really comes from the nomenclature of
functions, and I wanted quite a bit more than functions. I made up a
term "genericity" for dealing with generic behaviors in a
quasi-algebraic form.
- I didn't like the way Simula I or Simula 67 did inheritance
(though I thought Nygaard and Dahl were just tremendous thinkers and
designers). So I decided to leave out inheritance as a built-in
feature until I understood it better.
My original experiments with this architecture were done using a
model I adapted from van Wijngaarten's and Wirth's "Generalization of
Algol" and Wirth's Euler. Both of these were rather LISP-like but
with a more conventional readable syntax. I didn't understand the
monster LISP idea of tangible metalanguage then, but got kind of
close with ideas about extensible languages draw from various
sources, including Irons' IMP.
The second phase of this was to finally understand LISP and then
using this understanding to make much nicer and smaller and more
powerful and more late bound understructures. Dave Fisher's thesis
was done in "McCarthy" style and his ideas about extensible control
structures were very helpful. Another big influence at this time was
Carl Hewitt's PLANNER (which has never gotten the recognition it
deserves, given how well and how earlier it was able to anticipate
Prolog).
The original Smalltalk at Xerox PARC came out of the above. The
subsequent Smalltalk's are complained about in the end of the History
chapter: they backslid towards Simula and did not replace the
extension mechanisms with safer ones that were anywhere near as
useful.
>
> What does "object-oriented [programming]" mean to you?
> (No tutorial-like introduction is needed, just a short
> explanation [like "programming with inheritance,
> polymorphism and encapsulation"] in terms of other concepts
> for a reader familiar with them, if possible. Also, it is
> not neccessary to explain "object", because I already have
> sources with your explanation of "object" from
> "Early History of Smalltalk".)
(I'm not against types, but I don't know of any type systems that
aren't a complete pain, so I still like dynamic typing.)
OOP to me means only messaging, local retention and protection and
hiding of state-process, and extreme late-binding of all things. It
can be done in Smalltalk and in LISP. There are possibly other
systems in which this is possible, but I'm not aware of them.
Cheers,
Alan
>
> Thank you,
>
> Stefan Ram
--
E-Mail of 2003-07-26
Clarification of "object-oriented", 1 [E-Mail]
Date: Sat, 26 Jul 2003 13:47:59 -0800
To: Stefan Ram [removed for privacy]
From: Alan Kay [removed for privacy]
Subject: Re: Clarification of "object-oriented"
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One of the things I should have mentioned is that there were two main
paths that were catalysed by Simula. The early one (just by accident)
was the bio/net non-data-procedure route that I took. The other one,
which came a little later as an object of study was abstract data
types, and this got much more play.
If we look at the whole history, we see that the proto-OOP stuff
started with ADT, had a little fork towards what I called "objects"
-- that led to Smalltalk, etc.,-- but after the little fork, the CS
establishment pretty much did ADT and wanted to stick with the
data-procedure paradigm. Historically, it's worth looking at the USAF
Burroughs 220 file system (that I described in the Smalltalk
history), the early work of Doug Ross at MIT (AED and earlier) in
which he advocated embedding procedure pointers in data structures,
Sketchpad (which had full polymorphism -- where e.g. the same offset
in its data structure meant "display" and there would be a pointer to
the appropriate routine for the type of object that structure
represented, etc., and the Burroughs B5000, whose program reference
tables were true "big objects" and contained pointers to both "data"
and "procedures" but could often do the right thing if it was trying
to go after data and found a procedure pointer. And the very first
problems I solved with my early Utah stuff was the "disappearing of
data" using only methods and objects. At the end of the 60s (I think)
Bob Balzer wrote a pretty nifty paper called "Dataless Programming",
and shortly thereafter John Reynolds wrote an equally nifty paper
"Gedanken" (in 1970 I think) in which he showed that using the lamda
expressions the right way would allow data to be abstracted by
procedures.
The people who liked objects as non-data were smaller in number, and
included myself, Carl Hewitt, Dave Reed and a few others -- pretty
much all of this group were from the ARPA community and were involved
in one way or another with the design of ARPAnet->Internet in which
the basic unit of computation was a whole computer. But just to show
how stubbornly an idea can hang on, all through the seventies and
eighties, there were many people who tried to get by with "Remote
Procedure Call" instead of thinking about objects and messages. Sic
transit gloria mundi.
Cheers,
Alan
At 10:05 PM +0200 7/26/03, Stefan Ram wrote:
>On Wed, Jul 23, 2003 at 09:33:31AM -0800, Alan Kay wrote:
>> OOP to me means only messaging, local retention and protection and
>> hiding of state-process, and extreme late-binding of all things.
>
> Hi Alan,
>
> I just want to say "thank you" for your explanations
> (including the parts not quoted above)!
>
> "local retention" is a new notion to me in the context
> of OOP, and I assume it refers to state-process and
> means that an object is in possession of its state-process,
> so that the state of an object is kept locally with the
> object and not elsewhere.
>
> I have published your reply on the web, but have removed
> the E-Mail addresses and similar header lines for privacy.
>
> Thanks again,
>
> Stefan
--
Remarks
A memorandum on dataless programming:
http://www.rand.org/pubs/research_memoranda/2007/RM5290.pdf
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