Dr. Alan Kay explains when "object-oriented" was first used and what it means.

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Stefan Ram

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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