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Re: Jonathan Rees on OO
Why Proto has GOO (Generic Object-Orientation)?
This message is a response to Paul Graham's: "Why Arc Isn't Especially
Object-Oriented". I humbly submit this message at the risk of
creating a raging debate on par with the syntax wars. I also
acknowledge that GOO isn't the most conventional style of
object-orientation as recently detailed by Jonathan Rees. But first...
What is GOO?
The salient aspects of GOO are represented by Dylan and Proto's (soon
to be released) object system and comprise such qualities as: (1)
objects all the way down, (2) class heterarchy, and (3) multimethod
dispatch. Furthermore, GOO avoids being object-centric by providing
powerful functional programming facilities that allow users to ignore
the better portion of the object system if they choose and to instead
program in a functional style. In other words, the base Proto
provides a substrate upon which to write Scheme-style programs.
My overriding contention is that these GOO qualities are already
latent in Scheme and by bringing them to the surface we exact certain
benefits. Taking the various qualities one by one, Scheme already has
self identification of objects as the most basic level (e.g.,
integer?), has a class hierarchy or abstract data types organized into
a tower of subtypes (e.g., number, integer), and has protocols and
arguably rudimentary multimethod dispatch (e.g., +). Unfortunately,
there is no easy way to harness this latent power.
Equal rights for users
It has always been a language design principle of mine that anything
that's good for the language core libraries will most likely be good
for users. Now it is not always possible for designers to provide
mechanisms such that user libraries can look like system libraries,
but i maintain that such mechanisms will not only provide the sort of
advantages that recommended these designs to language designers in the
first place but will also make user code interoperate more seamlessly
with system code.
Of course Scheme could be written in terms of typecase expressions,
writing for example + as a big ole cascaded type case. Unfortunately,
it is my belief that this is an awkward definition of + that fails on
many counts, the first being that it does not allow for a factoring of
code into natural categories and the second being that it creates
Programmers and designers can not possibly think of all requirements
on a language or library up front, so it is crucial to design for
changing requirements. Guy Steele effectively articulated this point
in his 1998 OOPSLA invited lecture entitled "Growing a Language":
saying that it is potentially more (or at least equally) important
that language designers provide extensibility mechanisms than to think
of all possible features that might be used. Guy has said most of
what I'm saying here, but the point is that it is my contention that
GOO provides an excellent substrate with which to grow a language and
to write libraries.
Now going back to the running example of +, if Scheme implementers
were to write + as a moby typecase, it would be difficult for Scheme
users to extend + to handle new types of numbers, for instance. By
writing + as a generic function and a bunch of builtin methods, the
implementor has written + in such a way as to allow users to extend +
on equal footing.
Now say that despite what I've said up to this point, you think that
GOO is a fad and you really just want to provide a lambda-based
substrate like Scheme upon which users can write their own GOO.
This is certainly a worthy goal, but unfortunately, the sad fact is
that there are no Scheme implementations available that can do this
without exacting a cost.
I also contend that implementing GOO efficiently and writing the
system code in GOO forces a certain efficiency equality, that is, user
code can run as fast as system code. In other words, most of the
effort spent in bumming the GOO for system code will pay off user code
as well. The usual Scheme implementation trickery provides an unfair
advantage to system code.
GOO provides an elegant framework with which to level the playing
field giving users the power to define libraries in the spirit of the
core libraries. Furthermore, users' libraries and even the core
libraries can be extended without losing this original spirit. At the
very least, GOO is a way to describe what is already latent in Scheme.
Scheme would be an even smaller gem if it were entirely redefined in
GOO style. For example, many of the type-specific functions would be
replaced by protocols. In particular, integer? and number? would be
replaced with isa? and vector-ref and string-ref would be replaced by
ref. I'm not sure how to measure this shrinkage, but to my mind, it
would impose a certain structure and consistency that would make the
overall language and libraries simpler. Of course, there would be a
small increase in complexity by adding GOO itself but I think that
this would be a well spent one time cost.
This self-representation advantage is only the beginning. For
example, GOO protocols provide a mechanism to break free of the "use
lists for everything" (ULFE) mentality that defeats the advantages of
abstraction. Lisp and Scheme generally tends to encourage this
practice because it is inordinately difficult to define and manipulate
new abstract data types, certainly more difficult to do so than to
just use a list. In Proto, new collection classes can be easily
created by merely implementing a simple collection protocol. Paul
Graham seems to argue (perhaps in a tongue in cheek manner) that
creating new classes is busy work for mediocre programmers. I find
the whole ULFE philosophy to be not too different from using raw
integers for symbols or arrays for records (or objects). Sure you
might get short term wins by avoiding the "busy work" of defining new
data types, but don't you then create an unmaintainable code base.
Even SICP encourages the creation of abstact data types. Why not make
that even easier with GOO?
For more information on GOO, consult the Dylan Reference Manual:
Proto and Dylan both stand on the shoulders of several giants