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From: kers@hplb.hpl.hp.com (Chris Dollin)
Newsgroups: comp.object
Subject: Re: The Emperor Strikes Back
Message-ID: <KERS.91Feb22083952@cdollin.hpl.hp.com>
Date: 22 Feb 91 08:39:52 GMT
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In-Reply-To: steve@cs.qmw.ac.uk's message of 15 Feb 91 20:50:37 GMT
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Steve Cook writes a vigerous rebuttal of Scott Guthery's article (alas, I have
not read the original; like a *really* bad book, I might have enjoyed it). I
wish to respond to two points. Steve says:

   What is an object?

   According to Guthery, 'stripped of its fancy jargon, an object is a
   lexically-scoped subroutine with multiple entry points and persistent
   state'.  If that's not fancy jargon, I don't know what is; but let's have a
   closer look at it.
   The killer is 'lexically-scoped'.  It's a while since I heard the phrase, so
   I checked it in Abelson and Sussman's book, 'Structure and Interpretation of
   Computer Programs'.  According to them, lexical scoping means that free
   variables in procedures are statically bound to variables in
   textually-enclosing procedures, just like free variables in nested
   procedures in Pascal.  But the most fundamental idea of object-oriented
   programming is dynamic  binding of operation names to operations at
   run-time.  Lexical scoping has nothing whatever to do with objects.  Can
   somebody who's failed so completely to grasp the basic idea be trusted to
   criticise the whole technology?

Guthery is "right" (ie, that is indeed one way of regarding an object; I'm not
sure that it says anything new). I think Guthery is thinking of an objectish
implementation in a Lisp-like (actually this example is Pop-like, but the
principle's the same) language where we might say:

    define new_object( f1, f2, f3 ) as
        procedure ( method ) as
	    if method == "m1" then m1_expression( f1, f2, f3 )
	    elseif method == "m2" then m2_expression( f1, f2, f3 )
	    ...
        endif
    enddefine

the m1_expression (etc) are the expressions for the method bodies. The
if-expression encodes "multiple entry points". The fi are the persistent state,
because they are lexically-scoped variables, and full lexical scope is used to
smuggle them into the procedure representing the object. An object is thus
represented as a procedure which, when applied to an method-name argument, does
"the appropriate thing".

I seem to recall that such a method is described in Abelson and Sussman. It's
nice that it can be done at all, but it's not as nice as supporting the object
orientation (whatever that means ...) directly in the language. I *don't* think
you can use it as evidence that Guthery has "failed so completely to grasp the
basic idea" of OOP. (You may, or course, have other evidence.)

Steve also says:

   Consequences of Persistent State

   It seems as if what Scott Guthery means by persistent state is what most
   people mean by data abstraction.   If he thinks that data abstraction is a
   semantic minefield to be avoided, he's missed out on the last 20 years of
   programming methodology.

Not having read the original, I can't tell if Guthrey really does mean "data
abstraction" when he says "persistent state". I presume he means that objects
have little pieces of state inside them (I sometimes call them "statelets")
which persist for the lifetime of the object and result (in reference-based
OOLs, in any case) in the possibility of lots of aliasing (I send a message to
O1, and some O2 gets changed as a result). This is a source of great power, but
it can be a source of great confusion too (witness, for example, the advocates
of functional languages, who are so worried by the non-transparency of states
that they design state-free languages [*1]). I'm not sure that I'd call it a
"semantic minefield", however.

[*1] I myself am an advocate of functional languages, when I havn;t got my
Pop11 hat on ...
--

Regards, Kers.      | "You're better off  not dreaming of  the things to come;
Caravan:            | Dreams  are always ending  far too soon."


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