Re: Clean Object Class Design -- Circle/Ellipse

brangdon_at_cix.co.uk (Dave Harris) wrote in message news:<memo.20011001000009.35593A_at_brangdon.madasafish.com>...
> bbadour_at_golden.net (Bob Badour) wrote (abridged):
> > Ask him to point to any Smalltalk language definition that defines
> > "instance" as anything other than a variable which one can reference.
>
> The 1997 draft of the ANSI Smalltalk standard (which is the version of the
> standard I have to hand), says:
>
> A /variable/ is a computational entity that stores a single
> reference (the /value/ of the variable) to an object.
>
> [...] An /object/ is a computational entity which is capable of
> responding to a well defined set of messages. An object may
> also encapsulate some (possibly mutable) state.
>
> This is *not* saying an object is a variable.

"reference to an object" implies that "object" is variable. Values are self-identifying and one cannot reference a value. One can reference storage or memory that contains some representation of a value, but storage and memory are variable. That a variable is constrained to a single value for some period of time does not change its nature.

That's not to say that no such thing as an object value exists. Object value is an important concept just as object variable is an important concept.

If you had included a little more from the standard where it defines /message send/ and explains that "Each argument is a reference to an object", you would see that Smalltalk requires a variable in order to send a message. In other words, Smalltalk only pretends to have values and requires one to store a representation of a value in a variable prior to doing anything.

> (The ANSI standard does not define "instance". It uses "object" instead.
> Later it says that when an object does have state, it represents it with
> special kind of variables called "instance variables". In other words, an
> object is not a variable but an object can contain variables.)

How does one change part of something that cannot change?

> > An instance or variable can have the value 5. The value, however, is
> > not an instance.
>
> So sorry, but in Smalltalk the value 5 is an instance of the class
> SmallInteger.

Untrue. An instance of the class SmallInteger can contain a representation of the value 5, but the value 5 is not an instance of anything. I realize that Smalltalk confuses things by requiring variables for messaging and then pretending that variables are values.

> > One cannot reference a value. Values are self-identifying.
>
> Indeed 5 is self-identifying. We can send a message to the value 5 whether
> or not we have a variable referring to it.

Yes, I agree that one can have operations on values. These are, of course, different from update operations on variables. Have you not seen me state that previously? Of course, the Smalltalk standard requires all implementations to operate only on variables.

> > According to the definition of the Smalltalk language an instance is a
> > variable that one can reference.
>
> Again, so sorry, but my reading of the ANSI definition of Smalltalk says
> that an instance (or object) is something which responds to messages.

My reading of the same says that one can reference an instance (or object), which means it must have some physical existence. Variables have physical existence where each stores some representation of a value; values have no physical existence. Values exist regardless of physical representation.

> Perhaps you are talking about some other language, also called Smalltalk?

No. I just read with a little extra care and a little better understanding of the concepts involved.

> How confusing! Can you point me to a definition of your language?

The one above definition will suffice. You should also read section 3.3.2 very carefully.

> Definitions of ANSI Smalltalk are available from:
> http://minnow.cc.gatech.edu/squeak/172
>
> Dave Harris, Nottingham, UK | "Weave a circle round him thrice,
> brangdon_at_cix.co.uk | And close your eyes with holy dread,
> | For he on honey dew hath fed
> http://www.bhresearch.co.uk/ | And drunk the milk of Paradise."
Received on Sun Oct 07 2001 - 03:10:11 CEST