Re: Clean Object Class Design -- Circle/Ellipse

Use one I'm actually likely to stumble on in a real application I might write.

Bob Badour wrote:

> Using the real example of circle and ellipse:
> 
> How does Smalltalk:
> *   allow me to declare circle as a subtype of ellipse?

I subclass Circle from Ellipse

> * allow me to send the setFoci message to ellipse variables (that > incidentally contain circle values) with two different focal points?

I override the methods in the subclass so that each gets appropriate behavior. If the ellipse variable contain circle values, ,my application code is wrong>

> * prevent me from sending the setFoci message to circle variables?

See above.

If I find that I'm overriding enough methods, subclassing was a bad choice. You have created an example 'problem' that isn't much of a problem.

> 
> "James A. Robertson" <jarober_at_mail.com> wrote in message
> news:3BBFDD8E.4020802_at_mail.com...
> 

> 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
> 


>>>>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

> 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.

>>>>>

>>>>>

> 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.

>>>

>>>

>>>

>>>

> 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."