Re: Clean Object Class Design -- Circle/Ellipse

Using the real example of circle and ellipse:

How does Smalltalk:
* allow me to declare circle as a subtype of ellipse? * allow me to send the setFoci message to ellipse variables (that incidentally contain circle values) with two different focal points? * prevent me from sending the setFoci message to circle variables?

"James A. Robertson" <jarober_at_mail.com> wrote in message news:3BBFDD8E.4020802_at_mail.com...
> Using a real example, show me how any of your points matter in Smalltalk.
>
>
> Bob Badour wrote:
>
> > 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 Mon Oct 08 2001 - 09:09:25 CEST