Does it make sense to remove slots ?

[Todd Fleming]

Hi,

I have a related question for people on this list. I have the following 
philosophy about derivation and am curious if others here believe the 
same way. If A inherits from B, and you are able to do X with B, then 
you should also be able to do X with A. In this case, A = Circle, B = 
Ellipse, X = squish. If you squish an ellipse, then you still have an 
ellipse. If you squish a circle, then it's no longer a circle. By 
squish, I mean scale, with x scale different from y scale.

If you hold to this philosophy, and if your objects are mutable, then 
Circle can't be derived from Ellipse, unless your Circle can turn back 
into an Ellipse (yuck). If areas are considered immutable, then 
squishing returns a new ellipse and my argument doesn't apply. I don't 
know if Smalltalkers hold to this inheritance philosophy or not. I also 
don't know if areas are considered mutable or immutable in the library 
(I haven't looked closely at it).

Now I have a question for you, Pupeno. Does Ellipse subsume all of the 
functionality of Circle? If so, is Circle necessary?

Another question: will Ellipse eventually support rotation?

Todd


Pupeno wrote:

>I'm creating some objects for Ellipses and circles and I have the following 
>code:
>
>Graphics addPrototype: #Ellipse derivedFrom: {Area}.
>Ellipse addSlot: #center.
>Ellipse addSlot: #xRadius.
>Ellipse addSlot: #yRadius.
>
>Graphics addPrototype: #Circle derivedFrom: {Ellipse}.
>Circle addSlot: #radius.
>Circle removeSlot: #xRadius.
>Circle removeSlot: #yRadius.
>
>c@(Circle traits) xRadius [ c radius ].
>c@(Circle traits) xRadius: r [ c radius: r ].
>c@(Circle traits) yRadius [ c radius ].
>c@(Circle traits) yRadius: r [ c radius: r ].
>
>for me it makes sense since Circles are special cases of Ellipses where both 
>radius are the same, so, whatever method I can apply to an Ellipse should be 
>also available to Circle, except that a Circle can have only one radius. 
>That's why I removed the slots, created another one, and then created 
>accessors as if it was an Ellipse. Maybe the write accessors shouldn't be 
>there.
>Or, am I totally confused ? Does this make sense ?
>  
>