On 12.11.2009, at 14:30, Jecel Assumpcao Jr wrote:

> Levente Uzonyi wrote:
>>> Which message send would you save in
>>> 
>>> 	... == UnoccupiedSlot
>>> 
>> 
>> UnoccupiedSlot instance.
>> I wouldn't use a class, because I think we should be able create a set 
>> with any classes in it.
> 
> Indeed - I am sure there are several places in the image that deal with
> sets of all classes or all globals.

Good point.

> One example of a Smalltalk with a non nil object to mark empty slots in
> sets is Self, so it has been done before. On the other hand, Self
> doesn't use associations but instead implements dictionaries with two
> parallel arrays.

It has even been done in Squeak - see class WeakSet.

- Bert -