Dave Farber wrote:

>today's puzzle is:
>
>    Dictionary new
>        at: (Set with: 1) put: 'is';
>        at: (Set new add: 1; yourself) ifAbsent: ['isnot']
>
>why does this little snippet *sometimes* answer 'is' and *sometimes* answers
>'isnot'?

Because the implementation of Set makes the fundamental error of 
reimplementing #=, without reimplementing #hash, which remains inherited 
from Object.  This means that it cannot be used as a key for Sets or 
Dictionaries (as you have discovered), because its #hash value is not 
based upon the equality comparison, but rather upon a pseudo-random value 
kept in its header field.

The other classes in the image that do this are:

TextLine
LargePositiveInteger
CharacterBlock
TextStyle
SequenceableCollection
Text
CompiledMethod
TextLineInterval
SortedCollection
Set
TextColor
TextKern
TextFontChange
TextFontReference
TextAnchor
TextEmphasis

To be fair, it is doubtful that most of these would actually be used as 
Set or Dictionary keys, but one can certainly imagine uses for some of 
them (LargePositiveInteger, SequenceableCollection).  I guess this should 
be added to the official "cleanup list" of things to fix.



     -- tim