On 30.09.2011, at 13:05, Nicolas Cellier wrote:

> 2011/9/30 Nicolas Cellier <nicolas.cellier.aka.nice at gmail.com>:
>> 2011/9/30 Bert Freudenberg <bert at freudenbergs.de>:
>>> 
>>> On 29.09.2011, at 19:22, Rodney Polkinghorne wrote:
>>> 
>>>> Dear list
>>>> 
>>>> I wanted an array that lazily initialized elements 1 to n when you
>>>> asked for the nth one.  So I ran something like:
>>>> 
>>>> SequenceableCollection variableSubclass #Foo ...
>>>> 
>>>> When I started a workspace, and tried Foo new: 10, my image hung.
>>>> 
>>>> Eventually, I found the problem.  The workspace tried to print the
>>>> answered instance, which resulted in it being sent a do: message to
>>>> print its elements.  This ran the method
>>>> 
>>>> SequenceableCollection>>do: aBlock
>>>>       1 to: self size do:
>>>>               [:index | aBlock value: (self at: index)]
>>>> 
>>>> which ran
>>>> 
>>>> Collection>>size
>>>>       | tally |
>>>>       tally := 0.
>>>>       self do: [:each | tally := tally + 1].
>>>>       ^ tally
>>>> 
>>>> which sent do:, which sent size, which ...
>>>> 
>>>> Is this considered a bug, or a learning experience for new players?
>>> 
>>> The latter. You should not instantiate an abstract class. I don't think there is much that could be done about this.
>>> 
>>> - Bert -
>>> 
>> 
>> But Foo is a concrete class from Rodney, not Abstract.

No, it's still abstract since he did not override any methods.

>> Generally, if the subclass must implement a method, it is expressed
>> with an abstract implementation in the upper level
>>    ^self subclassResponsibility
>> 
>> Here, the subclass must implement a message, either #do: or #size but
>> there is no explicit guidance.
>> Personnally I consider this as very bad kernel code if not a bug.
>> Does any subclass really use Collection>>size ?
> 
> OK, I just opened an image to check and only one does: LinkedList.
> I suggest moving  Collection>>size implementation to LinkedList, and
> replace it with a subclassResponsibility.
> 
> Nicolas

That would be make the intention more clear, agreed. 

- Bert -