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.
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 ?

Nicolas


>> ArrayedCollection avoids it by restoring the primitive method for
>> size.
>>
>> Rodney
>>
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