I'm not sure I buy this change either.  It seems awfully special-casey...

I mean, the object *does* respond to that message - it does so by 
returning an error telling you that a subclass should have implemented 
the method in one case.  If the subclass hasn't implemented the method, 
this is an error that we should see - we shouldn't just silently ignore 
that method.

Obviously this depends on where #canUnderstand: is being called, but the 
appropriate behaviour to me seems to be to check whether the object 
responds to the message, not to consider what it is going to do *when* 
it responds to the message - that's a slippery slope that we can't go 
very far down unless we're going to somehow be able to come to a 
semantic understanding of all arbitrary code.

I agree with Peter that this might be better as another interface; at 
very least it demands some debate before inclusion.

Julian

ducasse wrote:
> Hi Peter
> 
> Good question. Nathanael will certainly reply but just two points:
> 
>     - if canUnderstand: #foo replies true when a class has a method
>     foo
>         ^ self subclassResponsibility  or shouldNotImplement
>     then there is something really wrong in smalltalk....because 
> normally people think that
>     they can call foo on the receiver but this is not the case with 
> those examples.
> 
>     This means that you will never be able to find code from VW or nay 
> other smalltalk that breaks what nathanael
>     proposes because else the code itself breaks in the other dialects 
> :) funny no!
>     
>     - The usage of canUnderstand: should really be restricted.
>     In fact this is a meta level interface functionality and we should 
> have a
>     task force to carefully evaluate the use in the current image as
>     usage of canUnderstand is 99 % of the time a sign of not so 
> well-designed code.
> 
> Thanks for letting me having fun! This was really cool.
> 
> Stef
> 
> On 12 déc. 03, at 18:05, Peter van Rooijen wrote:
> 
>> From: <n.schaerli at gmx.net>
>>
>>> from preamble:
>>>
>>> "Change Set: KCP-0112-FixCanUnderstand
>>> Date: 12 December 2003
>>> Author: Nathanael Schaerli
>>>
>>> Fixes canUnderstand so that it deals with abstract methods (i.e.,
>>> subclassResponsibility and shouldNotImplement) in the right way."!
>>
>>
>> Natanael,
>>
>> Are you sure this is wise? How exactly did you decide what is "the right
>> way"?
>>
>> How do other dialects implement #canUnderstand:? Are they also "wrong"?
>>
>> If you need the new semantics for something you are working on, why 
>> not add
>> another method that does exactly what you want instead of modifying this
>> old-timer?
>>
>> Regards,
>>
>> Peter van Rooijen
>>
>>
> 
>