[squeak-dev] Re: Exception return: 10 vs 10

Paolo Bonzini bonzini at gnu.org
Wed Jul 23 06:38:40 UTC 2008

Andreas Raab wrote:
> Paolo Bonzini wrote:
>> stephane ducasse wrote:
>>> I'm trying to understand the exact difference between using return: 
>>> and not as a last statement of a on:do: block
>> There is none.  Not using return: was less portable (VSE and GNU 
>> Smalltalk used resume: for the last statement if the exception is 
>> resumable), but with Seaside relying on the ANSI behavior many 
>> dialects are slowly changing.
> Hm ... I don't quite recall but didn't ANSI specify that the behavior is 
> undefined unless a specific handler response is invoked? In other words, 
> *unless* you are specific about either #return: or #resume: the result 
> may be implementation dependent?

No, though it is not explicitly defined as #return:. Message: on: selector do: action
If evaluation of the receiver terminates normally then the exception
environment is reset to the handler environment before returning to the 
sender of the #on:do: message.  If signaling of an exception results in 
evaluation of action the evaluation will occur in the context of the 
handler environment. The argument to the action will be an object that
conforms to the protocol <signaledException>. Message: signal
If the evaluation of the exception action returns normally (as if it had
returned from the #value: message), the handler environment is restored 
and the value returned from the exception action is returned as the 
value of the #on:do: message that created the handler. Before returning, 
any active #ensure: or #ifCurtailed: termination blocks created during 
evaluation of the receiver of the #on:do: message are evaluated.

See also http://snipurl.com/33hyb  [groups.google.com].


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