[squeak-dev] Exception return: 10 vs 10

[stephane ducasse]

Hello

I'm trying to understand the exact difference between using return:  
and not as
a last statement of a on:do: block
I read the excellent tests of ExceptionTester (Tx andreas they are  
cool).

returning
        "self new returning"

        ^ [self firstActionSignalingAnError.
        self secondActionReturning42]
                on: Error
                do: [:ex | Transcript show:  'An error has occurred: ', ex  
messageText ; cr.
                                 ex return: 69]



returning
        "self new returning"

        ^ [self firstActionSignalingAnError.
        self secondActionReturning42]
                on: Error
                do: [:ex | Transcript show:  'An error has occurred: ', ex  
messageText ; cr.
                               69]

[Paolo Bonzini-2]

stephane ducasse wrote:
> Hello
>
> 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.

Paolo

[Andreas Raab]

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?

Cheers,
   - Andreas

[Paolo Bonzini-2]

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:.

5.4.3.3 Message: on: selector do: action
[SNIP]
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>.

5.5.2.1 Message: signal
[SNIP]
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].

Paolo

[stephane ducasse]

I read several times the ansi before posting and I could not find a  
definitive answer.
I thought that return: had to have something special to be worth its  
existence, but I could not
find it. I thought that return: may clean the
On Jul 23, 2008, at 7:25 AM, 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?
>
> Cheers,
> - Andreas
>
>