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Subject: Re: [PHP-DEV] Re: Throwable::addSuppressed()
From: me@stephencoakley.com (Stephen Coakley)

On 08/01/2015 09:44 AM, Markus Malkusch wrote:
> Christoph Becker:
>> Markus Malkusch wrote:
>>>
>>> I'd like to revise that. I'd just learned that finally does indeed fit
>>> here, as it would automatically glue exceptions:
>
> [..]
>
>> Note that there's an open bug report (<https://bugs.php.net/68270>)
>> regarding this issue.
>
> Thank you for pointing to that bug. So I understand the authors of that
> bug are surprised that finally does magically attach exceptions. I did
> actually also not expect that, as it is not documented nor do I have
> that experience from other languages (did only check with Java).
>
> So I can assume that magic finally glue behaviour is not considered
> stable, which then again gives Throwable::addSupressed() IMO a
> justification.
>
> Markus Malkusch
>

So what should be the desired behavior, in regard to bug #68270? One 
possible behavior would be to, if an exception is thrown inside a 
finally block, to attach the uncaught exception from the try block via 
the addSupressed() method. The last exception is thrown, and the 
exception in the try block is still not lost. Such an alternative could 
be bundled with the RFC.

The alternative s to just drop the exception, never to be seen again. 
This is the behavior in both Java and C#. Python has a different 
approach. Python has two properties that contain previous exceptions: 
__context__, which holds implicitly chained exceptions, and __cause__, 
which holds explicitly chained exceptions. This system makes a ton of 
sense, and I think PHP exceptions should distinguish between the two as 
well.

(Relevant PEP: <https://www.python.org/dev/peps/pep-3134/>)

Currently, in the try/finally situation described, $previous is used to 
hold the missing exception. $previous should only hold explicitly passed 
exceptions, as described in the docs. addSupressed() could be defined as 
for implicit chained exceptions. The method could be paired with a 
Throwable::getSupressed() to retrieve the implicitly added exception.

So in a case like this:

try {
     try {
         throw new Exception("Exception 1");
     } finally {
         throw new Exception("Exception 2");
     }
} catch (Throwable $e) {
     var_dump($e->getSupressed());
}

we would have $e be exception 2, and $e->getSupressed() return exception 1.

The more I think about it, the better the idea sounds. Good thought, Markus!

-- 
Stephen Coakley