[phobos] Exception chaining

[Sean Kelly]

Two errors could still cause confusion though. 

Sent from my iPhone

On Jan 12, 2011, at 12:55 AM, Don Clugston <dclugston at googlemail.com> wrote:

> On 12 January 2011 09:10, Don Clugston <dclugston at googlemail.com> wrote:
>> How about this rule:
>> ---
>> If all collateral exceptions are derived from Exception, the 'boss' or
>> Master Exception is the first exception thrown.
>> Otherwise, the first Error (or Throwable non-Exception) is the boss.
>> The exception, including all collateral exceptions, will be caught by
>> the first handler who can catch the boss.
>> ---
>> 
>> Two issues:
>> * If the boss is a collateral exception, we still need a way to find
>> out what the first exception was. Maybe need to add a 'first' (or
>> 'prev') member to Throwable?
> 
> Or, the chain could simply be:
> boss -> exception1 -> exception2 -> BypassThrowable -> exception3.
> 
> Where BypassThrowable is a class which acts as a placeholder for the boss.
> 
> This would be the simplest interim solution, since it doesn't require
> any code changes elsewhere, and would allow the
> existing error messages to continue to work correctly.
> 
> 
>> * It is still possible to throw an Object which isn't derived from
>> Throwable. Can we disallow this?
>> (We could throw a "NoThrowableError" if such an object is found to
>> have been thrown (can happen via casting)). This would remove some
>> really nasty corner cases.
>> 
>> 
>> On 12 January 2011 01:10, Andrei Alexandrescu <andrei at erdani.com> wrote:
>>> On 1/11/11 4:03 PM, Jonathan M Davis wrote:
>>>> 
>>>> On Tuesday, January 11, 2011 15:50:52 Andrei Alexandrescu wrote:
>>>>> 
>>>>> I don't think that's helpful. It complicates the flow a lot because now
>>>>> understanding how the program acts depends not on the types anymore, but
>>>>> on what happens dynamically. Makes it more difficult, not easier, to
>>>>> write robust code.
>>>>> 
>>>>> If I throw a FileException, I must catch a FileException with
>>>>> catch(FileException) regardless of what collateral exceptions have
>>>>> happened.
>>>> 
>>>> I agree as long as it's other Exceptions that have been thrown. But
>>>> Errors?
>>>> Aren't they typically supposed to kill your program?
>>>> 
>>>> - Jonathan M Davis
>>> 
>>> I agree that non-Exception Throwables are an out-of-band method of
>>> communication that deserves special attention.
>>> 
>>> Don, would it be difficult to make a non-Exception Throwable thrown during
>>> unwinding essentially come to the top of the foodchain and save everything
>>> in its tail?
>>> 
>>> 
>>> Andrei
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>> 
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