Proposal: Add exception info

[Greg Weber]

On Tue, Apr 21, 2015 at 1:55 PM, Michael Sloan <mgsloan at gmail.com> wrote:

>
> On Tue, Apr 21, 2015 at 8:23 AM, Greg Weber <greg at gregweber.info> wrote:
>
>>
>> On Tue, Apr 14, 2015 at 11:38 AM, Michael Sloan <mgsloan at gmail.com>
>> wrote:
>>
>>>     data SomeException = forall e . Exception e =>
>>>         SomeExceptionWithInfo e [SomeExceptionInfo]
>>>
>>>     data SomeExceptionInfo = forall a . ExceptionInfo a =>
>>>         SomeExceptionInfo a
>>
>>
>> Is it necessary for SomeExceptionWithInfo to have a list of a forall data
>> type?
>> Are Exceptions really that mysterious, or can we more concretely describe
>> the information that should be attached to an exception?
>>
>>     SomeExceptionWithInfo e IsAsync CallStack ImplicitStack
>>
>
> I did consider this option, but I think as soon as a fixed set is
> selected, someone's going to put something else in it.  Usually we wouldn't
> want to use such a 'dynamic' mechanism in Haskell, but it's appropriate for
> something so global as the type used to throw exceptions.
>

The usual approach is to use information hiding. We are being rescued by
pattern synonyms right now because the constructor is directly exported.
Why not hide what needs to be hidden to make this more extensible and use
smart constructors, etc?.


>
>
>> I am still open to the idea of adding a forall data scratchpad, but can
>> we at least try to specify some standard fields?
>>
>>     SomeExceptionWithInfo e IsAsync CallStack ImplicitStack
>> [SomeExceptionInfo]
>>
>
> This is an interesting idea.  I particularly see value in having 'IsAsync'
> be a part of the Exception.  This is because `throwIO` / `throw` would need
> to set this to False when rethrowing async exceptions.
>
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