Proposal: Add exception info

[Michael Sloan]

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.


> 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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