Proposal: Add exception info

[Yitzchak Gale]

I wrote:
>>     Package foo uploads a new version with a critical bug fix.
>>     As is often the case, this new version also supports updated
>>     dependencies, including exceptions. The new exceptions
>>     breaks the old SomeException type, so foo is forced to
>>     specify a lower bound that excludes the old exceptions.

Michael Snoyman wrote:
>> I'm not seeing a situation where an author couldn't easily
>> be compatible with GHC <=7.8 by sticking to the
>> old API?

Felipe Lessa wrote:
> I think Michael Gale

heh

> is thinking about an author that may end up using the new
> SomeExceptionWithInfo constructor for some reason, thus
> leaving older GHCs out of any new updates.

Right, exactly what I was thinking. But perhaps
Michael Sn. is correct that explicit use of a SomeException
constructor is rare. (Which is what I think is what he means by
"sticking to the old API".) If that is really true, then my objection
is not valid.

And I'm certainly strongly in favor of the goals of this
proposal. So I'd be in favor even if *some* breakage
results from it, as long as the overall deprecation curve
isn't unreasonably steep.

Thanks,
Yitz