Proposal: Add exception info

[Michael Sloan]

Right, we have a choice between making a breaking change towards a more
standard API, or using a rather new language extension (bidirectional
pattern synonyms) to avoid breakage.

On Thu, Apr 23, 2015 at 6:30 AM, Greg Weber <greg at gregweber.info> wrote:

>
>
> 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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