Decorating exceptions with backtrace information

Carter Schonwald carter.schonwald at
Fri May 8 13:33:51 UTC 2020

On Fri, May 8, 2020 at 9:32 AM Carter Schonwald <carter.schonwald at>

> ben, could you please email the libraries list with this too? This seems
> like a core libraries / base change rather than a ghc-the-compiler change
> On Thu, May 7, 2020 at 6:57 PM Michael Sloan <mgsloan at> wrote:
>> Thanks so much for making a proposal for this, Ben!!  It's great to see
>> progress here.
>> I'm also glad that there is now a proposal process.  I made a fairly
>> similar proposal almost exactly 5 years ago to the libraries list -
>> -
>> but without the subtlety of particular backtrace representations.  Skimming
>> the ensuing thread may still be informative.
>> In particular, there is one thing I would like to highlight from that old
>> proposal.  I think it'd be good to have a standard way to represent a chain
>> of exceptions, and build this into `catch` and `finally`.  Python and Java
>> both have a mechanism for this, and both refer to it as a "cause"
>> exception.  When an exception is thrown during exception handling, the
>> exception being handled is preserved as its "cause".  I find this mechanism
>> to be incredibly useful in Java, it has made the underlying issue much
>> clearer in many cases, and in other cases at least provides helpful
>> context.  I have no doubt such a mechanism would have saved me many hours
>> of debugging exceptions in Haskell systems I've worked on in the past.
>> I considered commenting about that directly on the proposal, but I figure
>> this is a better place to suggest expanding the scope of the change :) .
>> Totally understandable if you want to keep this proposal focused on
>> stacktraces, but I think it'd be good to consider this as a potential
>> future improvement.
>> -Michael
>> On Thu, May 7, 2020 at 3:55 PM Ben Gamari <ben at> wrote:
>>> Hi everyone,
>>> After a nice discussion on IRC about the unfortunate state of error
>>> reporting in Haskell, I felt compelled to write down some long-lingering
>>> thoughts regarding backtraces on exceptions. The result is GHC proposal
>>> #330 [1]. I think the approach is viable and perhaps even
>>> straightforward. I have the sketch of an implementation here [2].
>>> Please have a look at the proposal and leave your comments. If there is
>>> consensus it is possible that we could have this done for 8.12.
>>> Cheers,
>>> - Ben
>>> [1]
>>> [2]
>>> _______________________________________________
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