Proposal: Add exception info
Michael Snoyman
michael at snoyman.com
Tue Apr 21 05:40:38 UTC 2015
The typical case for wanting to distinguish sync from async exceptions that
I'm aware of is using timeout, e.g.:
timeout (try someAction :: IO (Either SomeException SomeActionResult)
A more concrete example is available in the second code snippet at [1].
[1]
https://www.fpcomplete.com/user/snoyberg/general-haskell/exceptions/catching-all-exceptions
On Tue, Apr 21, 2015 at 8:27 AM John Lato <jwlato at gmail.com> wrote:
> I don't have a strong opinion towards this proposal. It definitely seems
> useful, but I think most of the uses would be either somewhat niche in
> practice, or better addressed by other mechanisms (the call stack stuff).
>
> However I am opposed to adding IsAsync, because I do not think it is
> useful. In every case I know where a user wants to differentiate async
> exceptions, what they actually want is to differentiate RTS control
> exceptions from everything else. I think the proposal would easily
> accommodate that, and you could have IsAsync too if there is a use case for
> it.
>
> On Thu, Apr 16, 2015, 8:08 PM Carter Schonwald <carter.schonwald at gmail.com>
> wrote:
>
>> if you can patch prelude error to include stack traces, i will owe you a
>> >=1 beer each at the next two icfps. Thats all i want for christmas. :)
>>
>> i can't speak for how a different patch might work out, because thats not
>> what I'd tried at the time. If you have a go, please share the results!
>> -Carter
>>
>> On Wed, Apr 15, 2015 at 12:22 AM, Michael Sloan <mgsloan at gmail.com>
>> wrote:
>>
>>> Hi Carter!
>>>
>>> Interesting! This thread, right?
>>> https://mail.haskell.org/pipermail/libraries/2014-December/024429.html
>>>
>>> I haven't tried this as a patch to base, but I'm certain that the core
>>> of the proposal has no extra dependencies. Note that the proposal isn't
>>> about stack traces in particular - that's just one application of being
>>> able to throw exceptions with extra information.
>>>
>>> Even if `throwTo` isn't modified to throw exceptions with stack traces,
>>> this functionality could be provided outside of `Control.Exception`
>>> (though, that does seem like the right place to put it). I'm surprised
>>> that the circularity was so problematic, though. Why isn't it sufficient
>>> to have an hs-boot file for `GHC.Stack`, which exports `currentCallStack`?
>>>
>>> -Michael
>>>
>>> On Tue, Apr 14, 2015 at 7:55 PM, Carter Schonwald <
>>> carter.schonwald at gmail.com> wrote:
>>>
>>>> Hey Michael,
>>>> I actually proposed something along these lines that got OK'd by
>>>> libraries early this past fall, the main challenge we hit was actually
>>>> doing the enginering to add the stack traces to exceptions! theres some
>>>> nasty module cycles in base that happen when you try to weave things around
>>>> so that the standard error "message here" call includes some stack trace
>>>> info. Have you tried to do that simple starter patch to base?
>>>>
>>>> Chris Allen and I spent like 2 days trying to get it to work and just
>>>> gave up because of the cycles. We (and others) would probably love some
>>>> headway on that front.
>>>>
>>>> Theres also some in progress work to use the dwarf debugging info data
>>>> in >7.10 to provide useful stack traces in the default builds for GHC
>>>> afaik, 'cause the stack trace functionality you're pointing at currenlty
>>>> only work on profiled builds
>>>>
>>>> cheers
>>>> -Carter
>>>>
>>>> On Tue, Apr 14, 2015 at 2:38 PM, Michael Sloan <mgsloan at gmail.com>
>>>> wrote:
>>>>
>>>>> Control.Exception currently lacks a good way to supply extra
>>>>> information along with exceptions. For example, exceptions could be
>>>>> thrown along with their callstack[1] or implicit stack[2], but we have
>>>>> no generic way to include this information with exceptions.
>>>>>
>>>>> Proposed Solution
>>>>> =================
>>>>>
>>>>> The proposed solution is to add a list of `SomeExceptionInfo` to the
>>>>> `SomeException` datatype. This list stores additional information
>>>>> about the exception. These `ExceptionInfo` instances use a mechanism
>>>>> which is pretty much identical to the dynamic way the `Exception` type
>>>>> works:
>>>>>
>>>>> data SomeException = forall e . Exception e =>
>>>>> SomeExceptionWithInfo e [SomeExceptionInfo]
>>>>>
>>>>> data SomeExceptionInfo = forall a . ExceptionInfo a =>
>>>>> SomeExceptionInfo a
>>>>>
>>>>> class Typeable a => ExceptionInfo a where
>>>>> displayExceptionInfo :: a -> String
>>>>>
>>>>> addExceptionInfo
>>>>> :: (ExceptionInfo a, Exception e)
>>>>> => a -> e -> SomeException
>>>>> addExceptionInfo x (toException -> SomeExceptionWithInfo e xs) =
>>>>> SomeExceptionWithInfo e (SomeExceptionInfo x : xs)
>>>>>
>>>>> `ExceptionInfo` lacks the to / from functions that `Exception` has,
>>>>> because I don't see much point in supporting a hierarchy for exception
>>>>> info. The `Typeable` superclass constraint supplies the necessary
>>>>> casting.
>>>>>
>>>>> `SomeExceptionInfo` could validly instead just use the constraint
>>>>> `(Typeable a, Show a)`. However, I believe it's good to have a new
>>>>> class for this so that:
>>>>>
>>>>> * The user can specify a custom `displayExceptionInfo`
>>>>> implementation, for when this extra info is presented to the user.
>>>>> This function would be invoked by the `show` implementation for
>>>>> `SomeException`.
>>>>>
>>>>> * Types need to opt-in to be usable with `SomeExceptionInfo`.
>>>>> Similarly to exceptions, I imagine that a type with a
>>>>> `ExceptionInfo` instance won't be used for anything but acting as
>>>>> such an annotation. Having a class for this allows you to ask GHCI
>>>>> about all in-scope exception info types via `:info ExceptionInfo`.
>>>>>
>>>>> Backwards Compatibility
>>>>> =======================
>>>>>
>>>>> GHC 7.10 adds support for bidirectional pattern synonyms. This means
>>>>> that this change could be made without breaking code:
>>>>>
>>>>> pattern SomeException x <- SomeExceptionWithInfo x _ where
>>>>> SomeException x = SomeExceptionWithInfo x []
>>>>>
>>>>> Note that consumers of this do not need to enable `-XPatternSynonyms`.
>>>>>
>>>>> Applications
>>>>> ============
>>>>>
>>>>> Callstacks
>>>>> ----------
>>>>>
>>>>> As mentioned at the beginning, this can be used to add callstacks to
>>>>> exceptions:
>>>>>
>>>>> newtype ExceptionCallStack =
>>>>> ExceptionCallStack { unExceptionCallStack :: [String] }
>>>>> deriving Typeable
>>>>>
>>>>> instance ExceptionInfo ExceptionCallStack where
>>>>> displayExceptionInfo = unlines . unExceptionCallStack
>>>>>
>>>>> throwIOWithStack :: Exception e => e -> IO a
>>>>> throwIOWithStack e = do
>>>>> stack <- currentCallStack
>>>>> if null stack
>>>>> then throwIO e
>>>>> else throwIO (addExceptionInfo (ExceptionCallStack stack)
>>>>> e)
>>>>>
>>>>> I see little downside for making something like this the default
>>>>> implementation `throwIO`. Each rethrowing of the `SomeException`
>>>>> would add an additional stacktrace to its annotation, much like the
>>>>> output of `+RTS -xc`. Unlike this debug output, though, the
>>>>> stacktraces would be associated with the exception, rather than just
>>>>> listing locations that exceptions were thrown. This makes it
>>>>> tractable to debug exceptions that occur in concurrent programs, or in
>>>>> programs which frequently throw exceptions during normal functioning.
>>>>>
>>>>> Throwing Exceptions in Handlers
>>>>> -------------------------------
>>>>>
>>>>> Example:
>>>>>
>>>>> main =
>>>>> throwIO InformativeErrorMessage `finally`
>>>>> throwIO ObscureCleanupIssue
>>>>>
>>>>> While `InformativeErrorMessage` got thrown, the user doesn't see it,
>>>>> since `ObscureCleanupIssue` is thrown instead. This causes a few
>>>>> issues:
>>>>>
>>>>> 1. If the exception is handled by the default handler and yielded to
>>>>> the user, then the more informative error is lost.
>>>>>
>>>>> 2. Callers who expect to catch the "Informative error message" won't
>>>>> run their handlers for this exception type.
>>>>>
>>>>> Problem 1 can now easily be resolved by adding some info to the
>>>>> exception:
>>>>>
>>>>> data ExceptionCause = ExceptionCause
>>>>> { unExceptionCause :: SomeException }
>>>>> deriving Typeable
>>>>>
>>>>> instance ExceptionInfo ExceptionCause where
>>>>> displayExceptionInfo fe =
>>>>> "thrown while handling " ++
>>>>> displayException (unExceptionCause fe)
>>>>>
>>>>> catch :: Exception e => IO a -> (e -> IO a) -> IO a
>>>>> catch f g = f `oldCatch` handler
>>>>> where
>>>>> handler ex = g ex `oldCatch` \(ex' :: SomeException) ->
>>>>> throwIO (addExceptionInfo info ex')
>>>>> where
>>>>> info = ExceptionCause (toException ex)
>>>>>
>>>>> This implementation of `catch` is written in a backwards-compatible
>>>>> way, such that the exception thrown during finalization is still the
>>>>> one that gets rethrown. The "original" exception is recorded in the
>>>>> added info. This is the same approach used by Python 3's
>>>>> `__context__` attribute[3]. This was brought to my attention in a
>>>>> post by Mike Meyer[4], in a thread about having bracket not suppress
>>>>> the original exception[5].
>>>>>
>>>>> This doesn't directly resolve issue #2, due to this backwards
>>>>> compatibility. With the earlier example, a `catch` handler for
>>>>> `InformativeErrorMessage` won't be invoked, because it isn't the
>>>>> exception being rethrown. This can be resolved by having a variant of
>>>>> catch which instead throws the original exception. This might be a
>>>>> good default for finalization handlers like `bracket` and `finally`.
>>>>>
>>>>> Asynchronous Exceptions
>>>>> -----------------------
>>>>>
>>>>> Currently, the only reliable way to catch exceptions, ignoring async
>>>>> exceptions, is to fork a new thread. This is the approach used by the
>>>>> enclosed-exceptions[6] package. I think it's quite ugly that we need
>>>>> to go to such lengths due to the lack of one bit of information about
>>>>> the exception! This would resolve ghc trac #5902[7].
>>>>>
>>>>> base-4.7 added the `SomeAsyncException` type, but this doesn't enforce
>>>>> anything. Any exception can be thrown as a sync or async exception.
>>>>> Instead, we ought to have a reliable way to know if an exception is
>>>>> synchronous or asynchronous. Here's what this would look like:
>>>>>
>>>>> data IsAsync = IsAsync
>>>>> deriving (Typeable, Show)
>>>>>
>>>>> instance ExceptionInfo IsAsync where
>>>>> displayExceptionInfo IsAsync = "thrown asynchronously"
>>>>>
>>>>> throwTo :: Exception e => ThreadId -> e -> IO ()
>>>>> throwTo tid = oldThrowTo tid . addExceptionInfo IsAsync
>>>>>
>>>>> The details of this get a bit tricky: What happens if `throwIO` is
>>>>> used to rethrow a `SomeException` which has this `IsAsync` flag set?
>>>>> I'm going to leave out my thoughts on this for now as the interactions
>>>>> between unsafePerformIO and the concept of "rethrowing" async
>>>>> exceptions. Such details are explained in a post by Edsko de Vries[8]
>>>>> and ghc trac #2558[9].
>>>>>
>>>>> Issue: fromException loses info
>>>>> ===============================
>>>>>
>>>>> I can think of one main non-ideal aspect of this proposal:
>>>>>
>>>>> Currently, the `toException` and `fromException` methods usually form
>>>>> a prism. In other words, when `fromException` yields a `Just`, you
>>>>> should get the same `SomeException` when using `toException` on that
>>>>> value.
>>>>>
>>>>> For example,
>>>>>
>>>>> fail "testing 1 2 3" `catch` \(ex :: SomeException) -> throwIO ex
>>>>>
>>>>> is equivalent to
>>>>>
>>>>> fail "testing 3 4 5" `catch` \(ex :: IOError) -> throwIO ex
>>>>>
>>>>> However, with exception info added to just `SomeException`, and no
>>>>> changes to existing `Exception` instances, this
>>>>> doesn't hold. Exceptions caught as a specific exception type get
>>>>> rethrown with less information.
>>>>>
>>>>> One resolution to this is be to add `[SomeExceptionInfo]` as a field
>>>>> to existing `Exception` instances. This would require the use of
>>>>> non-default implementations of the `toException` and `fromException`
>>>>> instances.
>>>>>
>>>>> Another approach is to have variants of `catch` and `throw` which also
>>>>> pass around the `[SomeExceptionInfo]`.
>>>>>
>>>>> [1]
>>>>> https://hackage.haskell.org/package/base-4.8.0.0/docs/GHC-Stack.html#currentCallStack
>>>>> [2]
>>>>> https://ghc.haskell.org/trac/ghc/wiki/ExplicitCallStack/ImplicitLocations
>>>>> [3] https://www.python.org/dev/peps/pep-3134/
>>>>> [4]
>>>>> https://mail.haskell.org/pipermail/haskell-cafe/2014-July/114987.html
>>>>> [5]
>>>>> https://mail.haskell.org/pipermail/haskell-cafe/2014-July/114986.html
>>>>> [6] https://hackage.haskell.org/package/enclosed-exceptions
>>>>> [7] https://ghc.haskell.org/trac/ghc/ticket/5902
>>>>> [8] http://www.edsko.net/2013/06/11/throwTo/
>>>>> [9] https://ghc.haskell.org/trac/ghc/ticket/2558
>>>>>
>>>>> _______________________________________________
>>>>> Libraries mailing list
>>>>> Libraries at haskell.org
>>>>> http://mail.haskell.org/cgi-bin/mailman/listinfo/libraries
>>>>>
>>>>>
>>>>
>>>
>> _______________________________________________
>> Libraries mailing list
>> Libraries at haskell.org
>> http://mail.haskell.org/cgi-bin/mailman/listinfo/libraries
>>
> _______________________________________________
> Libraries mailing list
> Libraries at haskell.org
> http://mail.haskell.org/cgi-bin/mailman/listinfo/libraries
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mail.haskell.org/pipermail/libraries/attachments/20150421/da02baef/attachment-0001.html>
More information about the Libraries
mailing list