Proposal: Add exception info
Michael Sloan
mgsloan at gmail.com
Tue Apr 21 22:24:01 UTC 2015
No, this proposal is not specifically about stack traces, that is just one
of the usecases. Instead, this is about a general mechanism for including
extra information with exceptions. The core of this proposal is still
relevant even if the behavior of error / throw / throwTo / etc remain
unchanged.
I'm not familiar with how the new dwarf stuff will interact with throwing /
displaying exceptions. It seems like this would require having the
debugger break at the throw site, and exceptions would still lack stack
traces. Having informative stack traces is quite orthogonal to having a
good place to store them.
Note that in my original proposal text I mentioned that this is agnostic of
the particular source of the stack trace. In particular, this could be
used with a profiling stack trace, implicit callstack, or, indeed, these
traces via dwarf.
-Michael
On Tue, Apr 21, 2015 at 3:06 PM, Carter Schonwald <
carter.schonwald at gmail.com> wrote:
> On a more important note: assuming ghc 7.12 has support for informative
> stack traces via dwarf by default, wouldn't that eliminate the need for
> this proposal? Namely : there perhaps should be some reasonable way to talk
> about concatting stack traces perhaps?
>
> Phrased differently: how is the info that should perhaps be in informative
> stack traces not subsuming the info of this proposal?
>
>
> On Tuesday, April 21, 2015, Michael Sloan <mgsloan at gmail.com> wrote:
>
>> Ah, but it looks like Niklas does have a patch which adds implicit
>> locations to such functions: https://phabricator.haskell.org/D861
>>
>> However, there are some issues with changing the API of these functions:
>> https://phabricator.haskell.org/D861#23250
>>
>> (as mentioned in the "Backporting srcLoc to the GHC 7.10 branch" thread)
>>
>> On Tue, Apr 21, 2015 at 2:04 PM, Michael Sloan <mgsloan at gmail.com> wrote:
>>
>>> Hmm, that patch doesn't appear to add stack traces to 'Prelude.error',
>>> which is what Carter wants here. Also, I think it would be done with
>>> profiling callstacks rather than implicit callstacks. But it's certainly
>>> also useful to have functions which do the same with implicit callstacks!
>>>
>>>
>>> On Tue, Apr 21, 2015 at 1:55 PM, Evan Laforge <qdunkan at gmail.com> wrote:
>>>
>>>> Maybe I'm missing something, but isn't this already implemented?
>>>>
>>>> https://phabricator.haskell.org/D578
>>>>
>>>> On Tue, Apr 21, 2015 at 1:37 PM, Michael Sloan <mgsloan at gmail.com>
>>>> wrote:
>>>> > On Thu, Apr 16, 2015 at 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.
>>>> :)
>>>> >
>>>> >
>>>> > Sounds good! No promises, but I'll be giving this a try soon.
>>>> Looking
>>>> > forward to ICFP beers either way :D
>>>> >
>>>> >> 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
>>>> >>>>>
>>>> >>>>> _______________________________________________
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>>>> >>>>>
>>>> >>>>
>>>> >>>
>>>> >>
>>>> >
>>>> >
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>>>> >
>>>>
>>>
>>>
>>
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