Exceptions with Context Re: Proposal: Add exception info
Michael Sloan
mgsloan at gmail.com
Mon Apr 27 00:07:18 UTC 2015
Sure, but the code for "throwWithCallStack" would be the code in my post.
Do you disagree that all usages of throw and catch would need to change if
we want stack traces?
I don't see why changes to the exception machinery are dependent on the new
stack trace pieces being in place. DWARF stack traces aren't available on
windows, and are only available with -g. Profiling based stack traces are
only available with -prof. Implicit location traces require a change to
the API which make them problematic for some usecases (edsko found some
interesting corners where they interact with unboxed types, can't find the
link atm).
Point is, we're not going to have a single source for callstacks. Maybe we
can have a single type for them. Surely it isn't so hard to come up with a
nice datatype to handle rendering these callstacks to the user?
Avoiding these sorts of dependencies on in-progress work is one of the
reasons I opted for a dynamic / open datatype for the extra info.
-Michael
On Sun, Apr 26, 2015 at 4:57 PM, Carter Schonwald <
carter.schonwald at gmail.com> wrote:
> no, the idea / premise is there'd be a new "throwWithCallStack" and or
> "throwWithExtraInfo"
>
> the POINT here is there no point in doing changes to exception machinery
> UNTIL those various new stack trace pieces ÅRE PRESENT.
>
> :)
>
> On Sun, Apr 26, 2015 at 7:52 PM, Michael Sloan <mgsloan at gmail.com> wrote:
>
>> So, based on discussion in IRC, it's clear that with Davean's proposal
>> the thrower would need to call a function which does something like:
>>
>> stack <- currentCallStack
>> throwIO (WithStack stack e)
>>
>> However, this requires that every thrower change their code if they want
>> to provide callstacks. Moreover, every exception handler that wants to
>> handle this exception would need to change as well. This would be a
>> breaking API change for every library that adds support for callstacks.
>> Perhaps this is our only viable option..
>>
>> With my proposal, we can have every use of throw provide a callstack, but
>> lose the information preservation of fromException.
>>
>> -Michael
>>
>> On Tue, Apr 21, 2015 at 9:08 PM, Carter Schonwald <
>> carter.schonwald at gmail.com> wrote:
>>
>>> no michael, Daveans proposal is that we add certain catchWithExtraInfo
>>> :: Exception e => IO a -> (ExtraInfo -> e -> IO a ) -> IO a style
>>> operations to the exceptions modules, for various choices of "extraInfo"
>>>
>>> The idea being, NO current exception codes should have to change. Nor
>>> does SomeException need to change.
>>> i will try to articulate my concerns about your proposed design in more
>>> details on the other proposal thread after i've had a bit more sleep and
>>> thought about it more
>>>
>>>
>>> On Tue, Apr 21, 2015 at 11:50 PM, Michael Sloan <mgsloan at gmail.com>
>>> wrote:
>>>
>>>> Either I am misunderstanding davean's proposal, or you are
>>>> misunderstanding mine. Namely:
>>>>
>>>> * How is his proposal more extensible? His specializes it to just
>>>> passing callstack information.
>>>>
>>>> * I'm not sure exactly how he proposes to change the catch. If it's
>>>> changed to always preserve the extra exception info, then this will be a
>>>> massive API breakage.
>>>>
>>>> -Michael
>>>>
>>>> On Tue, Apr 21, 2015 at 7:23 PM, Carter Schonwald <
>>>> carter.schonwald at gmail.com> wrote:
>>>>
>>>>> hrm, i like this proposal more, and it seems like with some fleshing
>>>>> out it can be strictly more extensible yet backwards compatible than
>>>>> michael's
>>>>>
>>>>> I'll need to mull it a bit more before I cast my vote, but this seems
>>>>> to sketch out a design that provides the same information, but in a more
>>>>> extensible/backwards compatible fashion (at least in a first cut of
>>>>> thinking about it)
>>>>>
>>>>> (i'm splitting this into a new thread so the discussions dont get
>>>>> mixed up)
>>>>>
>>>>> On Tue, Apr 21, 2015 at 9:40 PM, davean <davean at xkcd.com> wrote:
>>>>>
>>>>>> So, I've had a number of issues with exceptions. This has been one of
>>>>>> them. I don't really like this proposal as it stands though as it seems to
>>>>>> make catch a specific exception with said extra info more difficult.
>>>>>>
>>>>>> This is data Control.Exception can move around on its own though,
>>>>>> right? The problem really isn't passing it internal, we could just make a
>>>>>> (Stack, SomeException) tuple just fine, in theory I think (I'll admit I've
>>>>>> not actually reviewed the code, and this isn't meant as a complete proposal
>>>>>> but more a thought experiment). The problem is code handling the data and
>>>>>> working with old code while not losing any of the power of the current
>>>>>> system.
>>>>>>
>>>>>> So we start with: catch :: Exception e => IO a -> (e -> IO a) -> IO a
>>>>>>
>>>>>> Now this proposal allows: catch :: IO a -> (SomeException -> IO a) ->
>>>>>> IO a
>>>>>> If we want access to the new information, but that's not really
>>>>>> satisfactory.
>>>>>>
>>>>>> Real code regularly wants to (picking an arbitrary instance of
>>>>>> Exception) do: catch :: IO a -> (IOError -> IO a) -> IO a
>>>>>> only we still want new data.
>>>>>>
>>>>>> Now one could do something like: catch :: IO a -> (Stack -> IOError
>>>>>> -> IO a) -> IO a
>>>>>> but that is not very upgradable and it breaks existing code.
>>>>>>
>>>>>> But this is just a matter of requesting information, so one could do
>>>>>> something like: catch :: IO a -> (WithStack IOError -> IO a) -> IO a
>>>>>> where: data WithStack e = WithStack Stack e
>>>>>> Or maybe one just addes: catchWithContext :: Exception e => IO a ->
>>>>>> (Context -> e -> IO a) -> IO a
>>>>>> Or: catchWithContext :: Exception => IO a -> (Context e -> IO a) ->
>>>>>> IO a
>>>>>>
>>>>>> Now existing code continues to run and we can feed our exception
>>>>>> handlers the data they want, even when we want some specific exception
>>>>>> instead of just any exception.
>>>>>>
>>>>>> Now that still leave a hole in what I want out of exceptions. We're
>>>>>> still short of programmatic interrogating them, or even telling what the
>>>>>> exception was if we didn't expect it!
>>>>>>
>>>>>> Consider AssertionFailed
>>>>>> <https://hackage.haskell.org/package/base-4.8.0.0/docs/src/GHC-IO-Exception.html#AssertionFailed>.
>>>>>> Its show instance is "showsPrec _ (AssertionFailed err) = showString err",
>>>>>> so if we print out the SomeException, we get whatever string is in
>>>>>> AssertionFailed. Which is great if that string makes sense. But you see
>>>>>> that on your console and its a bit baffling if it doesn't. It could even be
>>>>>> a lie, I can make that say something that looks like its a different
>>>>>> exception. We can use the Typeable instance so the program can tell them
>>>>>> apart at least though. Which works as long as the exception is
>>>>>> single-constructor, or has a well-behaved show instance. What if we come
>>>>>> across a monstrosity like
>>>>>> http://hackage.haskell.org/package/http-conduit-2.1.5/docs/Network-HTTP-Conduit.html#g:12
>>>>>> and it doesn't have a nice show instance that says which on it is? If
>>>>>> Exception added a Data constraint we could actually pull apart these
>>>>>> exceptions and start to make proper sense of them reliably.
>>>>>>
>>>>>> Once you have that there are quite a few useful things you can do
>>>>>> with the exceptions you didn't expect. Currently you could only do them by
>>>>>> enumerating every possible exception which of course doesn't work for the
>>>>>> unexpected.
>>>>>>
>>>>>> On Tue, Apr 21, 2015 at 6:24 PM, Michael Sloan <mgsloan at gmail.com>
>>>>>> wrote:
>>>>>>
>>>>>>> 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
>>>>>>>>>>> >>>>>
>>>>>>>>>>> >>>>> _______________________________________________
>>>>>>>>>>> >>>>> 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
>>>>>>>>>>> >
>>>>>>>>>>>
>>>>>>>>>>
>>>>>>>>>>
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