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#current... [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