I had a problem last night where I was parsing a lot of non-trivial and annoyingly inconsistent json. These json documents have a very bad habit of replacing lists with single objects when a particular document contains a singleton list, which is maddening, but I can work around it when I know to expect it. That's just one example of the kinds of failures due to inconsistent documents. My parsing code looks like this: (Data.Aeson, btw) instance FromJSON Whatever where parseJSON (Object o) = do a <- parseSomething o b <- parseSomethingElse o c <- ... andManyMore <- ... return $ Whatever a b c ... parseJSON _ = fail "dude" In my code I might have 10 sub parsers so any of them might fail. When the parser fails I see a failure message like "expected [a] but got Object" and no additional context. And of course if I was in a language with a stack I could just look at the stack. (And I'd be miserable in other ways. So I'm not exactly complaining...) These documents are long enough that I burned a lot of time comparing Aeson dumps looking for where I got an object instead of an array. Wouldn't it be neato if I could do this... instance FromJSON Whatever where parseJSON (Object o) = do a <- failureContext "branchA" $ parseSomething o b <- failureContext "branchB" parseSomethingElse o return $ Whatever a b parseJSON _ = fail "dude" Now when it dies, the error message will be "branchB: expected [a] but got Object". This seems like a useful utility. Does it already exist and I don't know it? Would it be easy to write? I'm a little bit out of my depth here. I've never tried to handle exceptions in Haskell before. Seems like it would be something like: failureContext :: (MonadIOSomething m) => String -> m a -> m a failureContext errorContextName = -- Catch exceptions and rewrite messages -- or wrap the exception in something and rethrow like Erlang or Java -- Darrin