[Haskell-cafe] Failing to catch exception

Michael Jones mike at proclivis.com
Thu Oct 8 17:22:18 UTC 2015


Ok, I need some conceptual help on this one. If the action “in" the catch is not evaluated, I would think the catch itself cannot be fully evaluated. Once IO demands the value from the work “in” the catch, how can it escape the catch?

To me it seemed like the exception triggered the death of the wx callback without an evaluation of the catch, like the runtime let it escape.


> On Oct 8, 2015, at 7:41 AM, Erik Hesselink <hesselink at gmail.com> wrote:
> Perhaps the value you're calculating is lazily computed, so the
> exception occurs only when using it, and by then you're already
> outside the catch? Try forcing things with `seq` or `deepseq` and see
> if that helps in catching the exception.
> Erik
> On 8 October 2015 at 15:26, Michael Jones <mike at proclivis.com> wrote:
>> I’m having trouble with an exception that won’t catch and looking for suggestions. Basically, a “fail” inside a “catch” inside a unsafeIOToSTM, inside a wx callback, trying to catch SomeException or IOException.
>> Details:
>> I’m using Text.Regex.PCRE
>> and the exception comes from a “fail" in unwrap:
>> unwrap :: (Show e) => Either e v -> IO v
>> unwrap x = case x of Left err -> fail ("Text.Regex.PCRE.String died: "++ show err)
>>                    Right v -> return v
>> The catch is in this function, catching SomeException:
>> filterTableRow _ record fils = do
>>   s <- showRecord record
>>   vs <- Control.Exception.catch (
>>             return $ map (\(fil,col) -> let fil' = if fil == "" then "^" else fil in
>>                                                    ((splitOn "," s)!!(fromIntegral (col+1))) =~ fil') (elems fils)
>>             )
>>             ((\e -> do return $ replicate (length $ elems fils) False) :: SomeException -> IO [Bool])
>>   return $ foldl (.&.) True vs
>> filterTableRow is called from an unsafeIOToSTM:
>> unsafeIOToSTM $ filterTableRow gridData row’ fil
>> which is called from a wx callback from a button press
>> Before I added the catch, the application was working for over a year, so there is no general problem, All I added was the catch.
>> Adding -xc to the ghc runtime shows the error:
>> *** Exception (reporting due to +RTS -xc): (THUNK_1_0), stack trace:
>> Text.Regex.PCRE.String.matchTest,
>> called from Text.Regex.PCRE.String.unwrap,
>>>> user error (Text.Regex.PCRE.String died: (4,"unmatched parentheses"))
>> Which indicates the text in unwrap, as if the exception is not caught. I have tried to catch it as IOException, and SomeException.
>> Any ideas why it is not caught or some way to debug it?
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