[Haskell-cafe] Failing to catch exception

[Michael Jones]

Erik,

deepseq was the trick. It was not front of mind that a catch would not enforce evaluation, as it “naturally" is supposed to catch. Retuned my intuition :-)

Mike


> 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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