[Haskell-cafe] Failing to catch exception

[Erik Hesselink]

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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