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?