It Seemed Like A Good Idea, At The Time

Haskell’s absolute worst feature, in my opinion, is its system of exceptions. I have yet to see a single use of exceptions that wouldn’t be better served by the use of a “Maybe” or an “Either” type. Luckily, in a small nod to sanity, Haskell provides ‘Control.Exception.Base.try’, which returns an Either type, with the exception left, and the value right.

Once you have that, it becomes easy to implement some sane functionality for exception handling, such as my current favorite functions, ‘defaultOnError’ and ‘errorToMaybe’:

defaultOnError :: a -> IO a -> IO a
defaultOnError d a = do tryValue <- try a
                        case tryValue of
                             Left  _ -> return d
                             Right v -> return v

errorToMaybe :: IO a -> IO (Maybe a)
errorToMaybe a = do tryValue <- try a
                    case tryValue of
                         Left  _ -> return $ Nothing
                         Right v -> return $ Just v