Luminescent Dreams

Error Handling in Haskell

 January 01, 0001

The Kinds of Error Reports

  • Exceptions
  • Either
  • ErrorT or EitherT

Parsing a File

  • parseImage :: ByteString -> Either ParseError (Image PixelRGB8)
  • readFile :: FilePath -> IO ByteString
  • readImageFile fname = readFile fname »= return . parseImage :: FilePath -> IO (Either ParseError (Image PixelRGB8))

Exceptions

  • throw :: Exception e => e -> a
  • throwIO :: Exception e => e -> IO a
  • catch :: Exception e => IO a -> (e -> IO a) -> IO a
  • handle :: Exception e => (e -> IO a) -> IO a -> IO a
  • try :: Exception e => IO a -> IO (Either e a)
  • throwTo :: Exception e => ThreadId -> e -> IO ()
  • error :: String -> a

Exceptions

\footnotesize

readFileExc :: FilePath -> IO ByteString
readFileExc = handle silenceENoEnt . readFile
  where
    silenceENoEnt :: IOException -> IO ByteString
    silenceENoEnt exc | isDoesNotExistError exc = return empty
                      | otherwise = throw exc

readImageFileExc :: FilePath -> IO Image
readImageFileExc fn = do
    bs <- readFileExc fn
    either throw return (parseImage bs)

\normalsize

Either

\footnotesize

data ReadImageError = ParseError ParseError | ReadError IOException

readFileEither :: FilePath -> IO (Either IOException ByteString)
readFileEither fn = try (readFile fn) >>= return . either silenceENoEnt Right
  where
    silenceENoEnt :: IOException -> Either IOException ByteString
    silenceENoEnt exc | isDoesNotExistError exc = Right empty
                      | otherwise = Left exc

readImageFileEither :: FilePath -> IO (Either ReadImageError Image)
readImageFileEither fn = do
    mBs <- readFileEither fn
    return $ case mBs of
        Left err -> Left (ReadError err)
        Right bs -> either (Left . ParseError) Right (parseImage bs)

\normalsize

ErrorT and EitherT

  • newtype ErrorT e m a = ErrorT { runErrorT :: m (Either e a) }
  • newtype EitherT e m a = EitherT { runEitherT :: m (Either e a) }

ErrorT and EitherT

  • newtype ErrorT e m a = ErrorT { runErrorT :: m (Either e a) }
  • newtype EitherT e m a = EitherT { runEitherT :: m (Either e a) }
  • EitherT :: m (Either e a) -> EitherT e m a
  • runEitherT :: EitherT e m a -> m (Either e a)

EitherT

  • catch :: Exception e => IO a -> (e -> IO a) -> IO a
  • catch :: Exception e => IO (Either e a) -> (e -> IO (Either e’ a)) -> IO (Either e’ a)
  • catchT :: Monad m => EitherT e m a -> (e -> EitherT e’ m a) -> (EitherT e’ m a)

EitherT

  • handle :: Exception e => (e -> IO a) -> IO a -> IO a
  • handleT :: Monad m => (e -> EitherT e’ m a) -> EitherT e m a -> (EitherT e’ m a)

EitherT

  • try :: Exception e => IO a -> IO (Either e a)
  • tryIO :: MonadIO m => IO a -> EitherT IOException m a
  • EitherT . try :: Exception e => IO a -> EitherT e IO a

EitherT

\footnotesize

catchT :: forall e e' m a. Monad m
          => EitherT e m a
          -> (e -> EitherT e' m a)
          -> EitherT e' m a
catchT action handler = EitherT $ do
    res <- runEitherT action :: m (Either e a)
    case res of
        Right val -> return (Right val)
        Left err -> runEitherT (handler err)

\normalsize

MonadError

\footnotesize

class Monad m => MonadError e m | m -> e where
    throwError :: e -> m a
    catchError :: m a -> (e -> m a) -> m a

\normalsize

Revisiting readImageFileEither

\footnotesize

data ReadImageError = ParseError ParseError | ReadError IOException

readImageFileEither :: FilePath -> IO (Either ReadImageError Image)
readImageFileEither fn = do
    mBs <- readFileEither fn
    return $ case mBs of
        Left err -> Left (ReadError err)
        Right bs -> either (Left . ParseError) Right (parseImage bs)

\normalsize

Revisiting readImageFileEither

\footnotesize

data ReadImageError = ParseError ParseError | ReadError IOException

readImageFileEither :: FilePath -> EitherT ReadImageError IO Image
readImageFileEither fn =

\normalsize

Revisiting readImageFileEither

\footnotesize

readImageFileEither' :: FilePath -> EitherT ReadImageError IO Image
readImageFileEither' fn = do
    bs <- readFileEither' fn :: EitherT IOException IO ByteString

\normalsize

Revisiting readImageFileEither

\footnotesize

readImageFileEither' :: FilePath -> EitherT ReadImageError IO Image
readImageFileEither' fn = do
    bs <- handleT (throwError . ReadError) (readFileEither' fn)
                :: EitherT ReadImageError IO ByteString

\normalsize

Revisiting readImageFileEither

\footnotesize

readImageFileEither' :: FilePath -> EitherT ReadImageError IO Image
readImageFileEither' fn = do
    bs <- handleT (throwError . ReadError) (readFileEither' fn)
    case parseImage bs of
        Right val -> return val
        Left exc -> throwError (ParseError exc)

\normalsize

Revisiting readImageFileEither

\footnotesize

readImageFileEither' :: FilePath -> EitherT ReadImageError IO Image
readImageFileEither' fn = do
    bs <- handleT (throwError . ReadError) (readFileEither' fn)
    either (throwError . ParseError) return (parseImage bs)

\normalsize

Revisiting readImageFileEither

\footnotesize

readFileExc :: FilePath -> IO ByteString
readFileExc = handle silenceENoEnt . readFile
  where
    silenceENoEnt :: IOException -> IO ByteString
    silenceENoEnt exc | isDoesNotExistError exc = return empty
                      | otherwise = throw exc

readFileEither' :: FilePath -> EitherT IOException IO ByteString
readFileEither' = handleT silenceENoEnt . tryIO . readFile
  where
    silenceENoEnt :: IOException -> EitherT IOException IO ByteString
    silenceENoEnt exc | isDoesNotExistError exc = return empty
                      | otherwise = throwError exc

\normalsize

Revisiting readImageFileEither

\footnotesize

silenceENoEnt' :: (MonadError IOException m, MonadIO m)
               => IOException
               -> m ByteString
silenceENoEnt' exc
    | isDoesNotExistError exc = return empty
    | otherwise = throwError exc

readFileExc :: FilePath -> IO ByteString
readFileExc = handle silenceENoEnt' . readFile

readFileEither' :: FilePath -> EitherT IOException IO ByteString
readFileEither' = handleT silenceENoEnt' . tryIO . readFile

\normalsize

Building an Application Stack

\footnotesize

data DiskStore = DiskStore { root :: FilePath }

newtype DiskStoreM a =
        DSM {
            uDSM :: ReaderT DiskStore
                            (EitherT DataStoreError IO)
                            a }
    deriving ( Functor, Applicative, Monad, MonadIO,
             , MonadReader DiskStore, MonadError DataStoreError )

\normalsize

Put an Object In

\footnotesize

putObject :: DataObject obj
          => Path
          -> obj
          -> Maybe ObjVersion
          -> DiskStoreM ObjVersion
putObject path obj mVer = do
    DiskStore{..} <- ask
    let fsPath = root </> "objects" </> T.unpack (T.intercalate "/" elems)
    createDirectoryIfMissing True (dropFileName fsPath)
    BS.writeFile fsPath (content obj)
    return (ObjVersion T.empty)

\normalsize

  • createDirectoryIfMissing :: Bool -> FilePath -> IO ()
  • writeFile :: FilePath -> ByteString -> IO ()

hoistIO

(IOException -> EitherT DataStoreError IO a) -> IO a -> DiskStoreM a

hoistIO

\footnotesize

hoistIO :: (IOException -> EitherT DataStoreError IO a)
        -> IO a
        -> EitherT IOException IO a
hoistIO handler action =
    tryIO action

\normalsize

hoistIO

\footnotesize

hoistIO :: (IOException -> EitherT DataStoreError IO a)
        -> IO a
        -> EitherT DataStoreError IO a
hoistIO handler action =
    handleT handler (tryIO action)

\normalsize

hoistIO

\footnotesize

hoistIO :: (IOException -> EitherT DataStoreError IO a)
        -> IO a
        -> ReaderT DiskStore (EitherT DataStoreError IO) a
hoistIO handler action =
    ReaderT (\_ -> handleT handler
                           (tryIO action))

hoistIO handler action =
    lift (handleT (throwError . handler)
                  (tryIO action))

\normalsize

hoistIO

\footnotesize

hoistIO :: (IOException -> EitherT DataStoreError IO a)
        -> IO a
        -> DiskStoreM a
hoistIO handler action =
    DSM (lift (handleT handler (tryIO action)))

\normalsize

Back to adding the object

\footnotesize

putObject :: DataObject obj => Path
                            -> obj
                            -> Maybe ObjVersion
                            -> DiskStoreM ObjVersion
putObject (Path elems) obj mVer = do
    DiskStore{..} <- ask
    let fsPath = root </> "objects" </> T.unpack (T.intercalate "/" elems)
    hoistIO (throwError . trIOExc) $ do
        createDirectoryIfMissing True (dropFileName fsPath)
        BS.writeFile fsPath (content obj)
    return (ObjVersion T.empty)

\normalsize

Catching multiple types of Exceptions

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Catching and handling inside the monad

\footnotesize

putObject :: DataObject obj => Path
                            -> obj
                            -> Maybe ObjVersion
                            -> DiskStoreM ObjVersion
putObject (Path elems) obj mVer = do
    DiskStore{..} <- ask
    let fsPath = root </> "objects" </> T.unpack (T.intercalate "/" elems)
    res <- liftIO $ do
        createDirectoryIfMissing True (dropFileName fsPath)
        BS.writeFile fsPath (content obj)
    return (ObjVersion T.empty)

\normalsize

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