[Haskell-cafe] ContT and MonadSafe

[PICCA Frederic-Emmanuel]

Hello,

I have some code like this

data Hdf5Path sh e
  = H5RootPath (Hdf5Path sh e)
  | H5GroupPath ByteString (Hdf5Path sh e)
  | H5GroupAtPath Int (Hdf5Path sh e)
  | H5DatasetPath ByteString
  | H5DatasetPathAttr (ByteString, ByteString)
  | H5Or (Hdf5Path sh e) (Hdf5Path sh e)
    deriving (Show)

Then I implement this kind of method

withHdf5PathP :: (MonadSafe m, Location l) => l -> Hdf5Path sh e -> (Dataset -> m r) -> m r
withHdf5PathP loc (H5RootPath subpath) f = withHdf5PathP loc subpath f
withHdf5PathP loc (H5GroupPath n subpath) f = withGroupP (openGroup loc n Nothing) $ \g -> withHdf5PathP g subpath f
withHdf5PathP loc (H5GroupAtPath i subpath) f = withGroupAtP loc i $ \g -> withHdf5PathP g subpath f
withHdf5PathP loc (H5DatasetPath n) f = withDatasetP (openDataset' loc n Nothing) f
withHdf5PathP loc (H5DatasetPathAttr (a, c)) f = withDatasetP (openDatasetWithAttr loc a c) f
withHdf5PathP loc (H5Or l r) f = withHdf5PathP loc l f `catchAll` const (withHdf5PathP loc r f)

I decided to switch to the ContT transfomer and try to implement this like this

withHdf5PathP :: (MonadSafe m, Location l) => l -> Hdf5Path sh e -> ContT r m Dataset
withHdf5PathP loc (H5RootPath subpath) = withHdf5PathP loc subpath
withHdf5PathP loc (H5GroupPath n subpath) = do
  g <- withGroupP (openGroup loc n Nothing)
  withHdf5PathP g subpath
withHdf5PathP loc (H5GroupAtPath i subpath) = do
  g <- withGroupAtP loc i
  withHdf5PathP g subpath
withHdf5PathP loc (H5DatasetPath n) = withDatasetP (openDataset' loc n Nothing)
withHdf5PathP loc (H5DatasetPathAttr (a, c)) = withDatasetP (openDatasetWithAttr loc a c)
withHdf5PathP loc (H5Or l r) = ???


during the process, I also changed all the withXXX method to use Continuation.

-bracket' :: MonadSafe m => (a -> IO ()) -> IO a -> (a -> m r) -> m r
-bracket' r a = bracket (liftIO a) (liftIO . r)
+bracket' :: MonadSafe m => (a -> IO ()) -> IO a -> ContT r m a
+bracket' r a = ContT (bracket (liftIO a) (liftIO . r))
 
-withBytes :: MonadSafe m => Int -> (ForeignPtr a -> m r) -> m r
+withBytes :: MonadSafe m => Int -> ContT r m (ForeignPtr a)
 withBytes n = bracket' touchForeignPtr (mallocForeignPtrBytes n)
 
-withFileP :: MonadSafe m => IO File -> (File -> m r) -> m r
+withFileP :: MonadSafe m => IO File -> ContT r m File
 withFileP = bracket' closeFile
 
-withGroupP :: MonadSafe m => IO Group -> (Group -> m r) -> m r
+withGroupP :: MonadSafe m => IO Group -> ContT r m Group
 withGroupP = bracket' closeGroup
 
-withGroupAtP :: (Location l, MonadSafe m) => l -> Int -> (Group -> m r) -> m r
-withGroupAtP l i f = do
+withGroupAtP :: (Location l, MonadSafe m) => l -> Int -> ContT r m Group
+withGroupAtP l i = do
   es <- liftIO $ nxEntries' l
-  withGroupP (openGroup l (es !! i) Nothing) f
+  withGroupP (openGroup l (es !! i) Nothing)
 
-withDatasetP :: MonadSafe m => IO Dataset -> (Dataset -> m r) -> m r
+withDatasetP :: MonadSafe m => IO Dataset -> ContT r m Dataset
 withDatasetP = bracket' closeDataset
 
-withDataspaceP :: MonadSafe m => IO Dataspace -> (Dataspace -> m r) -> m r
+withDataspaceP :: MonadSafe m => IO Dataspace -> ContT r m Dataspace
 withDataspaceP = bracket' closeDataspace


The H5Or is a sort of Alternative which  try to extract the info from l and if any exception is triggered switch to the right part r.

Would you be so kind to help me write the H5Or part with the continuation. I am not sure that I understand all the ContT arcanes.

thanks for your help

Frederic