utils/Exception.hs

    {-# OPTIONS_GHC -fno-warn-deprecations #-}
    module Exception
        (
        module Control.Exception,
        module Exception
        )
        where

    import Control.Exception
    import Control.Monad.IO.Class

    catchIO :: IO a -> (IOException -> IO a) -> IO a
    catchIO = Control.Exception.catch

    handleIO :: (IOException -> IO a) -> IO a -> IO a
    handleIO = flip catchIO

    tryIO :: IO a -> IO (Either IOException a)
    tryIO = try

    -- | A monad that can catch exceptions.  A minimal definition
    -- requires a definition of 'gcatch'.
    --
    -- Implementations on top of 'IO' should implement 'gmask' to
    -- eventually call the primitive 'Control.Exception.mask'.
    -- These are used for
    -- implementations that support asynchronous exceptions.  The default
    -- implementations of 'gbracket' and 'gfinally' use 'gmask'
    -- thus rarely require overriding.
    --
    class MonadIO m => ExceptionMonad m where

      -- | Generalised version of 'Control.Exception.catch', allowing an arbitrary
      -- exception handling monad instead of just 'IO'.
      gcatch :: Exception e => m a -> (e -> m a) -> m a

      -- | Generalised version of 'Control.Exception.mask_', allowing an arbitrary
      -- exception handling monad instead of just 'IO'.
      gmask :: ((m a -> m a) -> m b) -> m b

      -- | Generalised version of 'Control.Exception.bracket', allowing an arbitrary
      -- exception handling monad instead of just 'IO'.
      gbracket :: m a -> (a -> m b) -> (a -> m c) -> m c

      -- | Generalised version of 'Control.Exception.finally', allowing an arbitrary
      -- exception handling monad instead of just 'IO'.
      gfinally :: m a -> m b -> m a

      gbracket before after thing =
        gmask $ \restore -> do
          a <- before
          r <- restore (thing a) `gonException` after a
          _ <- after a
          return r

      a `gfinally` sequel =
        gmask $ \restore -> do
          r <- restore a `gonException` sequel
          _ <- sequel
          return r

    instance ExceptionMonad IO where
      gcatch    = Control.Exception.catch
      gmask f   = mask (\x -> f x)

    gtry :: (ExceptionMonad m, Exception e) => m a -> m (Either e a)
    gtry act = gcatch (act >>= \a -> return (Right a))
                      (\e -> return (Left e))

    -- | Generalised version of 'Control.Exception.handle', allowing an arbitrary
    -- exception handling monad instead of just 'IO'.
    ghandle :: (ExceptionMonad m, Exception e) => (e -> m a) -> m a -> m a
    ghandle = flip gcatch

    -- | Always executes the first argument.  If this throws an exception the
    -- second argument is executed and the exception is raised again.
    gonException :: (ExceptionMonad m) => m a -> m b -> m a
    gonException ioA cleanup = ioA `gcatch` \e ->
                                 do _ <- cleanup
                                    liftIO $ throwIO (e :: SomeException)