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September 2003

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bracket, (un)block and MonadIO
by Lunar 02 Sep '03

02 Sep '03

-----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1 Hello, I'm trying to make generic helper functions for database querying. I would like to be able to define functions that work for every monad that holds a database connection and is built on top of the IO monad. For this, I have defined the following type class: > class MonadIO m => ConnectedM m where > getConnection :: m Connection The 'query' function from the database library I am using returns a Statement which has to be closed under all circumstances (especially when an exception is thrown); I am using bracket to ensure this, in the following helper function: > withQuery :: ConnectedM m => (Statement -> m a) -> String -> m a > withQuery f q = > do conn <- getConnection > bracket > (liftIO $ query conn q) > (\ s -> liftIO $ closeStatement s) > f However, the definition of bracket relies on block and unblock, which have the following types: > bracket :: IO a -> (a -> IO b) -> (a -> IO c) -> IO c > block :: IO a -> IO a > unblock :: IO a -> IO a This forces f to be an IO action in the above withQuery function. If bracket, block, and unblock had these types, my helper function would be well typed: > bracket :: MonadIO m => m a -> (a -> m b) -> (a -> m c) -> m c > block :: MonadIO m => m a -> m a > unblock :: MonadIO m => m a -> m a Would anything prevent block, unblock, bracket (and other similar functions working on IO actions) from being generalized to all intances of MonadIO? Lunar. -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.0.6 (GNU/Linux) Comment: For info see http://www.gnupg.org iD8DBQE/VJ11d1rcjNWgdWQRAqeGAJ9tUgXPkOwAfyWj6wTHEGNQ9UYnGgCfSn9S Tf9lH2Tq5pMZVhTM/WVO1/o= =fPqV -----END PGP SIGNATURE-----

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