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September 2003
- 60 participants
- 91 discussions
-----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.
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