On Thu, Sep 04, 2003 at 10:45:17AM +0100, Simon Marlow wrote: > > 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? > > I'm afraid I can't see a way to generalise the types of block and > unblock, since they are based on underlying primitives that really do > have type (IO a -> IO a). Perhaps if your monad is isomorphic to IO, it > could be done, but otherwise I don't think it's possible. Unless I'm > missing something.
this is a problem I have noticed too. I posted something to the libraries list asking for 'catch' to be added to MonadIO. however it was unclear how to best go about it, although a suitable catch can be written for most all mondads it was not certain whether there were some for which it couldn't be. http://haskell.org/pipermail/libraries/2003-February/000774.html -- --------------------------------------------------------------------------- John Meacham - California Institute of Technology, Alum. - [EMAIL PROTECTED] --------------------------------------------------------------------------- _______________________________________________ Glasgow-haskell-users mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/glasgow-haskell-users
