Richard Silverman wrote:
I'm puzzled by something. Suppose I have some code that does lots of IO, and also occasionally refers to some global state. No problem, use ReaderT for the state, combining with the IO monad. Except... since IO is on the bottom, simple uses of do-notation such as "foo <- ask" work in the Reader monad, and to access the IO monad, I need to lift, e.g. (bar <- liftIO getLine). If my code does lots of IO, this is *very* ugly -- the code is littered with lift functions! Is there no cleaner way to do this?
Not that I know of. However, you can usually group IO-statements together in one bigger block: blah = do foo <- ask biz <- liftIO $ do meep bop foo bleep foo zap biz etc. And if there is a limited number of specific functions you need to lift often, you can write generalised MonadIO-based versions of these so that you can call them directly from the monad transformer: getLine :: MonadIO m => m String getLine = liftIO SystemIO.getLine Assuming an appropriate MonadIO instance of ReaderT, of course. Does that help? Martijn.