Putting a constraint on the MonadIO class actually makes your code less general rather than more since it prevents your code from ever being used in a purely functional setting. If you leave out this constraint, you can actually turn your code into a pure function by doing things like feeding it a wrapper monad that supplies a list of strings as input and returns the list of written strings at the end. The only reason why you would add a MonadIO constraint would be if you wanted to do other things that require the IO monad (e.g., connecting to a database, working with files, etc.) and so you wanted to require that the IO monad be ultimately present somewhere in the type so that you could use it. Cheers, Greg On Oct 6, 2009, at 8:22 AM, Floptical Logic wrote:
class StreamMonad m where fetchLine = m sendLine = String -> m ()
instance StreamMonad IO where fetchLine = getLine sendLine = putLine
fetchLineFromStream = lift fetchLine sendLineToStream = lift . sendLine
This approach makes more sense to me. The equivalent to printing something to the screen in the Net (IRC) monad is privmsg and it is easy to make that connection using type classes. I don't see how to make this connection using MonadIO.