[Haskell-cafe] Monad Input/Output and Monad Transformers

[Maciej Piechotka]

On Thu, 2009-07-02 at 15:43 +0400, Bulat Ziganshin wrote:
> Hello Maciej,
> 
> Thursday, July 2, 2009, 3:31:59 PM, you wrote:
> 
> > class (Monad m, Monoid v) => MonadInput v m where
> >     -- | Gets an element from input (line of text [with \n], 4096 bytes,
> >     --   or something like that). mzero on end
> >     getChunk :: m v
> > class (Monad m, Monoid v) => MonadOutput v m where
> >     -- | Puts an element
> >     putChunk :: v -> m ()
> 
> how about interact function?
> 

Well. As far as I know there is no way of using it with network.
Additionally there is hard to put monadic code in it:

myFunc :: (MonadInput i, MonadOutput o) =>
          (String -> m a) -> MyMonad i o m [a]

If m == IO - which may be a case in normal code it requires
unsafePerformIO with all it's problems. In testing I can use pipes and
Identity simplifying the whole testing - allowing user to use it's own
monads.

The other problem is that the order sometimes matters. Consider:
main = interact (\x -> "What's your name?\n" ++ "Hello:\n" ++ x)

For human being it is annoying but sometimes it is for example against
RFC.

Regards