Gregory Collins
I'm trying to solve a very practical problem: I need a stateful iteratee monad transformer. Explicit state passing is very inconvenient and would destroy the elegance of my library.
There are two approaches to this:
1. type MyT a m = Iteratee a (StateT MyConfig m) 2. type MyT a m = StateT MyConfig (Iteratee a m)
Both work well except in two very specific corner cases:
- I need to convert the transformer to 'Iteratee a m', i.e. remove the state layer. This is obviously trivial with the second variant, but seems very difficult with the first one, if it's possible at all.
Why can't you use #1 and do this when you call "run_"?
Because that runs the iteratee and leaves me with a StateT. Even though I use a CPS-based StateT, I doubt that it can be converted back to Iteratee easily. With the first variant, I would need a function like this: runMyApp :: Iteratee a (StateT MyConfig m) b -> Iteratee a m b I think, this function is impossible to write. The reason behind this requirement is that I have multiple monad transformers of this kind, each in different libraries, each with different state types, and I need to compose them. But I have another idea in mind. I could do the following instead: -- First library. class Monad m => OneStateMonad m where mapOneConfig :: (OneConfig -> OneConfig) -> m OneConfig oneComp :: OneStateMonad m => Iteratee Input m Output -- Second library. class TwoStateMonad m where mapTwoConfig :: (TwoConfig -> TwoConfig) -> m TwoConfig twoComp :: TwoStateMonad m => Iteratee Input m Output Then the user of the library has to build the monad transformer stack by themselves, like this: instance Monad m => OneStateMonad (StateT (OneConfig, TwoConfig) m) instance Monad m => TwoStateMonad (StateT (OneConfig, TwoConfig) m) appComp :: Monad m => Iteratee Input (StateT (OneConfig, TwoConfig) m) Output That might work, but it seems to be cumbersome. Greets, Ertugrul -- nightmare = unsafePerformIO (getWrongWife >>= sex) http://ertes.de/