On Thu, 2011-04-07 at 19:04 +0200, Ertugrul Soeylemez wrote:
Hello fellow Haskellers,
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.
- I need to use control structures of Iteratee like catchError. This is obviously trivial with the first variant, but very inconvenient with the second, because I would need to reinvent many wheels.
Does someone know a cleaner, more elegant solution? Encapsulating the state in the iteratee's input type is not an option.
Many thanks in advance.
The first thing that come to my mind. runWithState :: Iteratee a (StateT s m) b -> s -> Iteratee a m (b, s) runWithState i s = do let onDone v st = return (Right (v, st)) onCont c err = return (Left (c, err)) (i', s') <- runStateT (runIter i onDone onCont) s case i' of Left (c, err) -> icont (\str -> runWithState (c str) s') err Right (v, st) -> idone (v, s') st I believe it is equivalent to: runWithState :: Iteratee a (StateT s m) b -> s -> Iteratee a m (b, s) runWithState i s = do let onDone v st = do s' <- get return (idone (v, s') st) onCont c err = do s' <- get return (icont (\str -> runWithState (c str) s') err) joinIM $ evalStateT (runIter i onDone onCont) s I haven't tested but it compiles so it should work.