On Thu, Apr 7, 2011 at 7:35 PM, Ertugrul Soeylemez <e...@ertes.de> wrote:
>> 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
Let me rephrase my question: why do you need a function like this?
Anyways, something like this is definitely doable (using enumerator
formulation, sorry), but your type needs to be this:
runStateIteratee :: Monad m => Iteratee a (StateT s m) r -> s ->
Iteratee a m (r, s)
See runStateT for a parallel. The implementation isn't even too bad:
-----------------------------------------------------------------------------------------------------
{-# LANGUAGE OverloadedStrings #-}
import qualified Data.ByteString.Char8 as S
import Data.ByteString.Char8 (ByteString)
import Data.Enumerator hiding (head)
import qualified Data.Enumerator.List as EL
import Control.Monad.State
import Control.Monad.Trans
mapStep :: Monad m => Step a (StateT s m) r -> s -> Step a m (r, s)
mapStep (Yield x r) st = Yield (x, st) r
mapStep (Error e) _ = Error e
mapStep (Continue k) st = Continue $ \str -> Iteratee $ do
(step, st') <- runStateT (runIteratee (k str)) st
return $ mapStep step st'
runStateIteratee :: Monad m => Iteratee a (StateT s m) r -> s ->
Iteratee a m (r, s)
runStateIteratee iter st = do
(step, st') <- lift $ runStateT (runIteratee iter) st
returnI $ mapStep step st'
-- example
byteCounter :: Monad m => Iteratee ByteString (StateT Int m) ()
byteCounter = do
EL.head >>= maybe (return ())
(\x -> do
lift $ modify (S.length x +)
byteCounter)
main :: IO ()
main = do
(_, c) <- run_ $ enumList 1 input $$ iter
putStrLn $ "count was " ++ show c
where
iter = runStateIteratee byteCounter 0
input = [ "The quick "
, "brown fox "
, "jumped "
, "over the lazy "
, "dog" ]
-----------------------------------------------------------------------------------------------------
G.
--
Gregory Collins <g...@gregorycollins.net>
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