Re: [Haskell-cafe] Conduit Error Output: Control.Monad.Trans.Resource.stateCleanup

On Tue, Feb 21, 2012 at 7:40 AM, Michael Snoyman wrote:

On Tue, Feb 21, 2012 at 5:46 AM, Lyndon Maydwell wrote:

...

Hi Michael, Café.

I'm writing some code using the conduit library and am encountering the following error output (while the program appears to function correctly) when using Data.Conduit.Lazy.

The error given is:

...

profile_simple_test_data: Control.Monad.Trans.Resource.stateCleanup: There is a bug in the implementation. The mutable state is being accessed after cleanup. Please contact the maintainers.

A reduced code snippet that generates this error is (also attached):

...

import Control.Monad import System.Environment import Control.Monad.IO.Class (liftIO) import System.IO import Data.Conduit.Lazy import Data.List (sort)

import Data.Conduit

import Prelude hiding (map)

main = getArgs >>= process

process args = mapM_ sorted args

sorted x = runResourceT (lazyConsume $ sourceFeed x) >>= (mapM_ print . id)

sourceFeed :: ResourceIO m => FilePath -> Source m String sourceFeed file = sourceIO     (openFile file ReadMode)     hClose     (\h -> liftIO $ do         eof <- hIsEOF h         if eof             then return IOClosed             else fmap IOOpen $ hGetLine h)

when run over any text file.

I may be doing something inconsistent with the correct use of sourceIO or lazyConsume, however, I tried to follow the example at http://www.yesodweb.com/home/snoyberg/blogs/conduit/conduit/source/source.di... as closely as possible.

Is this a bug, or simply an incorrect use of Conduit?

I haven't fully debugged this yet. There's certainly a bug in the implementation of ResourceT, but the sample program is also wrong. You can't pass the result from a call to lazyConsume outside the scope of its ResourceT; the correct way to write sorted would be:

   sorted x = runResourceT $ lazyConsume (sourceFeed x) >>= mapM_ (liftIO . print)

My guess is that this is a fallout from the transition away from mutable variables: lazyConsume no longer has any way of knowing that its ResourceT has already been terminated. Perhaps a simple solution would be to expose a primitive that checks if the ResourceT block has already been finalized.

Michael

I've added a test case for this bug, and fixed it. The commit is: https://github.com/snoyberg/conduit/commit/87e890fe7ee58686d20cabba15dd37f18... The basic idea is to add an extra constructor to represent when the ResourceT has already been closed, and expose a function resourceActive to check the state. Can you check if this solves your problem? Michael