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

Hi Michael, The behaviour of my original code has now changed to output nothing with no errors. I'm not sure of the significance of this as my code was incorrect, however, using the code you demonstrated gives the desired results. Thanks for the blindingly quick response! On Tue, Feb 21, 2012 at 3:30 PM, Michael Snoyman wrote:

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

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

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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:

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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):

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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