Re: [Haskell-cafe] How to write Source for TChan working with LC.take?

Thanks! I just read your article. I think your proposal is rational, useful and so brilliant! The new yield/await style would make writing conduits much easier. Thank you again for taking so much time for this problem! On 2012/05/29, at 22:14, Michael Snoyman wrote:

OK, after thinking on this for the past week, I've come up with a proposal to make this kind of code easier to write (and more of an explanation on why the behavior was unintuitive in the first place).

http://www.yesodweb.com/blog/2012/05/next-conduit-changes

Do you think the modified yield/await would be a good solution to the problem?

Michael

On Mon, May 21, 2012 at 6:07 AM, Michael Snoyman wrote:

...

I agree that this behavior is non-intuitive, but still believe it's the necessary approach. The short answer to why it's happening is that there's no exit path in the yield version of the function. To understand why, let's expand the code a little bit. Realizing that

liftIO = lift . liftIO

and

lift mr = PipeM (Done Nothing `liftM` mr) (Finalize mr)

we can expand the yield version into:

sourceTChanYield2 ch = forever $ do let action = liftIO . atomically $ readTChan ch ans <- PipeM (Done Nothing `liftM` action) (FinalizeM action) yield ans

So the first hint that something is wrong is that the finalize function is calling the action. If you try to change that finalize action into a no-op, e.g.:

sourceTChanYield3 :: MonadIO m => TChan a -> Source m a sourceTChanYield3 ch = forever $ do let action = liftIO . atomically $ readTChan ch ans <- PipeM (Done Nothing `liftM` action) (return ()) yield ans

then you get an error message:

test.hs:36:53: Could not deduce (a ~ ())

The problem is that, as the monadic binding is set up here, the code says "after running the PipeM, I want you to continue by yielding, and then start over again." If you want to expand it further, you can change `forever` into a recursive call, expand `yield`, and then expand all the monadic binding. Every finalization call is forcing things to keep running.

And remember: all of this is the desired behavior of conduit, since we want to guarantee finalizers are always called. Imagine that, instead of reading data from a TChan, you were reading from a Handle. In the code above, there was no way to call out to the finalizers.

Not sure if all of that rambling was coherent, but here's my recommended solution. What we need is a helper function that allows you to branch based on whether or not it's time to clean up. `lift`, `liftIO`, and monadic bind all perform the same actions regardless of whether or not finalization is being called. The following code, however, works correctly:

liftFinal :: Monad m => m a -> Finalize m () -> (a -> Source m a) -> Source m a liftFinal action final f = PipeM (liftM f action) final

sourceTChanYield :: Show a => MonadIO m => TChan a -> Source m a sourceTChanYield ch = liftFinal (liftIO . atomically $ readTChan ch) (return ()) $ \ans -> do yield ans sourceTChanYield ch

Michael

On Sun, May 20, 2012 at 4:22 PM, Hiromi ISHII wrote:

...

Oops, sorry. The last case's behaviour was not as I expected... A correct log is below:

~~~~ ghci> sourceTChanRaw ch $$ LC.isolate 10 =$= LC.mapM_ print () () () () () () () () () () ghci> sourceTChanState ch $$ LC.isolate 10 =$= LC.mapM_ print () () () () () () () () () () ghci> sourceTChanYield ch $$ LC.isolate 10 =$= LC.mapM_ print () () () () () () () () () () *blocks* ~~~~

So again, sourceTChanYield blocks here even if it is already supplied with enough values!

-- Hiromi ISHII konn.jinro@gmail.com

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-- Hiromi ISHII konn.jinro@gmail.com