Re: Control.Concurrent.Chan: how do i close a channel?

Aaron Denney wrote:

On 2007-07-17, David Menendez wrote:

...

On 7/16/07, Claus Reinke wrote:

...

...

I often use Chan (Maybe a), with Nothing to tell the reader thread that EOF is reached -- perhaps something like that is what you're looking for? yes. but that would add another slight indirection, and it still doesn't make getChanContents itself any more useable. if you "often" have to modify/expand the API when you use it, perhaps there is something missing in that API? the "much like hGetContents" comment does seem to suggest that as well. If STM is available, you could use a TChan for content and a TVar for signalling.

That seems excessive. STM has nice composable properties but if you're not composing it with other STM usages, there's not much reason to buy those properties.

...

getClosableChanContents :: Chan (Maybe a) -> IO [a] getClosableChanContents ch = unsafeInterleaveIO $ do x <- readChan ch case x of Nothing -> return [] Just y -> do ys <- getClosableChanContents ch return (y : ys)

Untested of course. With the corresponding

...

writeList2CChan :: Chan (Maybe a) -> [a] -> IO () writeList2CChan ch ls = do sequence_ (map (writeChan ch) . (Just)) ls) writeChan ch Nothing

Am I missing something that makes this "not lazy enough"?

The above costs the construction of the Maybe data for each item going through the channel. Using an MVar instead of a TVar costs taking the MVar on each read. Using STM is optimistic, it performs the read on the channel and then a check that nothing was committed in the mean time (unlikely given the short atomic block). Benchmarking the three idioms would be a useful service, but I don't have time. -- Chris