Ah yes of course. I was just wondering if there was a way to do it without the overhead of using Maybe. I guess it makes sense that there isn't. On Fri, May 13, 2011 at 4:03 PM, Christopher Done wrote:

On 12 May 2011 12:20, Lyndon Maydwell wrote:

...

Hi Beginners.

I'm trying to implement a program that accumulates a list of items through many IO actions, then use these in another part of the program as a list.

I've found Control.Concurrent.Chan which seems to provide a nice abstraction with getChanContents :: Chan a -> IO [a], but there doesn't seem to be a way for the source to close the channel.

Are channels the right abstraction for something like this? Should look in to (itter|enumer)at(or|ee)s instead... They seem to encompass this model, but look quite involved.

For what it's worth you can implement this abstraction ontop of channels. module Control.Concurrent.Chan.Closeable where import           Control.Concurrent.Chan (Chan) import qualified Control.Concurrent.Chan as C import           Data.Maybe getChanWhileJust :: Chan (Maybe a) -> IO [a] getChanWhileJust ch = fmap (catMaybes . takeWhile isJust) (C.getChanContents ch) -- λ> chan <- newChan :: IO (Chan (Maybe Integer)) -- λ> writeList2Chan chan (map Just [1..10] ++ [Nothing]) -- λ> getChanWhileJust chan -- [1,2,3,4,5,6,7,8,9,10]