[Haskell-beginners] Closing a channel

[Christopher Done]

On 12 May 2011 12:20, Lyndon Maydwell <maydwell at gmail.com> 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]
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