[Haskell-beginners] Closing a channel

[Lyndon Maydwell]

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
<chrisdone at googlemail.com> wrote:
> 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]