[Haskell-cafe] Running an action periodically

[Erèbe]

You have the auto-update package that provide that kind of functionnality
https://hackage.haskell.org/package/auto-update-0.1.4/
docs/Control-AutoUpdate.html

2017-11-24 21:53 GMT+01:00 Tristan Seligmann <mithrandi at mithrandi.net>:

> I just wrote this helper:
>
> > import Control.Concurrent
> > import Control.Concurrent.STM
> > import Control.Exception.Safe
> >
> > pollT :: Int -> IO a -> IO (STM (Maybe a), Async b)
> > pollT delay act = do
> >   tv <- atomically (newTVar Nothing)
> >   as <-
> >     async . forever $ do
> >       r <- tryAny act
> >       case r of
> >         Left _ -> pure ()
> >         Right r' -> atomically (writeTVar tv (Just r'))
> >       threadDelay delay
> >   pure (readTVar tv, as)
>
> I was sort of surprised not to find something like this in an existing
> library.
>
>    1. Did I miss an existing implementation?
>    2. Any problems with this one?
>    3. Any suggestions for a better name?
>    4. Any thoughts on the general idea? That is, "run an action
>    periodically, updating a TVar with the result".
>
> There's a couple of obvious variations that can be built on top of this,
> like retrying if the TVar is Nothing when reading from it, or writing
> Nothing to the TVar when the action fails rather than keeping the old
> value. Maybe passing in the old value to the action?
>
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