[Haskell-cafe] Caching Actions

[Michael Snoyman]

I believe the intention is to ensure that two threads do not perform the
action at the same time. If you look at the implementation of mfix for
IO[1], it's using an MVar. I'm not 100% certain, but I think the `cache`
function above could be rewritten to use MVars explicitly and avoid both
monadic do and unsafe IO functions.

[1] The fixIO function,
https://www.stackage.org/haddock/lts-10.3/base-4.10.1.0/src/System-IO.html#fixIO

On Thu, Jan 25, 2018 at 11:50 AM, Yotam Ohad <yotam2206 at gmail.com> wrote:

> Hi,
>
> I've been digging around the source code of reactive-banana and I found
> this code
> <https://github.com/HeinrichApfelmus/reactive-banana/blob/master/reactive-banana/src/Reactive/Banana/Prim/Cached.hs>:
>
>
> data Cached m a = Cached (m a)
>
> runCached :: Cached m a -> m a
> runCached (Cached x) = x
>
> -- | An action whose result will be cached.
> -- Executing the action the first time in the monad will
> -- execute the side effects. From then on,
> -- only the generated value will be returned.
> {-# NOINLINE cache #-}
> cache :: (MonadFix m, MonadIO m) => m a -> Cached m a
> cache m = unsafePerformIO $ do
>     key <- liftIO $ newIORef Nothing
>     return $ Cached $ do
>         ma <- liftIO $ readIORef key    -- read the cached result
>         case ma of
>             Just a  -> return a         -- return the cached result.
>             Nothing -> mdo
>                 liftIO $                -- write the result already
>                     writeIORef key (Just a)
>                 a <- m                  -- evaluate
>                 return a
>
> I'm trying to understand the reasom behind the use of mdo. Can't it be like this:
>
> do
>   a <- m
>   liftIO $ writeIORef key (Just a)
>   return a
>
> Removing the need for a recursive definition?
>
> Yotam
>
>
> _______________________________________________
> Haskell-Cafe mailing list
> To (un)subscribe, modify options or view archives go to:
> http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe
> Only members subscribed via the mailman list are allowed to post.
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mail.haskell.org/pipermail/haskell-cafe/attachments/20180125/c7cf88f0/attachment.html>