Proposal: Add atomic IORef operations to Data.IORef

[Alexey Kuleshevich]

Despite that this one is easy `atomicSwapIORef' ref !a = atomicSwapIORef ref a` I think it is important to include it. I also think `atomicWriteIORef'` should be added as well. Point is that most of the time in a concurrent setup it is the thread that does the IORef modification should be responsible for computing the value that is being written. This is important not only for performance but also for error handling.

Alexey.


‐‐‐‐‐‐‐ Original Message ‐‐‐‐‐‐‐
On Saturday, January 16, 2021 12:01 AM, David Feuer <david.feuer at gmail.com> wrote:

> Cool. Do you also want a strict atomicSwapIORef'?
>
> On Fri, Jan 15, 2021 at 3:38 PM Alexey Kuleshevich alexey at kuleshevi.ch wrote:
>
> > I totally agree with everything that you just said. Sorry, I should have said originally "-1 for adding JUST these lazy versions". As long as there are strict versions (whatever the names they might get, IMHO Lazy suffix is nice) and a warning on lazy versions about leaking memory, then I am 100% on board as well. Also +1 on not forcing the result.
> > Alexey.
> > ‐‐‐‐‐‐‐ Original Message ‐‐‐‐‐‐‐
> > On Friday, January 15, 2021 11:29 PM, David Feuer david.feuer at gmail.com wrote:
> >
> > > I disagree. I believe we should offer both atomicModifyIORef2 and an
> > > atomicModifyIORef2'. The latter should force the new value in the
> > > IORef but, unlike atomicModifyIORef', should not force the returned
> > > value. Or if you and others prefer, we could offer a strict
> > > atomicModifyIORef and a lazy atomicModifyIORef2Lazy. As a Haskell
> > > programmer, I really don't want totally polymorphic values getting
> > > forced behind my back. It's actually possible to write an even lazier
> > > version that doesn't even force the function result pair, but I don't
> > > think that's a very useful idea.
> > > One other thing: the natural approach to atomicModifyIORef2' uses a
> > > "half-strict pair" type
> > > data HSPair a b = HSPair !a b
> > > atomicModifyIORef2' :: IORef a -> (a -> HSPair a b) -> IO (a, HSPair a b)
> > > I don't know to what extent users are willing to tolerate such an
> > > extra datatype.
> > > Side note: the peculiarly strict behavior of atomicModifyIORef' is a
> > > result of the poor design of the old atomicModifyMutVar# primop.
> > > On Fri, Jan 15, 2021 at 3:19 PM Alexey Kuleshevich alexey at kuleshevi.ch wrote:
> > >
> > > > Both of these functions are lazy with respect to the new value being written into the IORef, which is a horrible default for atomic operations. That is why atomicModifyIORef is a source of memory leaks and terrible performance, which is also why atomicModifyIORef' is almost always used instead.
> > > > So +1 from me on adding strict versions of these functions that force new value to whnf, but -1 for adding these lazy versions as they are currently defined in GHC.IORef
> > > > Alexey.
> > > > ‐‐‐‐‐‐‐ Original Message ‐‐‐‐‐‐‐
> > > > On Friday, January 15, 2021 11:05 PM, David Feuer david.feuer at gmail.com wrote:
> > > >
> > > > > GHC.IORef currently exports
> > > > > atomicModifyIORef2 :: IORef a -> (a -> (a,b)) -> IO (a, (a, b))
> > > > > atomicSwapIORef :: IORef a -> a -> IO a
> > > > > atomicModifyIORef2 is a lot like atomicModifyIORef, but it returns
> > > > > both the old value in the IORef and the full result of applying the
> > > > > user-supplied function to that. This is a pretty thin wrapper around
> > > > > the newish atomicModifyMutVar2# primop, which has replaced the less
> > > > > powerful atomicModifyMutVar#.
> > > > > atomicSwapIORef atomically installs a user-supplied value in an IORef
> > > > > and returns the old value. It is currently implemented using
> > > > > atomicModifyIORef2, but it can and should be reimplemented using its
> > > > > own, more efficient primop.
> > > > > I propose to add both of these functions to Data.IORef.
> > > > > David
> > > > > Libraries mailing list
> > > > > Libraries at haskell.org
> > > > > http://mail.haskell.org/cgi-bin/mailman/listinfo/libraries