Re: Proposal: Add atomic IORef operations to Data.IORef

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 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 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

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