Provide loop wrapper for casArrayElem? about haskell-lockfree HOT 2 OPEN

Unfortunately, this can't be implemented terribly efficiently using the low-level atomic operations. Some CMM code will be needed to do it optimally. Given atomicModifyIORef ref f, GHC creates a closure (let's call it q) representing f applied to the old value and two closures to fetch the first and second components of q. Writing in Haskell, we'd have to create all three of these closures anew each time through the loop, but the primop implementation can actually just mutate q to replace the function argument. We could, I believe, simulate something like this by using an extra IORef, but that's not likely to be terribly efficient.

from haskell-lockfree.

I just realized you were probably looking for something like atomicModifyIORefCAS, but for arrays. Silly me. That only works okay for very cheap functions, but it's an option.

from haskell-lockfree.