Proposal: Add an atomic IORef replacement operation

David Feuer david.feuer at
Sat Jun 23 08:14:51 UTC 2018

Currently, atomicWriteIORef is implemented using atomicModifyIORef. This is
not ideal: the latter allocates memory while the former shouldn't need to.
In the process of writing alternative implementations, I realized that we
can efficiently support an atomic operation that replaces the value in an
IORef, returning the previous value. I propose that we add, to Data.IORef,
a function to do so:

atomicReplaceIORef :: IORef a -> a -> IO a
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