Proposal: Add an atomic IORef replacement operation

Dr.Koster drkoster at
Sun Jun 24 23:38:06 UTC 2018

+1, BTW. let's call it atomicSwapIORef to keep sync with MVar/TVar's counterpart.


------------------ Original ------------------
From: David Feuer <david.feuer at>
Date: Sat,Jun 23,2018 4:15 PM
To: Haskell Libraries <libraries at>
Subject: Re: Proposal: Add an atomic IORef replacement operation

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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