What is the purpose of the extra result parameter of atomicModifyIORef?

Question

The signature of modifyIORef is straightforward enough:

modifyIORef :: IORef a -> (a -> a) -> IO ()

Unfortunately, this is not thread safe. There is an alternative that adresses this issue:

atomicModifyIORef :: IORef a -> (a -> (a,b)) -> IO b

What exactly are the differences between these two functions? How am I supposed to use the b parameter when modifying an IORef that might be read from another thread?

Answer 1

As you stated in a comment, without concurrency you'd be able to just write something like

modifyAndReturn ref f = do
  old <- readIORef ref
  let !(new, r) = f old
  writeIORef r new
  return r

But in a concurrent context, someone else could change the reference between the read and the write.