Deprecated re-exports of IORef functions under the old CRef names.

In a concurrent program, CRef operations may appear out-of-order to another thread, depending on the memory model of the underlying processor architecture. For example, on x86 (which uses total store order), loads can move ahead of stores. Consider this example:

crefs :: MonadConc m => m (Bool, Bool)
crefs = do
  r1 <- newCRef False
  r2 <- newCRef False

  x <- spawn $ writeCRef r1 True >> readCRef r2
  y <- spawn $ writeCRef r2 True >> readCRef r1

  (,) <$> readMVar x <*> readMVar y

Under a sequentially consistent memory model the possible results are (True, True), (True, False), and (False, True). Under total or partial store order, (False, False) is also a possible result, even though there is no interleaving of the threads which can lead to this.

We can see this by testing with different memory models:

> autocheckWay defaultWay SequentialConsistency relaxed
[pass] Never Deadlocks
[pass] No Exceptions
[fail] Consistent Result
       (False,True) S0---------S1----S0--S2----S0--

       (True,True) S0---------S1-P2----S1---S0---

       (True,False) S0---------S2----S1----S0---
False

> autocheckWay defaultWay TotalStoreOrder  relaxed
[pass] Never Deadlocks
[pass] No Exceptions
[fail] Consistent Result
        (False,True) S0---------S1----S0--S2----S0--

        (False,False) S0---------S1--P2----S1--S0---

        (True,False) S0---------S2----S1----S0---

        (True,True) S0---------S1-C-S2----S1---S0---
False

Traces for non-sequentially-consistent memory models show where writes to CRefs are committed, which makes a write visible to all threads rather than just the one which performed the write. Only writeCRef is broken up into separate write and commit steps, atomicModifyCRef is still atomic and imposes a memory barrier.