Can we improve reschedule?

[treeowl]

reschedule queue@Sched{ workpool } = do
  e <- atomicModifyIORef workpool $ \ts ->
         case ts of
           []      -> ([], Nothing)
           (t:ts') -> (ts', Just t)
 --[...]

The pairs will actually be realized, because atomicModifyIORef needs them. But I suspect that atomicModifyIORef is actually overkill in this case. In particular, we only deconstruct ts; we don't have to construct any new values. I suspect, therefore, that casMutVar# can do the job more cheaply. Benchmarking will be needed to test this hypothesis.

[treeowl]

Essentially (making up a casIORef function because I can't remember the argument order or result semantics of casMutVar#)

reschedule queue@Sched{ workpool0 } = do
  go workpool0
  -- ....

  where
     go workpool = do
       ts <- readIORef workpool
       case ts of
         [] -> pure Nothing
         t : ts' -> do
           (r, _) <- casIORef workpool ts ts'
           case r of
             CasSucceeded -> pure (Just t)
             CasFailed -> go workpool