On Thu, Jan 12, 2012 at 7:48 AM, Johan Brinch
Hi all,
I'm seeing the "Control.Concurrent.STM.atomically was nested" error, but I just can't figure out what's happening. I'm not using any unsafe IO (only for debug printing), and the test program is only running one thread. Where is a transaction being nested?
What are the scenarios where this error is reported?
I might as well state the obvious. The type system prevents this from happening under normal circumstances. However, it is possible to circumvent the type system using, for example, unsafePerformIO or unsafeInterleaveIO: nest1 :: IO () nest1 = let x = unsafePerformIO $ atomically $ return () in atomically (x `seq` return ()) nest2 :: IO () nest2 = do x <- unsafeInterleaveIO $ atomically $ return () atomically (x `seq` return ()) In both nest1 and nest2, x is a thunk whose evaluation performs an STM transaction. In both cases, this produces an "atomically was nested" error. On GHC, the Debug.Trace functions internally call a C function called debugBelch (defined in RtsMessages.c). These don't appear to use 'atomically' at all. Thus, I doubt using trace will produce an "atomically was nested" error. - Joey