On Thu, Jan 12, 2012 at 6:48 AM, Johan Brinch wrote:

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?

Where is 'evalCacheSTM' defined?

The behaviour is consistent on GHC 7.4.0 RC and GHC 7.2.2 (stable).

That function that's being run can be found here: https://gist.github.com/30b94760abc27b05ec7c

And here is the last output from the runtime system: https://gist.github.com/4356ae541895becb4169

Any ideas to track this down will be greatly appreciated!

-- Johan Brinch, Dept. of Computer Science, University of Copenhagen

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On 12/01/2012 12:48 PM, Johan Brinch wrote:

I'm not using any unsafe IO

OK, good...

(only for debug printing)

...ah. It seems we have reached a contradiction.

Where is a transaction being nested?

My guess is that your "debug printing" is causing something to be evaluated when it otherwise wouldn't be, causing a transaction to begin within a transaction. I could, however, be horribly wrong about that...

On Thu, Jan 12, 2012 at 7:48 AM, Johan Brinch wrote:

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