In the stm package, the docs for atomically read:

> You cannot use 'atomically' inside an 'unsafePerformIO' or 'unsafeInterleaveIO'. Any attempt to do so will result in a runtime error. (Reason: allowing this would effectively allow a transaction inside a transaction, depending on exactly when the thunk is evaluated.)

This doesn't seem to be true. The following program runs fine:

import Control.Monad.STM

import Control.Concurrent.STM.TVar

import System.IO.Unsafe

main :: IO ()

main = do

v <- atomically $ newTVar (7 :: Int)

print $ unsafePerformIO $ atomically $ do

readTVar v

I suspect that the runtime only gives you an error if you actually create a nested transaction. Is my understanding correct?

-Andrew Thaddeus Martin