Re: Inaccurate docs for atomically

Thanks everyone for feedback on this. I’ve opened a PR at https://github.com/ghc/ghc/pull/87 and any commentary on the haddock changes there is appreciated. Sent from my iPhone

On Nov 12, 2017, at 11:17 AM, Andrew Martin wrote:

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