Stephan Friedrichs-2 wrote:

it's unsafe to perform IO inside of a transaction as it can't be undone, when rolling it back. I guess, unsafeIOToSTM has been designed in order to allow us to inject debugging output into a transaction, but you really shouldn't use it to perform "real" IO (like writing files, etc.).

Simon Peyton Jones provides a good example of this in http://research.microsoft.com/~simonpj/papers/stm/beautiful.pdf

atomically (do { x <- readTVar xv ; y <- readTVar yv ; if x>y then launchMissiles else return () })

where launchMissiles :: IO () causes serious international side-effects.

;-) -- Ricardo Guimarães Herrmann "There are only two industries that refer to their customers as 'users'" -- Edward Tufte -- View this message in context: http://www.nabble.com/question-about-STM-and-IO-tp15439579p15481669.html Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

On 2/20/08, Bulat Ziganshin wrote:

STM operations can be repeated if first transaction was unsuccessful. so, you may se here only operations that may be safely repeated - say, reading/writing memory areas, or reading/writing files, or even sending network message as long as its duplication is ok

Actually, you have to be even more careful than that; you may only include not just operations that may be safely repeated, but operations that may be erroneously executed. Consider the following snippet: badNews:: TVar Int -> TVar Int -> IO () badNews xRef yRef = atomically $ do x <- xRef y <- yRef if (x < y) then unsafeIOToSTM launchMissiles else return () (where launchMissiles has serious side effects, but can be called repeatedly without problem; the missiles will already have been launched in the second call). Even if (x < y) is never "atomically" true, launchMissiles could get executed during the evaluation of this STM action if it was run concurrently with another action that wrote new values to x and y in some order, such as the following snippet safe :: TVar Int -> TVar Int -> IO () safe xRef yRef = do atomically $ do writeTVar xRef 15 writeTVar yRef 13 main :: IO () main = do xRef <- newTVar 10 yRef <- newTVar 8 forkIO safe forkIO badNews If "badNews" runs to the point of reading from xRef, then "safe" runs in its entirety and commits successfully, then badNews resumes, launchMissiles will get called and then badNews will fail to commit and be restarted. The second runthrough of badNews will read the values, determine that x >= y, and just return (), but it's too late, the missiles have already been launched. -- ryan

Hello Ryan, Thursday, February 21, 2008, 5:02:52 AM, you wrote:

values, determine that x >= y, and just return (), but it's too late, the missiles have already been launched.

it seems that "asymmetrical answer" of mr.Putin is just to hire a bit more Haskell Hackers :) -- Best regards, Bulat mailto:Bulat.Ziganshin@gmail.com