Re[2]: [Haskell-cafe] question about STM and IO
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, Bulatmailto:[EMAIL PROTECTED] ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] question about STM and IO
Hello John, Tuesday, February 12, 2008, 9:28:22 PM, you wrote: I was recently looking at the STM library, and I have a question about the function unsafeIOToSTM. Can anyone explain to me what is unsafe about it, and what sort of use would be considered safe? 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 -- Best regards, Bulatmailto:[EMAIL PROTECTED] ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] question about STM and IO
On 2/20/08, Bulat Ziganshin [EMAIL PROTECTED] 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 ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] question about STM and IO
I take it that this follows from the lack of any mechanism to rollback IO? If so, I think that the following guidelines suffice for when it's acceptable to use unsafeIOtoSTM: 1. The IO action must be able to be safely repeated. 2. The IO action must be able to be safely performed with possibly-incorrect arguments, even if it isn't supposed to be performed. 3. Don't try to nest transactions. If I understand it correctly, I think that covers it. Thanks to everyone who answered; I really appreciate it. John On Wed, Feb 20, 2008 at 8:02 PM, Ryan Ingram [EMAIL PROTECTED] wrote: On 2/20/08, Bulat Ziganshin [EMAIL PROTECTED] 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: ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] question about STM and IO
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 xy then launchMissiles
else return () })
where launchMissiles :: IO () causes serious international side-effects.
;-)
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[Haskell-cafe] question about STM and IO
Hello, I was recently looking at the STM library, and I have a question about the function unsafeIOToSTM. Can anyone explain to me what is unsafe about it, and what sort of use would be considered safe? Thanks, John ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] question about STM and IO
Thanks. Both this answer and Luke's make perfect sense. John On Feb 12, 2008 12:41 PM, Stephan Friedrichs [EMAIL PROTECTED] wrote: Hello, John Lato wrote: I was recently looking at the STM library, and I have a question about the function unsafeIOToSTM. Can anyone explain to me what is unsafe about it, and what sort of use would be considered safe? 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.). HTH - Stephan -- Früher hieß es ja: Ich denke, also bin ich. Heute weiß man: Es geht auch so. - Dieter Nuhr ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] question about STM and IO
Hello, John Lato wrote: I was recently looking at the STM library, and I have a question about the function unsafeIOToSTM. Can anyone explain to me what is unsafe about it, and what sort of use would be considered safe? 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.). HTH - Stephan -- Früher hieß es ja: Ich denke, also bin ich. Heute weiß man: Es geht auch so. - Dieter Nuhr signature.asc Description: OpenPGP digital signature ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] question about STM and IO
On Feb 12, 2008 6:28 PM, John Lato [EMAIL PROTECTED] wrote: Hello, I was recently looking at the STM library, and I have a question about the function unsafeIOToSTM. Can anyone explain to me what is unsafe about it, and what sort of use would be considered safe? A few things. First, this will crash: atomically $ unsafeIOToStm $ atomically $ return () The implementation requires that transactions must not be nested, and this function allows you to nest transactions. Also, it's unclear what retry means in the context of arbitrary IO actions. A memory transaction is really only defined for exactly that: memory stuff. When you start talking about user I/O, network, etc. the transactional idea breaks down. So mostly the way I think of it is that it's semantically nonsense. Luke ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
