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 wrote:

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: