IO and STM

[Robert van Herk]

Hi all,

I have a question about IO in a STM transaction.

I want to make an atomic algorithm that does the following:

1. read an IORef
2. write some changes to the value you get from 1
3. save the IORef again

Currently, I did this, roughly like this:

doSomething :: IORef [s] -> s -> IO ()
doSomething ref s =
  atomically (
    do { ss <- unsafeIOToSTM (readIORef s);
         do something with ss;
         unsafeIOToSTM (writeIORef s ss);
         return ()
       }
  )

Since I read that STM uses a revertable log, I was wondering what 
happens with the IO actions, when a rerun on the atomic part is 
performed... I guess it will just redo them? Is this the unsafe-part of 
unsafeIOToSTM, or are there more caveats?

Regards,
Robert