I think it would be nice to summarise the current state of play. This really needs to be a WiKi. Ian's ACIO proposal http://www.haskell.org//pipermail/haskell-cafe/2004-November/007680.html seems in fact to achieve much the same as the proposal I originally posted to http://www.haskell.org//pipermail/haskell/2004-November/014894.html and which Benjamin Franksen has explained and simplified here http://www.haskell.org//pipermail/haskell-cafe/2004-November/007691.html You can in fact do much the same with both. With Haskell 98, Data.Dynamic and ACIO and, if your implementation has concurrency at all, thread identifiers, I can implement execution contexts. On the other hand ACIO allows you to create variables with top-level declarations such as x <- newIORef 0 which can then be accessed in a pure way. With Execution Contexts you cannot do this; you would instead need a function getX :: IO (IORef Int) which would get the global variable x. In practice I don't think this distinction very important, since there's not much you could do with x except using the IO monad. Disadvantages of execution contexts ----------------------------------- 1. You are only allowed one value of each type in an execution context. Thus it is dangerous to try to use a global value of type (IORef Int); people would have to be advised to create a special newtype for each value. 2. They are likely to be less efficient than just allowing <- declarations at top level. Certainly they are in my implementation. The vast majority of this inefficiency can be eliminated by using hash tables, but you can't escape doing at least one hash-table lookup each time you want to access a global value. The bare minimum then would be one division by a constant followed by an indirection. 3. In a concurrent system, you also need forkIO to be modified, so that you can work out which execution context applies to you. Disadvantages of ACIO --------------------- 1. The ACIO proposal preserves one bad feature of unsafePerformIO, namely that you only have one "execution context". You cannot for example run two copies of the same program simultaneously, since they will clobber each other's global variables. 2. I don't know exactly how ACIO is to be implemented, but it looks as if at the least it requires an extension to the Haskell syntax. 3. I think that if the ACIO proposal were implemented, it would in any case be a good idea to implement execution contexts or something like them on top of it, so that people can create initialisation actions which do other things than the very strict subset allowed as ACIO actions. Questions --------- 1. Is there anything people would like to do with ACIO for which execution contexts would not be practical? 2. Which if either of the proposals would the gods responsible for implementing Haskell compilers favour?