Anyone have thoughts to share? I'd love to read others' experiences but there isn't much coming up with searches or on redditt ... I was happiest with the VM I implemented. Sadly, I wasn't able to solve any of the scenarios, but my VM ran damn fast. That didn't seem to matter so much this year as it did two years ago - I think I am still scarred by Endo's DNA .. Anyways, for those who care, the heart of my VM implementation was a monadic fold over the program, with a mutable array representing the machine's memory, all inside ''runSTUArray.'' I used a simple data type to represent the machine: data Machine = Machine { instrs :: [OpCode] , dataMem :: UArray Int Val , outputs :: UArray Int Val , status :: !Int , inputs :: IntMap Val } where ''dataMem'' holds the data memory area. To execute a program, I folded over the ''instrs'' list and updated memory as I went: exec :: Machine -> Input -> Machine exec m@(Machine { instrs = instrs }) inps = let m' = m { inputs = readInputs inps } newMem = runSTUArray $ do mem' <- thaw (dataMem m) foldM (step m') mem' (zip instrs [0..]) I considered ''unsafeThaw'' in the above, but performance didn't hurt me much and it seemed safer. Since I was recording all output I was worried that would break referential integrity. In any case, when the fold was finished I just assigned ''newMem'' to the new machine and moved to the next iteration. ''step'' takes the mutable array and updates it while the program executes. I won't show the whole body, as it just a separate definition for each opcode, just the type: step :: Machine -> STUArray s Int Val -> (OpCode, Addr) -> ST s (STUArray s Int Val) The only downside to this approach was the machine had three mutuable states: memory, output, and status. runSTUArray only wants to return a single array so inside step I had to hold all values in one array. Not a big deal but a little ugly. I kept a diary of my experience over the weekend at http://icfp2009.codeslower.com. I started out with such optimism ... Stil, I'm glad to have done it. Lots of fun! My thanks to the organizers!