On 2/23/07, Greg Fitzgerald
I want to write a program where a user would update a bunch of variables, and everything that depends on those variables (and nothing else) are recalculated. Basically, a spreadsheet, but generalized for any computation. Could someone recommend an elegant way to do it or some good reading material?
Off the top of my head, one cool way of doing it would be to have each variable be a separate thread (they're lightweight, so it's okay! :-)), and you would have a list of input connections and output connections, in the form of TVars. In its resting state you would have a transaction which simply reads all of the input variables, and returns the new list of inputs if and only if either of them are changed, something like: -- not compiled, consider it pseudocode :-) getInputs :: (Eq a ) => [( a, TVar a)] -> STM [a] getInputs inp = do let (vals, vars) = unzip inp newVals <- mapM readTVar vars when ( newVals /= vals ) retry -- at least one input must've been changed return vals This takes a list of "current values" zipped with the corresponding TVars, and would block until at least one input has changed. So once you get the values back from the function above, you know one of them has change, and you simply recompute the new value from the inputs, and plug the output into the output TVars and all the dependent "nodes" will automatically be unblocked and propagate its changes by virtue of the GHC scheduler. No coding required! At this stage you'd probably want to update some GUI control as well to display the new state. -- Sebastian Sylvan +46(0)736-818655 UIN: 44640862