Re: [Haskell-cafe] How to build a generic spreadsheet?

On 2/23/07, Sebastian Sylvan wrote:

On 2/23/07, Sebastian Sylvan wrote:

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On 2/23/07, Greg Fitzgerald wrote:

...

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

Bah, this should be:

when ( newVals == vals ) retry

Of course...

*sigh* and you should return "newVals" not "vals".. It's too late :-) -- Sebastian Sylvan +46(0)736-818655 UIN: 44640862