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

[Sebastian Sylvan]

On 2/23/07, Sebastian Sylvan <sebastian.sylvan at gmail.com> wrote:
> On 2/23/07, Greg Fitzgerald <garious at gmail.com> 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...

-- 
Sebastian Sylvan
+46(0)736-818655
UIN: 44640862