[Haskell-cafe] Re: Simple Fudgets-question

Henrik Berg wrote:

Thomas Hallgren writes:

...

...

... All I want to do is to resend the input out on the output.

If that is all you want, this combinator is the right choice:

throughF :: F a b -> F a (Either b a)

Yes, but (Either b a) won't give me the input _together_ with the output, will it? I need a fudget that accepts some input, does some work on it, and outputs the result _and_ the original input for further processing by other fudgets. As far as I can see, this means I need a tupple (a, b), and not the disjoint sum (Either a b).

Aha, I suspected you wanted something more :-) Fudgets are asynchronous in nature, so there is no single right way to merge two streams into a stream of pairs, and I guess that is why there is no combinator for it in the library. Here is some code I found in an old example, illustrating one way to do it... collectF :: F (Either a b) (a,b) collectF = absF (collectSP Nothing Nothing) collectSP :: Maybe a -> Maybe b -> SP (Either a b) (a,b) collectSP x y = case (x,y) of (Just x,Just y) -> putSP (x,y) (collectSP Nothing Nothing) _ -> getSP (\msg -> case msg of Right y -> collectSP x (Just y) Left x -> collectSP (Just x) y ) -- This is how collectF was used: --fileF = collectF>==<throughF fileDialogF>==