[Haskell-cafe] multiple computations, same input

[Greg Fitzgerald]

Thanks Neil.  How do I add in another ~10 computations, or map a list of a
100 computations to the same input?

Isn't there a way to do this without one computation having to be aware of
the other?

This feels like a situation Parsec users would find themselves in all the
time.  When you have a bunch of parsers in a 'choice', does the start of the
input stream linger until the last parser is executed?

Thanks,
Greg


On 3/27/06, Neil Mitchell <ndmitchell at gmail.com> wrote:
>
> Hi,
>
> > Here would be a better example then.
> >
> >     f lst = show (sum (filter (> 1) lst), sum (filter (> 2) lst))
>
> I suspected that you actually wanted to do something "cleverer" with
> the list, for the sake of argument, I'm going to change >1 to p1 and
> >2 to p2 - to show how this can be done in the general case. With the
> specific information you know about >1 vs >2 you can do better, but
> this gets across the general point:
>
> f lst = show (sumPairs (>1) (>2) lst)
>
> sumPairs :: (Int -> Bool) -> (Int -> Bool) -> [Int] -> (Int, Int)
> sumPairs p1 p2 [] = (0, 0)
> sumPairs p1 p2 (x:xs) = (add p1 a, add p2 b)
>     where
>        (a,b) = sumPairs xs
>        add pred value = if pred x then x+value else value
>
> [Untested, something like this should work]
>
> You can actually arrive at this solution entirely be reasoning on the
> program, i.e. not coming up with a fresh definition.
>
> The above code essentially follows your imperative pseudo code - I
> think its constant space, but I'm not too sure...
>
> Thanks
>
> Neil
>
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