[Haskell-cafe] vector operations

[Jake McArthur]

Have you already verified that stream fusion won't just do this for you?
On May 23, 2012 12:35 AM, "Evan Laforge" <qdunkan at gmail.com> wrote:

> So I wanted to find the first index in a vector whose running sum is
> greater than a given number.
>
> The straightforward way is to create the running sum and then search:
>
> Vector.findIndex (>=target) (Vector.scanl' (+) 0 vector)
>
> But vectors are strict so it could do extra work, and what if I don't
> want to generate garbage?  I could do it with a fold, but it would
> have to have the ability to abort early.  Of course I could write such
> a fold myself using indexing:
>
> import qualified Data.Vector.Generic as Vector
>
> fold_abort :: (Vector.Vector v a) => (accum -> a -> Maybe accum) -> accum
>    -> v a -> accum
> fold_abort f accum vec = go 0 accum
>    where go i accum = maybe accum (go (i+1)) $ f accum =<< vec Vector.!? i
>
> find_before :: (Vector.Vector v a, Num a, Ord a) => a -> v a -> Int
> find_before n = fst . fold_abort go (0, 0)
>    where
>    go (i, total) a
>        | total + a >= n = Nothing
>        | otherwise = Just (i+1, total+a)
>
> So it's bigger and clunkier, but I would think it would be much more
> efficient (provided using Data.Vector.Generic won't inhibit inlining
> and unboxing).  But I'm a bit surprised there isn't already something
> like fold_abort... or is there?
>
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