On 17/04/2010, at 13:32, Ben wrote:
module Main where
import qualified Data.Vector.Generic as V import qualified Data.Vector.Unboxed as UV
type Vec = UV.Vector Double
axpy :: Double -> Vec -> Vec -> Vec axpy a x y = V.zipWith (+) (V.map (* a) x) y
sumVecs :: [(Double, Vec)] -> Vec sumVecs axs = let (a, x) = head axs in foldl adder (V.map (* a) x) (tail axs) where adder :: Vec -> (Double, Vec) -> Vec adder v1 (a, x) = axpy a x v1
how to i write this in a way which ensures recycling / fusion, e.g. in-place updates?
Unfortunately, recycling won't help you here. It is a purely local optimisation which doesn't work across function boundaries (inlining notwithstanding) and recursive calls. Your best bet is to use a mutable vector and do the fold in the ST monad. That said, it would be quite possible to provide something like the following: fold_inplace :: Vector v a => (v a -> b -> v a) -> v a -> [b] -> v a This could use the recycling framework to safely do as much in-place as possible while still preserving a purely functional interface. I have to think about it. Really, this looks like just a poor man's substitute for linear types. Roman