2008/2/7, Jeff φ
I played with your foldl1MArray' last night. I noticed it can be reduced to . . .
foldl1MArray' :: (MArray a e m, Ix i) => (e -> e -> e) -> a i e -> m e foldl1MArray' f a = do (l,u) <- getBounds a foldl1' (liftM2 f) (map (readArray a) (range (l,u)))
Unfortunately, my new version consumes a lot of stack and heap space. Why is this so inefficient? Is there a simple change that will make it efficient?
This code don't compute the results incrementally, it can't because foldl1' is not aware of the monad, it only construct a huge action in m which is then run. foldM advantage is that it can run incrementally. Which is not to say my code was the best possible (far from it), already the following would have been better :
foldlA f a arr = getBounds arr >>= foldM (\a->a `seq` liftM $ f a) a . map (readArray arr) . range
foldl1A f arr = getBounds arr >>= readArray arr . fst >>= flip (foldlA f) arr
-- Jedaï