Re: [Haskell-cafe] how to optmize this code?

Am 31.03.2011 05:59, schrieb Felipe Almeida Lessa:

On Wed, Mar 30, 2011 at 2:39 PM, Gilberto Garcia wrote:

...

fkSum :: Int -> [Int] -> Int fkSum a [] = 0 fkSum a (b) = foldl (+) 0 (filter (\x -> isMultiple x b) [1..a])

Daniel Fischer and Yves Parès gave you good suggestions about implementing a different, better algorithm for you problem. However, there's one small thing about your current code. Instead of foldl, you should use foldl' (use "import Data.List"), which is strict in the accumulator. Most of the time you want foldl' instead of foldl. You can learn more about the list folds here [1].

Since we don't have a function sum' in the Prelude (should we have it?) I wonder what happens if you just use "sum". Will the "sum" (based on sum' so without -DUSE_REPORT_PRELUDE) be strict enough? #ifdef USE_REPORT_PRELUDE sum = foldl (+) 0 product = foldl (*) 1 #else sum l = sum' l 0 where sum' [] a = a sum' (x:xs) a = sum' xs (a+x) product l = prod l 1 where prod [] a = a prod (x:xs) a = prod xs (a*x) #endif Cheers C. P.S. isMultiple a = any ((== 0) . mod a)

HTH,

[1] http://www.haskell.org/haskellwiki/Foldr_Foldl_Foldl%27