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

Hi Guys, Thanks all for the suggestions, I have certainly improved my knowledge. I made a blog post to show all the possible solution a problem can have. you can check it out at katacoder.blogspot.com Giba On Sun, Apr 10, 2011 at 3:35 AM, Johan Tibell wrote:

Hi Gilberto,

On Wed, Mar 30, 2011 at 4:39 PM, Gilberto Garcia wrote:

...

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

isMultiple :: Int -> [Int] -> Bool isMultiple a [] = False isMultiple a (x:xs) = if (mod a x == 0) then True else isMultiple a xs

You can make both these functions a little bit more efficient by making them strict in the first argument, like so:

{-# LANGUAGE BangPatterns #-}

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

isMultiple :: Int -> [Int] -> Bool isMultiple !a [] = False isMultiple a (x:xs) = if (mod a x == 0) then True else isMultiple a xs

This change ensures that the first argument is always evaluated. Before `fkSum undefined []` would return 0, now it results in an error. The upside is that when a function is strict in an argument, GHC can use a more efficient calling convention for the function. In this case it means that instead of passing the first argument as a pointer to a machine integer, it can pass the machine integer directly (in a register).

This optimization is particularly worthwhile for accumulator parameters.

Johan