Thank you very much. You are right. foldl performs much better here. I thought foldr always has a performance advantage due to its tail recursion. On Feb 3, 2012, at 5:36 AM, Chaddaï Fouché wrote:
let go a b = b `minus` (multiplies a c) foldr go cs' (p:p':ps) ==> foldr go cs' (p':ps) `minus` multiplies p c ==> (minus needs his first argument to start removing stuff) (foldr go cs' ps `minus` multiplies p' c) `minus` multiplies p c And so on and so forth... In other words, contrary to your first version, this function must develop the entire list before making its first suppression...
Your version rather correspond to a foldl (be careful of strictness, using foldl it's pretty easy to get big thunks).
Foldr is very useful for functions that are lazy in their second argument like (||) , (&&), (:) or others but if the function is strict in its second argument like yours (strict in b)...
-- Jedaï
Best regards, Zhi-Qiang Lei zhiqiang.lei@gmail.com