Concerning the laziness support problem, I thank people for explanations about foldl and foldr.

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I wonder how to avoid these numerous cost pitfalls. Maybe, the complier could do more optimization?

Duncan Coutts writes

There are important differences between foldl, foldl' and foldr. It is quite important to choose the right one. I don't think this can be done automatically.

In my experience, the choice is almost always between foldl' and foldr.

[..]

I do not see foldl' in the standard library. Is it of the GHC lib extension? has it strictness annotation?

So as Lemmih says, in this case you want to use foldr:

import List (union) main = let n = 10^4 :: Int in putStr (shows (take 2 $ unionMany [[1 .. i] | i <- [1 .. n]]) "\n")

unionMany = foldr union []

I see. Thank you. I have impression that something is here besides the intuition for the foldl/foldr choice. Here is a contrived example which is more close to my real situation. ----------------------------------------------------------------- import qualified Data.Set as Set (Set(..), empty, member, insert) import List (union, find) main = let n = 10^6 :: Int in putStr (shows (g1 n) "\n") f :: Int -> (Set.Set Int, [Int]) f n = -- original version, I write so because it is easy to program -- foldl add (Set.empty, []) [[1 .. i] | i <- [1 .. n]] where add (s, xs) ys = (Set.insert (sum xs) s, union xs ys) {- attempt to optimize (fails) -- h (Set.empty, []) [[1 .. i] | i <- [1 .. n]] where h (s, xs) [] = (s, xs) h (s, xs) (ys: yss) = h (Set.insert (sum xs) s, union xs ys) yss -} g1, g2 :: Int -> Bool -- client functions g1 n = case snd $ f n of x: _ -> even x _ -> False g2 n = let (set, xs) = f n in case find (> 100) xs of Just x -> Set.member (2*x) set _ -> False ----------------------------------------------------------------- Evidently, g1 n must have the cost of O(1). But in ghc-6.6 -O, it has O(n). How to improve f ? I tried foldr, and failed. The situation is so that some clients are as g1, and others are as g2, and, at least, g1 must be O(1). Regards, ----------------- Serge Mechveliani mechvel@botik.ru