On Sat, May 17, 2008 at 10:40 PM, Daniel Fischer
However, more than can be squished out of fiddling with these versions could be gained from a better algorithm.
Just for fun and there probably should be better implementation for the same idea: module Main where data Tree a = Nil | Tree { el :: a, lft :: Tree a, rgt :: Tree a } deriving (Eq, Ord, Show) fromDistinctAscListN :: Int -> [a] -> ([a], Tree a) fromDistinctAscListN 0 xs = (xs, Nil) fromDistinctAscListN n xs = let ((e:xs'), l) = fromDistinctAscListN (n - 1) xs in let (xs'', r) = fromDistinctAscListN (n - 1) xs' in (xs'', Tree { el = e, lft = l, rgt = r }) branch :: Ord a => a -> a -> (a -> b) -> (a -> b) -> (a -> b) -> b branch x y lt eq gt = case (compare x y) of LT -> lt x EQ -> eq x GT -> gt x dispatch :: Ord a => a -> a -> (a -> Bool) -> (a -> Bool) -> Bool dispatch x y lt gt = branch x y lt (const True) gt member :: Ord a => a -> Tree a -> Bool member _ Nil = False member x t = dispatch x (el t) (`member` (lft t)) (`member` (rgt t)) type Forest a = [(a, Tree a)] memberOfForest :: Ord a => a -> Forest a -> Bool memberOfForest x ((y, t):fs) = dispatch x y (`member` t) (`memberOfForest` fs) fromDistAscList :: [a] -> Forest a fromDistAscList l = go 0 l where go n xs = let ((x:xs'), t) = fromDistinctAscListN n xs in (x, t):go (n + 1) xs' primes :: [Int] primes = [1..] primes' = fromDistAscList primes isPrime :: Int -> Bool isPrime = (`memberOfForest` primes') main = print $ length (filter isPrime [1..5000]) yours, anton.