Recently, I'm trying to solve some problems in project euler using haskell. When it came to problem 10, calculating the sum of all primes below 20000000, I try to write a program which can generate primes. In my memory Eratosthenes is faster than just whether a number can be divided by the number less then the square root of it. Firstly, I wrote the following programs: module Main where isPrime x = isPrime' 3 x (round . sqrt. fromIntegral $ x) isPrime' d target maxd | d > maxd = True | mod target d == 0 = False | otherwise = isPrime' (d + 2) target maxd main = print $ (sum (filter isPrime [3,5..2000000]) + 2) And it consume about 11s in my computer. Then, I tried to figure out how to solve the problem by Eratosthenes, but failed. Later, I find a program implemented by others, meeting my purpose and I've used it to solve the problem: primes :: [Int] primes = primes' [2..] primes' :: [Int] -> [Int] primes' [] = [] primes' (n:ns) = n : primes' (filter (\v -> v `mod` n /= 0) ns) solve x = sum $ primes' [2..x] main = print $ solve 2000000 Well, although the code is beautiful, it is slow. Even waiting for a minute, no answer was printed. In C version, Eratosthenes is faster than the method implemented in my earlier code, which only consume 0.3s(the earlier method consume 1.6s). So I want to know, why Eratosthenes implemented in Haskell is slow than the ugly code implemented by me. Could anyone tell me? Thank you Yi Cheng