**Praxis 2**: Write a function that calculates the list of prime numbers using the Sieve of Eratosthenes.

Ummm, yeah. Done it a couple dozen times before over at Project Euler. Never hurts to review. I did manage to use more function composition this time around. Oh, and don’t forget to install module data.ordlist using Cabal.

```
import Data.List.Ordered
primes :: Integer -> [Integer]
primes m = 2 : sieve [3,5..m]
where
sieve [] = []
sieve (p:xs) = p : sieve (minus xs [p*p, p*p+2*p..m])
main = do
putStrLn "Enter n: "
input <- getLine
putStrLn $ "# of primes: " ++ (show . length . primes $ read input)
```