A simple program for calculating the number of prime numbers from 2 to 250000. Using `ghc -02 primes.hs` gave me around 2.9s per run.

primes.hs

isPrime :: Int -> Int
isPrime = helper 2
  where helper :: Int -> Int -> Int
        helper count num
          | prime = if count <= div num 2
                    then helper (count+1) num
                    else 1
          | otherwise = 0
          where prime = (num == 2) || (mod num count /= 0)

sumResult :: Int
sumResult = sum $ map isPrime [2..250000]

main :: IO ()
main = print sumResult