I replaced the standard random number generated with the one from
mersenne-random. On my system this makes the resulting program about
14 times faster than the original. I also made a change to
accumulateHit because it doesn't need to count to total. That is
already known.
{-# LANGUAGE BangPatterns #-}
import System( getArgs )
import Data.List( foldl' )
import System.Random.Mersenne
pairs :: [a] -> [(a,a)]
pairs [] = []
pairs (x:[]) = []
pairs (x:y:rest) = (x, y) : pairs rest
isInCircle :: (Double, Double) -> Bool
isInCircle (x,y) = sqrt (x*x + y*y) <= 1.0
accumulateHit :: Int -> (Double, Double) -> Int
accumulateHit (!hits) pair | isInCircle pair = hits + 1
| otherwise = hits
countHits :: [(Double, Double)] -> Int
countHits ps = foldl' accumulateHit 0 ps
monteCarloPi :: Int -> [(Double, Double)] -> Double
monteCarloPi n xs = 4.0 * fromIntegral hits / fromIntegral n
where hits = countHits $ take n xs
main = do
args <- getArgs
let samples = read $ head args
randomNumberGenerator <- getStdGen
randomNumbers <- randoms randomNumberGenerator
let res = monteCarloPi samples $ pairs randomNumbers
putStrLn $ show $ res
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