calc_pi_by_monte_calro.hs

-- if this program stack space overflow,
-- ./monte_carlo +RTS -K200M
import System.Random
import Control.Monad

main :: IO ()
main = do
    randsx <- replicateM s $ (randomRIO(0,1.0)::IO Double)
    randsy <- replicateM s $ (randomRIO(0,1.0)::IO Double)
    print (getPI (isInTheCircle randsx randsy) sDouble)
    where
        s = 100000
        sDouble = realToFrac s
              
isInTheCircle :: [Double] -> [Double] -> Double 
isInTheCircle [] _ = 0
isInTheCircle _ [] = 0
isInTheCircle (x:xs) (y:ys)
    | l <= 1 = 1 + isInTheCircle xs ys
    | l > 1  = isInTheCircle xs ys
    where l = x^2 + y^2

getPI:: Double -> Double -> Double
getPI x s = x * 4 / s