purescript-sample

gistfile1.purs

module Exercises where

import Math

import Control.Monad.Eff.Random

import Control.Monad.Eff
import Control.Monad.ST

import Debug.Trace

main = do
        n <- approximatePi 1000
        print n

data Point = Point {x :: Number, y :: Number}

generateCoord :: forall eff. Eff (random :: Random | eff) Number
generateCoord = randomRange (-1) 1

point :: Number -> Number -> Point
point x y = Point {x: x, y: y}

randomPoint :: forall eff. Eff (random :: Random | eff) Point
randomPoint = point <$> generateCoord
                    <*> generateCoord

inUnitCircle :: Point -> Boolean
inUnitCircle (Point {x = x, y = y}) = (sqrt (x * x + y * y)) < 1


approximatePi :: forall eff. Number -> Eff (random :: Random | eff) Number
approximatePi n = runST (do
        count <- newSTRef 0
        forE 0 n $ \i -> do
                inCircle <- (inUnitCircle <$> randomPoint)
                modifySTRef count (\o -> if inCircle then (o + 1) else o)
                return unit
        total <- readSTRef count
        return (4*total/n))