Solution to exercise in PureScript by Example book to estimate pi

Pi.purs

module Pi where

import Control.Monad.Eff (Eff(..), forE)
import Control.Monad.Eff.Random (RANDOM(..), random)
import Control.Monad.ST (ST(..), modifySTRef, newSTRef, readSTRef)
import Data.Array ((:), filter, length)
import Data.Int (toNumber)
import Data.Tuple (Tuple(..))

import Prelude (($), (<=), (+), (*), (/), bind, return, unit)


pi :: forall eff r. Int -> Eff (st :: ST r, random :: RANDOM | eff) Number
pi n = do
  ref <- newSTRef ([] :: Array (Tuple Number Number))
  forE 0.0 (toNumber n) $ \i -> do
    x <- random
    y <- random
    modifySTRef ref (\o -> Tuple x y : o)
    return unit
  tuples <- readSTRef ref
  all <- return $ toNumber $ length tuples
  inside <- return $ toNumber $ length $ filter (\(Tuple x y) -> x*x + y*y <= 1.0) tuples
  return (4.0 * inside / all)

Ah... DUH... pfffft... what the hell was I thinking about needing all? (－‸ლ)