Help for a G+er

gistfile1.hs

{-# LANGUAGE BangPatterns #-}

piCalc :: Int -> Float
piCalc iterations = step * loop 0 iterations
    where step = 1.0 / fromIntegral iterations
          loop a i =
            if i == 0 then a
            else
                let x = (fromIntegral i - 0.5) * step
                    !a' =  a + (4.0 / (1.0 + x*x))
                in loop a' (i-1)


main :: IO ()
main = do
    putStrLn "please input the number of steps: "
    x <- getLine
    let iterations = read x
    print $ piCalc iterations