Pi approx using rationals

import Data.Ratio

myPi :: Int -> Double
myPi seriesLength = fromRational piRatApprox
    where piRatApprox = (* toRational 4)
                        . sum
                        . take seriesLength
                        . map (\n -> 1 % n)
                        . iterate (\n -> -1 * (n + signum n * 2))
                        $ 1