rec :: Rational -> Bool -> [Integer] -> Rational
rec currSum _   []                 = currSum
rec currSum add (n:ns) | add       = rec (currSum + (1%n)) (not add) ns
                       | otherwise = rec (currSum - (1%n)) (not add) ns
odds :: [Integer]
odds = [1,3..]

myPi2 seriesLength = fromRational $ 4 * rec 0 True (take seriesLength odds)