Sinc function

The sinc function, $\frac{\sin(x)}{x}$, is a useful function that is a little tricky to implement because it becomes $\frac{0}{0}$ as x approaches $0$. Here is an implementation taken from the Boost library.

epsilon :: RealFloat a => a
epsilon = encodeFloat 1 (fromIntegral $ 1-floatDigits epsilon)

{- Boosted from Boost http://www.boost.org/boost/math/special_functions/sinc.hpp -}
sinc :: (RealFloat a) => a -> a
sinc x =
   if abs x >= taylor_n_bound
     then sin x / x
     else 1 - x^2/6 + x^4/120
 where
  taylor_n_bound = sqrt $ sqrt epsilon