Square root calculated using the Newton - Raphson method in Haskell

nrSqrt.hs

module NewtonRaphson.Sqrt (nrSqrt) where 

nrSqrt :: Double -> Double
nrSqrt n = within 0.001 (approximations init n)
          where init = n / 2

approximations :: Double -> Double -> [Double]
approximations x0 n = iterate (next n) x0

next :: Double -> Double -> Double
next n x0 = (x0 + n / x0) / 2

within :: Double -> [Double] -> Double
within eps (x0:t@(x1:xs)) = if abs(x0 - x1) < eps then x1 else within eps t