Approximates pi using Archimedes' method

pigrec.hs

module Pigrec where

pigrec :: Int -> Float
pigrec x = foldl (ptwon) (2 * sqrt 2) (map (\x -> 2^x) [2..x])

ptwon :: Double -> Int -> Double
-- Given the perimeter pn of a n-sided polygon inscribed in a 
-- circle of diameter 1 returns the perimeter of the 2n-sided
-- polygon inscribed in the same circle
ptwon pn n = n' * ( sqrt (1 + l) - sqrt (1 - l))
  where
    n' = fromIntegral n
    l = pn / n'