A function that computes π

pi.hs

import Data.Number.CReal

piit accuracy = showCReal 100 (go 1 (1 / sqrt 2) (1 / 4) 1)
   where go a b t p | (a - b) >= accuracy = let aa = (a + b) / 2
                                               bb = sqrt (a * b)
                                               tt = t - p * (a - aa)^2
                                               pp = 2 * p
                                            in go aa bb tt pp
                    | otherwise           = (a + b)^2 / (4 * t)