Example of a pure function which isn't a pure function

e-mach.hs

-- | A pure function that isn't. Or is it?
--
eMach :: Double
eMach = eMach' 1.0
  where
    eMach' i
        | i' > 0    = eMach' i'
        | otherwise = i
      where
        i' = i / 2.0

main :: IO ()
main = putStrLn $ show eMach