showInt with fold, attempted tail recursion and normal recursion.

final2.hs

import Data.List

showInt :: Integer -> String
showInt number =
  let tostr = show number
  in let digitcount = length tostr
  in let b = zip tostr [digitcount, digitcount-1 ..1]
  in foldr (\x y -> 
    if snd x `mod` 3 == 0 && snd x /= digitcount 
      then " "++[fst x]++y 
    else [fst x] ++ y) "" b

-- Attempted tail recursive implementation of insertSpaces.
showInt2 :: Integer -> String
showInt2 n = ys ++ (if null ys || null zs then "" else " ") ++ drop 1 (reverse(insertSpaces zs))
			 where
			 xs = show n
			 (ys,zs) = splitAt (length xs `mod` 3) xs

insertSpaces str = insertSpaces2 str "" 

insertSpaces2 :: String -> String -> String
insertSpaces2 [] append = append
insertSpaces2 (x:y:z:xs) append = 
	insertSpaces2 (xs) ( [z,y,x, ' '] ++ append )


showInt3 :: Integer -> String
showInt3 n = ys ++ (if null ys || null zs then "" else " ") ++ insertSpaces3 zs
			 where
			 xs = show n
			 (ys,zs) = splitAt (length xs `mod` 3) xs

insertSpaces3 :: String -> String
insertSpaces3 (a:b:c:x:xs) = [a,b,c,' '] ++ insertSpaces3 (x:xs)
insertSpaces3 xs = xs

--main :: IO ()
--main = do
--	putStrLn $ show (length (showInt $ product [1..80000]) )
--	putStrLn $ show (length (showInt2 $ product [1..80000]) )
--	putStrLn $ show (length (showInt3 $ product [1..80000]) )