Hello All Alternatively, less dependencies and a bit less golf (but not much testing...): Best wishes Stephen import Data.List import Numeric formatDecimal :: RealFloat a => a -> String formatDecimal d = let s = showFFloat (Just 2) d "" (a,b) = break (=='.') s in (intersperseN 3 ',' a) ++ b intersperseN :: Int -> a -> [a] -> [a] intersperseN n a = snd . para phi (1,[]) where phi x ((_:_), (i,acc)) | i == n = (1, a : x : acc) phi x (_, (i,acc)) = (i+1, x : acc) -- paramorphism (generalizes fold) para :: (a -> ([a], b) -> b) -> b -> [a] -> b para phi b = step where step [] = b step (x:xs) = phi x (xs, step xs) On 8 February 2010 11:17, Anton van Straaten wrote:

There's a small function to do it here:

http://bluebones.net/2007/02/formatting-decimals-in-haskell/

Hello I'd overlooked that the left-to-right behaviour of the paramorphism when I needed to go right-to-left, here's a version with a rather horrible right fold (different seeds for the counter, ho-hum, but easily testable): import Data.List import Numeric formatDecimal :: RealFloat a => a -> String formatDecimal d | d < 0 = '-' : fmt (negate d) | otherwise = fmt d where fmt x = let s = showFFloat (Just 2) x "" (a,b) = break (=='.') s in (intersperseN_rl 3 ',' a) ++ b intersperseN_rl :: Int -> a -> [a] -> [a] intersperseN_rl n sep xs = snd $ foldr phi (0,[]) xs where phi a (i,acc) | i == n = (1, a : sep : acc) | otherwise = (i+1, a : acc) test01 :: IO () test01 = mapM_ (print . (intersperseN_rl 3 ',')) $ inits "abcdefghijklmno" *FormatDecimal> test01 "" "a" "ab" "abc" "a,bcd" "ab,cde" "abc,def" "a,bcd,efg" "ab,cde,fgh" "abc,def,ghi" "a,bcd,efg,hij" "ab,cde,fgh,ijk" "abc,def,ghi,jkl" "a,bcd,efg,hij,klm" "ab,cde,fgh,ijk,lmn" "abc,def,ghi,jkl,mno" On 8 February 2010 13:40, Stephen Tetley wrote:

Ahem...

*FormatDecimal> formatDecimal (888.005) ",888.01"

I'll post a revision shortly (that handles negatives as well) ...