Dabblings with Huffman encoding in Haskell

huff.hs

import Data.List (sortBy)
import Data.Ord (comparing)
import qualified Data.Map as M (Map, fromList)

data Freq a = V Int a | B Int (Freq a) (Freq a) deriving (Show)

size :: Freq a -> Int
size (V n _)   = n
size (B n _ _) = n

freq :: (a,Int) -> Freq a
freq (x, n) = V n x

freqs :: [(a,Int)] -> Freq a
freqs = merge . map freq
    where merge [x] = x
          merge xs = merge $ sortBy (comparing size) $ merged xs
          merged (x:y:ys) = B (size x + size y) x y : ys

huffs :: Freq a -> M.Map String a
huffs = M.fromList . huffs' ""
    where huffs' s (V _ x) = [(reverse s, x)]
          huffs' s (B _ x y) = huffs' ('0':s) x ++ huffs' ('1':s) y