http://programmingpraxis.com/2011/01/28/population-count/

popcount.hs

-- lazily produce the binary representation of a number as a list of [0,1]         
bin ∷ Integer → [Integer]                                                          
bin n = bin' n ((floor ∘ logBase 2 ∘ fromIntegral) n)                              
    where bin' 1 0 = [1]                                                           
          bin' 0 0 = [0]                                                           
          bin' a b = if a ≥ 2^b                                                    
                        then 1:bin' (a-2^b) (b-1)                                  
                        else 0:bin' a       (b-1)                                  
                                                                                   
popCount :: Integer -> Integer                                                     
popCount n = foldr (+) 0 (bin n)                                                   
                                                                                   
main = do                                                                          
      print $ popCount 23                                                          
      print $ popCount 48975823499389471387489274897123894718238947238947189