Number of 1 bits in a decimal number

Question

How can I write a function to return the number of 1 bits corresponding a decimal number? Maybe a square root function? The type should be this:

bits :: Int -> Int

EDIT: SOlved

uns :: Int -> Int
uns  0 = 0
uns 1 = 1
uns x | mod x 2 == 1 = 1 + uns (div x 2)
      | otherwise = uns (div x 2)

Answer 1

What about this old trick?

import Data.Bits

countOnes 0 = 0
countOnes x = 1 + countOnes (x .&. (x-1))

It only recurses n times where n is the number of one bits in x.

Of course, if you are going to import Data.Bits, then you might as well use popCount as @Cirdec suggested.