Solution for Programming Praxis, 6 Dec 2011 (http://programmingpraxis.com/2011/12/16/majority-voting/)

majority.hs

import Data.Function (on)
import Data.List (group, sortBy, sort)

data Vote = A | B | C deriving (Eq, Show, Ord)

majority:: (Ord a, Eq a) => [a] -> Maybe a
majority votes = if length majority > (length votes `div` 2)
                    then Just (head majority)
                    else Nothing
  where majority = (last . sortBy (compare `on` length) . group . sort) votes

main = do
    let votes = [A, A, A, C, C, B, B, C, C, C, B, C, C]
    print $ majority votes
    let votes' = [A, B, C, A, B, C, A]
    print $ majority votes'