Find the most frequently occurring item that occurs more than once. Idea from http://skipoleschris.blogspot.com.au/2010/11/functional-programming-challenge-most.html

most.hs

import qualified Data.Map as Map


-- Find the most frequently occurring item that occurs more than once
-- Idea from http://skipoleschris.blogspot.com.au/2010/11/functional-programming-challenge-most.html

most :: Ord a => [a] -> Maybe a
most xs = fst $ foldl most' (Nothing, Map.empty) xs where
  most' (winning, seen) x = (winning', seen') where
    winning' = if count > winningCount then Just x else winning
    seen' = Map.alter incSeen x seen
    count = seen' Map.! x
    winningCount = maybe 0 ((Map.!) seen) winning
    incSeen i = Just $ maybe 0 ((+) 1) i

test1 = ["a", "b", "c", "a", "b", "a"]
test2 = ["a", "b", "c"]
test3 = ["a", "b", "a", "b"]
test4 = ["b", "a", "b", "a"]
test5 = [] :: [String]
test6 = [1, 2, 3, 4, 2, 3, 3, 3, 2] 

toText :: Show a => Maybe a -> String
toText = maybe "None" show

mostText :: (Ord a, Show a) => [a] -> String
mostText = toText . most

main = do
  putStrLn $ mostText test1
  putStrLn $ mostText test2
  putStrLn $ mostText test3
  putStrLn $ mostText test4
  putStrLn $ mostText test5
  putStrLn $ mostText test6