yacht.hs

import Control.Arrow
import Data.List

data Category = Ones | Twos | Threes | Fours | Fives | Sixes | FullHouse
              | FourOfAKind | LittleStraight | BigStraight | Choice | Yacht
  deriving (Enum, Eq, Ord, Show)

allEqual []     = True
allEqual (c:cs) = all (==c) cs
counts :: (Eq a, Ord a) => [a] -> [(a, Int)]
counts = fmap (head &&& length) . group . sort
scoreN n = (* n) . length . filter (== n)
straightFrom b = and . zipWith (==) [b..] . sort

yacht FullHouse c | [2,3] == (sort $ map snd $ counts c) = sum c
yacht FourOfAKind c | not $ null c4 = sum c4
  where c4 = [x | (x, l) <- counts c, l >= 4]
yacht LittleStraight c | straightFrom 1 c = 30
yacht BigStraight c | straightFrom 2 c = 30
yacht Yacht c | allEqual c = 50
yacht Ones c = scoreN 1 c
yacht Twos c = scoreN 2 c
yacht Threes c = scoreN 3 c
yacht Fours c = scoreN 4 c
yacht Fives c = scoreN 5 c
yacht Sixes c = scoreN 6 c
yacht Choice c = sum c
yacht _ _ = 0