applbowl.hs

import Control.Applicative ((<|>))

type Bonus = Int

data Frame = Roll Int | Frame Int Int | Spare Int | Strike deriving (Show)

badnum :: Int -> Bool
badnum n = n < 0 || n > 10

strike, spare, frame :: [Int] -> Maybe (Frame, Bonus, [Int])

strike (10:a:b:rs) = Just (Strike, a + b, a : b : rs)
strike (10:rs)     = Just (Strike, 0, rs)
strike rs          = Nothing

spare (a:b:rs)     | a + b == 10  = Just (Spare a, case rs of (c:cs) -> c
                                                              []     -> 0, rs)
spare rs           = Nothing

frame (a:b:rs)     | badnum (a+b) = Nothing
                   | a == 10      = Nothing
                   | otherwise    = Just (Frame a b, 0, rs)
frame _            = Nothing

score1 :: [Int] -> Maybe (Frame, Bonus, [Int])
score1 rs = strike rs <|> spare rs <|> frame rs

scores :: [Int] -> Either String [(Frame, Bonus)]
scores = scores' (Right [])
    where scores' acc rs = case score1 rs of
                               Just (f,b,[])  -> fmap (reverse . ((f,b):)) acc
                               Just (f,b,rs') -> scores' (fmap ((f,b):) acc) rs'
                               Nothing -> Left $ "Illegal next roll(s): " ++ show rs

-- example game from https://github.com/hontas/bowling-game-kata
exampleGame = scores [10,9,1,5,5,7,2,10,10,10,9,0,8,2,9,1,10]