gistfile1.hs

module Sudoku where

import Control.Monad (sequence)

data Digit = D1 | D2 | D3 | D4 | D5 | D6 | D7 | D8 | D9
    deriving (Eq, Ord, Read, Enum)

mkDigit :: Int -> Maybe Digit
mkDigit n 
    | n > 0 && n < 10 = Just $ toEnum (n-1)
    | otherwise       = Nothing

instance Show Digit where show x = show $ fromEnum x+1

newtype Sudoku = Sudoku { toList :: [[Digit]] }
    deriving (Show)

mkSudoku :: [[Int]] -> Maybe Sudoku
mkSudoku xss = if isSudoku then buildSudoku else Nothing 
    where
        isSudoku :: Bool
        isSudoku = outerLength == 9 && (all (== outerLength) $ map length xss)
            where outerLength = length xss

        toDigits :: Maybe [[Digit]]
        toDigits = let listOfMaybes = [map mkDigit y | y <- xss]
                       in sequence . map sequence $ listOfMaybes

        buildSudoku :: Maybe Sudoku
        buildSudoku = let (Just digits) = toDigits
                      in Just $ Sudoku digits