gistfile1.hs

module Sudoku where

import Control.Monad (guard)

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 = return xss >>= checkDimensions >>= buildSudoku
    where
        checkDimensions :: [[Int]] -> Maybe [[Int]]
        checkDimensions xss = do 
            let lengthOuter = length xss
            lengthInner <- map length xss
            guard (lengthInner == lengthOuter)
            return xss

        buildSudoku :: [[Int]] -> Maybe Sudoku
        buildSudoku _ = Just $ Sudoku [[D1]]