sudoku solver now solves complex puzzles

src/sudoku-hs/src/Main.hs

@@ -33,14 +33,20 @@ printGrid = putStrLn . unlines . map (unwords . map showCell)
   where showCell = maybe "." show
 
 solve :: Grid -> Maybe Grid
-solve g = case nextCell g of
-  Nothing -> Just g
+solve g
+  | isComplete g = Just g
+  | otherwise = case nextCell g of
+      Nothing -> Nothing
       Just (r, c, opts) ->
         listToMaybe
           [ result
           | n <- opts
           , let g' = updateGrid g r c (Just n)
-      , Just result <- [solve g'] ]
+          , Just result <- [solve g']
+          , isComplete result ]
+
+isComplete :: Grid -> Bool
+isComplete = all (notElem Nothing)
 
 nextCell :: Grid -> Maybe (Int, Int, [Int])
 nextCell g =
@@ -62,7 +68,7 @@ getRow :: Int -> Grid -> [Cell]
 getRow r g = g !! r
 
 getCol :: Int -> Grid -> [Cell]
-getCol c g = map (!! c) g
+getCol c = map (!! c)
 
 getBox :: Int -> Int -> Grid -> [Cell]
 getBox r c g =
@@ -85,6 +91,6 @@ main = do
     [filePath] -> do
       grid <- readCSV filePath
       case solve grid of
-        Nothing -> putStrLn "No solution found."
+        Nothing -> putStrLn "No complete solution found."
         Just g  -> printGrid g
     _ -> putStrLn "Usage: sudoku-solver <sudoku.csv>"