check should return score or nothing if invalid

src/Types/GameMode.hs

@@ -8,7 +8,6 @@ import Data.Bifunctor (Bifunctor (first))
 import Data.Kind (Type)
 import Display
 import System.Random.Stateful (Random (randomR), RandomGen)
-import Types.BoardPosition
 
 {- | The type of game that is being played.
 
@@ -19,7 +18,7 @@ that sums up to 'n'.
 -}
 class (Display (Tile t), Show (Tile t), Eq (Tile t), Eq t, Show t) => GameMode t where
     data Tile t :: Type
-    check :: t -> [Tile t] -> Bool
+    check :: t -> [Tile t] -> Maybe Int
     gen :: (RandomGen g) => t -> g -> (Tile t, g)
 
 data SumTo = MkSumTo Int
@@ -28,8 +27,8 @@ data SumTo = MkSumTo Int
 instance GameMode SumTo where
     data Tile SumTo = IntTile Int deriving (Eq, Show)
 
-    check (MkSumTo n) [] = n == 0
-    check (MkSumTo n) lst = n == sum (map toInt lst)
+    check (MkSumTo n) [] = if n == 0 then Just 0 else Nothing
+    check (MkSumTo n) lst = if n == sum (map toInt lst) then Just (length lst) else Nothing
       where
         toInt (IntTile a) = a
     gen (MkSumTo n) = first IntTile . randomR (1, n)