Encode in the types that you can't run askPlayer with an empty list.

Startups/Game.hs

@@ -236,8 +236,12 @@ playAge age = do
     let recyclers = pm ^.. traverse . filtered (has (_2 . cardEffects . _Recycling)) . _1
     forM_ recyclers $ \pid -> do
         stt <- use id
-        card <- askCardSafe age pid (stt ^. discardpile) stt "Choose a card to recycle (play for free)"
-        playermap . ix pid . pCards %= (card :)
+        case stt ^? discardpile . _NonEmpty of
+            Just nedp -> do
+                card <- askCardSafe age pid nedp stt "Choose a card to recycle (play for free)"
+                playermap . ix pid . pCards %= (card :)
+                discardpile %= filter (/= card)
+            Nothing -> tellPlayer pid "The discard pile was empty, you can't recycle."
     -- resolve the "military" part
     resolvePoaching age
 
@@ -251,10 +255,12 @@ checkCopyCommunity = use playermap >>= itraverse_ checkPlayer
             mvioletCards <- forM (stt ^.. pNeighborhood . traverse) $ \nid ->
                 toListOf (pCards . traverse . filtered (has (cType . _Community))) <$> getPlayerState nid
             let violetCards = concat mvioletCards
-            unless (null violetCards) $ do
-                gs <- use id
-                card <- askCardSafe Age3 pid violetCards gs "Which community would you like to copy ?"
-                playermap . ix pid . pCards %= (card:)
+            case violetCards ^? _NonEmpty of
+                Just nevc -> do
+                    gs <- use id
+                    card <- askCardSafe Age3 pid nevc gs "Which community would you like to copy ?"
+                    playermap . ix pid . pCards %= (card:)
+                Nothing -> tellPlayer pid "There were no violet cards bought by your neighbors. You can't use your copy capacity."
 
 victoryPoints :: GameStateOnly m => m (M.Map PlayerId (M.Map VictoryType VictoryPoint))
 victoryPoints = use playermap >>= itraverse computeScore

Startups/GameTypes.hs

@@ -51,6 +51,17 @@ type Message = String
 data PlayerAction = PlayerAction ActionType Card
 data ActionType = Play | Drop | BuildCompany
 
+-- | Some types for non empty lists
+data NonEmpty a = NonEmpty a [a]
+
+_NonEmpty :: Prism' [a] (NonEmpty a)
+_NonEmpty = prism fromNonEmpty toNonEmpty
+    where
+        fromNonEmpty (NonEmpty x xs) = x : xs
+        toNonEmpty l = case l of
+                           [] -> Left l
+                           (x:xs) -> Right (NonEmpty x xs)
+
 -- | This describe the capabilities needed to write the rules, when no
 -- interaction with the player is required.
 type NonInteractive m = (MonadState GameState m, Monad m, MonadError Message m, Functor m, Applicative m)
@@ -61,15 +72,15 @@ class NonInteractive m => GameMonad m where
     playerDecision    :: Age -> Turn -> PlayerId -> [Card] -> GameState -> m (PlayerAction, Exchange)
     -- | Ask the player to chose a card, along with a descriptive message.
     -- This is used for the Recycling and CopyCommunity effects.
-    askCard           :: Age -> PlayerId -> [Card] -> GameState -> Message -> m Card
+    askCard           :: Age -> PlayerId -> NonEmpty Card -> GameState -> Message -> m Card
     tellPlayer        :: PlayerId -> Message -> m () -- ^ Tell some information to a specific player
     generalMessage    :: Message -> m () -- ^ Broadcast some information
 
 -- We define "safe" versions of the `askCard` function, that makes sure the
 -- player doesn't introduce a new card in the game.
 
-askCardSafe :: GameMonad m => Age -> PlayerId -> [Card] -> GameState -> Message -> m Card
+askCardSafe :: GameMonad m => Age -> PlayerId -> NonEmpty Card -> GameState -> Message -> m Card
 askCardSafe a p cl s m = do
     card <- askCard a p cl s m
-    when (card `notElem` cl) (throwError "The player tried to play a non proposed card")
+    when (card `notElem` (cl ^. re _NonEmpty)) (throwError "The player tried to play a non proposed card")
     return card