First, thank you Li-yao Xia for your clear and helpful response. I was able to continue thanks to you. Thanks for the question Anthony, I look forward to learning something. I hope I'll be explain this clearly. I am writing some games, which can share some code, such as logging in, and broadcasting messages to players. I started by writing just one game, with no consideration for how and what to share. This let to a gamestate which contained things like: newtype GameId = GameId Int data Game1 { _game1TimeStarted :: UTCTime, _game1PlayerId :: GameId, ... _game1RandomGen :: StdGen, _game1Commands :: [Game1Command] } the actual state is a more complicated than this, because I split it into a Static part and a Mutable part that is in a TVar (Map GameId Game1MutableState) Next I start writing Game2, which I discover will do the same kind of broadcasting of messages (Commands) as Game1. So I would like to use the same code in Game2 as in Game1 that handles the broadcasting. I came up with: (this is part of the actual class I am using) class Game a where type Command a :: * type Mutable a :: * type Static a :: * toGameId :: a -> GameId toMutable :: a -> IO (Maybe (Mutable a)) everyone :: Mutable a -> [Sink] instance Game Game1State where type Command Game1State = Game1Command type Mutable Game1State = Game1Mutable type Static Game1State = Game1Static toGameId ss = ss ^. game1Static . game1GameId toMutable ss = do let tVar = ss ^. game1Mutable gId = toGameId ss gm <- liftIO (readTVarIO tVar) return $ M.lookup gId (unGameMap gm) everyone = mapMaybe _game1Sink . M.elems . _game1SPMap similarly, I make Game2State an instance of Game. Now in the code that broadcasts commands I have: sendBroadCasts :: (Show (Command a) , Game a) => [BroadCast a] -> a -> IO () ... sendOneCommand :: (Show (Command a) , Game a) => a -> BroadCastReceiver -> Command a -> IO () sendOneCommand ss BroadCastToEveryone command = do mbMut <- toMutable ss case mbMut of Nothing -> return () Just mut -> do let sinks = everyone mut mapM_ (`sendCommand` command) sinks but GHC scolds me with: src/Games/BroadCasts.hs:53:12-23: error: … • Couldn't match type ‘Mutable a’ with ‘Mutable a0’ Expected type: IO (Maybe (Mutable a0)) Actual type: IO (Maybe (Mutable a)) NB: ‘Mutable’ is a non-injective type family The type variable ‘a0’ is ambiguous • In a stmt of a 'do' block: mbMut <- toMutable ss In the expression: ... After incorporating Li-yao Xia's suggestion, the code becomes: sendOneCommand :: forall a. (Show (Command a) , Game a) => ----------------- ^^^^^^^^^ this is added a -> BroadCastReceiver -> Command a -> IO () sendOneCommand _ (BroadCastToSome sinks) command = mapM_ (`sendCommand` command) sinks sendOneCommand ss BroadCastToEveryone command = do mbMut <- toMutable ss case mbMut of Nothing -> return () Just mut -> do let sinks = everyone @a mut -------------------------- ^^ this is added mapM_ (`sendCommand` command) sinks and GHC is happy. I hope I have explained this sufficiently clearly. I look forward to your insights. Best wishes, Henry Laxen

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"Anthony" == Anthony Clayden writes:

Anthony> Hi Henry, both replies you've received are excellent, I won't expand on them. Anthony> I am interested in how you got into such a pickle in the first place: Anthony> Why set `AllowAmbiguousTypes`? Did you understand what that means? ----------------------^^^^^^^^^^^^^^^^^^^^ I read https://stackoverflow.com/questions/49684655/how-dangerous-is-allowambiguous... which seems to indicate that using it is "a perfectly reasonable thing" Anthony> Did you think it odd that despite having that set, you get error Anthony> `type variable ... is ambiguous`? Anthony> There are ways to achieve what you want without Anthony> `AllowAmbiguousTypes` nor `TypeApplications`, but that would need Anthony> a rather different design. So what lead you into this design? Anthony> AntC -- Nadine and Henry Laxen The rest is silence Villa Alta #6 Calle Gaviota #10 Never try to teach a pig to sing Chapala It wastes your time +52 (376) 765-3181 And it annoys the pig