On Sun, Aug 28, 2022 at 09:36:56AM -0700, Henry Laxen wrote:
class State a where type StateKey a :: * type StateValue a :: * lensTmap :: Functor f => Lenstype f a (TMap (StateKey a) (StateValue a)) lensKey :: Functor f => Lenstype f a (StateKey a) lensCounter :: Functor f => Lenstype f (StateValue a) Int
updateState state f = liftIO $ do let key = state ^. lensKey -- read the key mutable = state ^. lensTmap -- find the TVar of the TMap withKey key (\(Just x) -> x & lensCounter %~ f) mutable -- update it
-- THE FIRST MYSTERY: If I use the inferred type explicitly for updateState -- above, this no longer compiles. I get a "could not deduce" error -- reproduced below:
-- xupdateState -- :: (MonadIO m, Ord (StateKey s), State s, State a, -- StateValue s ~ StateValue a) => -- s -> (Int -> Int) -> m () -- xupdateState state f = liftIO $ do -- let -- key = state ^. lensKey -- read the key -- mutable = state ^. lensTmap -- find the TVar of the TMap -- withKey key (\(Just x) -> x & lensCounter %~ f) mutable -- update it
AllowAmbiguousTypes is what's allowing you to get tangled into this knot. I suggest you don't enable that extension unless you feel confident you are equipped for axe warfare. The behaviour here is as though the type checker has turned `lensCounter` in the body into `lensCounter @a` for a fresh type variable `a`, and then applied the necessary constraints to the context. Normally the type checker would notice that `a` is ambiguous and refuse to continue. AllowAmbiguousTypes defers that check to the use site. That means I don't think it makes any difference to you, because even though `updateState` with inferred type signature will compile I bet you can't use it. Give it a try! I guess you'll get an "ambiguous type" error at the use site. Tom