Re: [Haskell-cafe] Two mysteries

I'm probably just missing something, but in this:

-- xupdateState -- :: (MonadIO m, Ord (StateKey s), State s, State a, -- StateValue s ~ StateValue a) => -- s -> (Int -> Int) -> m () ... -- • Could not deduce: StateValue a0 ~ StateValue a ... -- The type variable ‘a0’ is ambiguous

`a` is mentioned in the constraint but isn't used in the signature. Is that the problem? Jeff

On Aug 28, 2022, at 9:36 AM, Henry Laxen wrote:

-- run this with ghci -package microlens-platform -package stm -package containers SC.hs

{-# LANGUAGE TypeFamilies #-} {-# LANGUAGE AllowAmbiguousTypes #-} {-# LANGUAGE FlexibleContexts #-} {-# LANGUAGE TemplateHaskell #-}

module Games.SC where

import qualified Data.Map.Strict as M import GHC.IO.Unsafe ( unsafePerformIO ) import Lens.Micro.Platform import Control.Concurrent.STM.TVar import Control.Concurrent.STM import Control.Monad.IO.Class

type Lenstype f a b = (b -> f b) -> a -> f a instance Show a => Show (TVar a) where show x = unsafePerformIO $ do sx <- readTVarIO x return ("TVar: " <> show sx)

newtype TMap k v = TMap (TVar (M.Map k (TVar v))) deriving Show

-- TMap (TVar (Map k (TVar v))) -- ^^^^ modify this TVar to add/delete a key -- TMap (TVar (Map k (TVar v))) -- ^^^^ modify this TVar to change a value

withKey :: (MonadIO m, Ord k) => k -> (Maybe v -> v) -> TMap k v -> m () withKey k f (TMap x) = liftIO $ do atomically $ do tMap <- readTVar x let mbK = M.lookup k tMap case mbK of -- insert a new (key,value) into the map Nothing -> do v' <- newTVar (f Nothing) let newMap = M.insert k v' tMap writeTVar x newMap Just tv -> do -- modify the value of an existing key v' <- readTVar tv let newV = f (Just v') writeTVar tv newV return ()

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

-- /home/henry/nadineloveshenry/projects/nlh/src/Games/SC.hs:84:33-43: error: -- • Could not deduce: StateValue a0 ~ StateValue a -- from the context: (MonadIO m, Ord (StateKey s), State s, State a, -- StateValue s ~ StateValue a) -- bound by the type signature for: -- xupdateState :: forall (m :: * -> *) s a. -- (MonadIO m, Ord (StateKey s), State s, State a, -- StateValue s ~ StateValue a) => -- s -> (Int -> Int) -> m () -- at /home/henry/nadineloveshenry/projects/nlh/src/Games/SC.hs:(76,1)-(79,30) -- Expected type: ASetter (StateValue a) (StateValue a) Int Int -- Actual type: Lenstype -- Data.Functor.Identity.Identity (StateValue a0) Int -- NB: ‘StateValue’ is a non-injective type family -- The type variable ‘a0’ is ambiguous -- • In the first argument of ‘(%~)’, namely ‘lensCounter’ -- In the second argument of ‘(&)’, namely ‘lensCounter %~ f’ -- In the expression: x & lensCounter %~ f -- • Relevant bindings include -- x :: StateValue a -- (bound at /home/henry/nadineloveshenry/projects/nlh/src/Games/SC.hs:84:23) -- mutable :: TMap (StateKey s) (StateValue a) -- (bound at /home/henry/nadineloveshenry/projects/nlh/src/Games/SC.hs:83:5)

data SampleState = SampleState { _key :: String , _tMap :: TMap String SampleValue } deriving Show

data SampleValue = SampleValue { _counter :: Int , _other :: () } deriving Show

$(makeLenses ''SampleState) $(makeLenses ''SampleValue)

makeSampleState :: IO SampleState makeSampleState = do let sampleValue = SampleValue 1 () tvar1 <- newTVarIO sampleValue let sampleTMap = M.fromList [("a", tvar1)] tvar2 <- newTVarIO sampleTMap return (SampleState "a" (TMap tvar2))

-- I can show the result of makeSampleState: -- λ> makeSampleState -- SampleState {_key = "a", _tMap = TMap TVar: fromList [("a",TVar: SampleValue {_counter = 1, _other = ()})]}

bump1 :: IO () bump1 = do xss <- makeSampleState let xTmap = xss ^. tMap withKey "a" (\(Just x) -> x & counter %~ (+1)) xTmap print xTmap

-- Everything here is tickety-boo -- λ> bump1 -- TMap TVar: fromList [("a",TVar: SampleValue {_counter = 2, _other = ()})]

-- Now let's make SampleState an instance of State

instance State SampleState where type StateKey SampleState = String type StateValue SampleState = SampleValue lensTmap = tMap lensKey = key lensCounter = counter

-- THE SECOND MYSTERY: now lets try bump1 with the type family, but there is no joy

-- bump2 :: IO () -- bump2 = do -- xss <- makeSampleState -- let xTmap = xss ^. lensTmap -- withKey "a" (\(Just x) -> x & lensCounter %~ (+1)) xTmap -- print xTmap

-- SC.hs:163:33-43: error: -- • Couldn't match type ‘StateValue a0’ with ‘SampleValue’ -- Expected type: ASetter SampleValue SampleValue Int Int -- Actual type: Lenstype -- Data.Functor.Identity.Identity (StateValue a0) Int -- The type variable ‘a0’ is ambiguous -- • In the first argument of ‘(%~)’, namely ‘lensCounter’ -- In the second argument of ‘(&)’, namely ‘lensCounter %~ (+ 1)’ -- In the expression: x & lensCounter %~ (+ 1)

-- Can someone please explain what is going on, or point me at a book/paper -- Thanks in advance.

-- You can load this into ghci and uncomment to "bad" code to see for -- yourself what happens

-- Nadine and Henry Laxen The rest is silence Gral. Manuel Márquez de León 1301 Onix #2302 Zona Urban Rio Never try to teach a pig to sing; Tijuana It wastes your time +52 (333) 667-8633 And it annoys the pig _______________________________________________ Haskell-Cafe mailing list To (un)subscribe, modify options or view archives go to: http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe Only members subscribed via the mailman list are allowed to post.