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+{-# LANGUAGE PartialTypeSignatures #-}
+{-# LANGUAGE UndecidableInstances  #-}
+{-# LANGUAGE QuantifiedConstraints #-}
+{-# LANGUAGE StandaloneDeriving    #-}
+{-# LANGUAGE PolyKinds             #-}
+--------------------------------------------------------------------------------
+module Xanthous.Data.NestedMap
+  ( NestedMapVal(..)
+  , NestedMap(..)
+  , lookup
+  , lookupVal
+  , insert
+
+    -- *
+  , (:->)
+  , BifunctorFunctor'(..)
+  , BifunctorMonad'(..)
+  ) where
+--------------------------------------------------------------------------------
+import           Xanthous.Prelude hiding (lookup, foldMap)
+import qualified Xanthous.Prelude as P
+--------------------------------------------------------------------------------
+import           Test.QuickCheck
+import           Data.Aeson
+import           Data.Function (fix)
+import           Data.Foldable (Foldable(..))
+import           Data.List.NonEmpty (NonEmpty(..))
+import qualified Data.List.NonEmpty as NE
+--------------------------------------------------------------------------------
+
+-- | Natural transformations on bifunctors
+type (:->) p q = forall a b. p a b -> q a b
+infixr 0 :->
+
+class (forall b. Bifunctor b => Bifunctor (t b)) => BifunctorFunctor' t where
+  bifmap' :: (Bifunctor p, Bifunctor q) => (p :-> q) -> t p :-> t q
+
+class BifunctorFunctor' t => BifunctorMonad' t where
+  bireturn' :: (Bifunctor p) => p :-> t p
+
+  bibind' :: (Bifunctor p, Bifunctor q) => (p :-> t q) -> t p :-> t q
+  bibind' f = bijoin' . bifmap' f
+
+  bijoin' :: (Bifunctor p) => t (t p) :-> t p
+  bijoin' = bibind' id
+
+  {-# MINIMAL bireturn', (bibind' | bijoin') #-}
+
+--------------------------------------------------------------------------------
+
+data NestedMapVal m k v = Val v | Nested (NestedMap m k v)
+
+deriving stock instance
+  ( forall k' v'. (Show k', Show v') => Show (m k' v')
+  , Show k
+  , Show v
+  ) => Show (NestedMapVal m k v)
+
+deriving stock instance
+  ( forall k' v'. (Eq k', Eq v') => Eq (m k' v')
+  , Eq k
+  , Eq v
+  ) => Eq (NestedMapVal m k v)
+
+instance
+  forall m k v.
+  ( Arbitrary (m k v)
+  , Arbitrary (m k (NestedMapVal m k v))
+  , Arbitrary k
+  , Arbitrary v
+  , IsMap (m k (NestedMapVal m k v))
+  , MapValue (m k (NestedMapVal m k v)) ~ (NestedMapVal m k v)
+  , ContainerKey (m k (NestedMapVal m k v)) ~ k
+  ) => Arbitrary (NestedMapVal m k v) where
+  arbitrary = sized . fix $ \gen n ->
+    let nst = fmap (NestedMap . mapFromList)
+            . listOf
+            $ (,) <$> arbitrary @k <*> gen (n `div` 2)
+    in if n == 0
+       then Val <$> arbitrary
+       else oneof [ Val <$> arbitrary
+                  , Nested <$> nst]
+  shrink (Val v) = Val <$> shrink v
+  shrink (Nested mkv) = Nested <$> shrink mkv
+
+instance Functor (m k) => Functor (NestedMapVal m k) where
+  fmap f (Val v) = Val $ f v
+  fmap f (Nested m) = Nested $ fmap f m
+
+instance Bifunctor m => Bifunctor (NestedMapVal m) where
+  bimap _ g (Val v) = Val $ g v
+  bimap f g (Nested m) = Nested $ bimap f g m
+
+instance BifunctorFunctor' NestedMapVal where
+  bifmap' _ (Val v) = Val v
+  bifmap' f (Nested m) = Nested $ bifmap' f m
+
+instance (ToJSONKey k, ToJSON v, ToJSON (m k (NestedMapVal m k v)))
+       => ToJSON (NestedMapVal m k v) where
+  toJSON (Val v) = toJSON v
+  toJSON (Nested m) = toJSON m
+
+instance Foldable (m k) => Foldable (NestedMapVal m k) where
+  foldMap f (Val v) = f v
+  foldMap f (Nested m) = foldMap f m
+
+-- _NestedMapVal
+--   :: forall m k v m' k' v'.
+--     ( IsMap (m k v), IsMap (m' k' v')
+--     , IsMap (m [k] v), IsMap (m' [k'] v')
+--     , ContainerKey (m k v) ~ k, ContainerKey (m' k' v') ~ k'
+--     , ContainerKey (m [k] v) ~ [k], ContainerKey (m' [k'] v') ~ [k']
+--     , MapValue (m k v) ~ v, MapValue (m' k' v') ~ v'
+--     , MapValue (m [k] v) ~ v, MapValue (m' [k'] v') ~ v'
+--     )
+--   => Iso (NestedMapVal m k v)
+--         (NestedMapVal m' k' v')
+--         (m [k] v)
+--         (m' [k'] v')
+-- _NestedMapVal = iso hither yon
+--   where
+--     hither :: NestedMapVal m k v -> m [k] v
+--     hither (Val v) = singletonMap [] v
+--     hither (Nested m) = bimap _ _ $ m ^. _NestedMap
+--     yon = _
+
+--------------------------------------------------------------------------------
+
+newtype NestedMap m k v = NestedMap (m k (NestedMapVal m k v))
+
+deriving stock instance
+  ( forall k' v'. (Eq k', Eq v') => Eq (m k' v')
+  , Eq k
+  , Eq v
+  ) => Eq (NestedMap m k v)
+
+deriving stock instance
+  ( forall k' v'. (Show k', Show v') => Show (m k' v')
+  , Show k
+  , Show v
+  ) => Show (NestedMap m k v)
+
+instance Arbitrary (m k (NestedMapVal m k v))
+       => Arbitrary (NestedMap m k v) where
+  arbitrary = NestedMap <$> arbitrary
+  shrink (NestedMap m) = NestedMap <$> shrink m
+
+instance Functor (m k) => Functor (NestedMap m k) where
+  fmap f (NestedMap m) = NestedMap $ fmap (fmap f) m
+
+instance Bifunctor m => Bifunctor (NestedMap m) where
+  bimap f g (NestedMap m) = NestedMap $ bimap f (bimap f g) m
+
+instance BifunctorFunctor' NestedMap where
+  bifmap' f (NestedMap m) = NestedMap . f $ bimap id (bifmap' f) m
+
+instance (ToJSONKey k, ToJSON v, ToJSON (m k (NestedMapVal m k v)))
+       => ToJSON (NestedMap m k v) where
+  toJSON (NestedMap m) = toJSON m
+
+instance Foldable (m k) => Foldable (NestedMap m k) where
+  foldMap f (NestedMap m) = foldMap (foldMap f) m
+
+--------------------------------------------------------------------------------
+
+lookup
+  :: ( IsMap (m k (NestedMapVal m k v))
+    , MapValue (m k (NestedMapVal m k v)) ~ (NestedMapVal m k v)
+    , ContainerKey (m k (NestedMapVal m k v)) ~ k
+    )
+  => NonEmpty k
+  -> NestedMap m k v
+  -> Maybe (NestedMapVal m k v)
+lookup (p :| []) (NestedMap vs) = P.lookup p vs
+lookup (p :| (p₁ : ps)) (NestedMap vs) = P.lookup p vs >>= \case
+  (Val _) -> Nothing
+  (Nested vs') -> lookup (p₁ :| ps) vs'
+
+lookupVal
+  :: ( IsMap (m k (NestedMapVal m k v))
+    , MapValue (m k (NestedMapVal m k v)) ~ (NestedMapVal m k v)
+    , ContainerKey (m k (NestedMapVal m k v)) ~ k
+    )
+  => NonEmpty k
+  -> NestedMap m k v
+  -> Maybe v
+lookupVal ks m
+  | Just (Val v) <- lookup ks m = Just v
+  | otherwise                  = Nothing
+
+insert
+  :: ( IsMap (m k (NestedMapVal m k v))
+    , MapValue (m k (NestedMapVal m k v)) ~ (NestedMapVal m k v)
+    , ContainerKey (m k (NestedMapVal m k v)) ~ k
+    )
+  => NonEmpty k
+  -> v
+  -> NestedMap m k v
+  -> NestedMap m k v
+insert (k :| []) v (NestedMap m) = NestedMap $ P.insertMap k (Val v) m
+insert (k₁ :| (k₂ : ks)) v (NestedMap m) = NestedMap $ alterMap upd k₁ m
+  where
+    upd (Just (Nested nm)) = Just . Nested $ insert (k₂ :| ks) v nm
+    upd _ = Just $
+      let (kΩ :| ks') = NE.reverse (k₂ :| ks)
+      in P.foldl'
+         (\m' k -> Nested . NestedMap . singletonMap k $ m')
+         (Nested . NestedMap . singletonMap kΩ $ Val v)
+         ks'
+
+-- _NestedMap
+--   :: ( IsMap (m k v), IsMap (m' k' v')
+--     , IsMap (m (NonEmpty k) v), IsMap (m' (NonEmpty k') v')
+--     , ContainerKey (m k v) ~ k, ContainerKey (m' k' v') ~ k'
+--     , ContainerKey (m (NonEmpty k) v) ~ (NonEmpty k)
+--     , ContainerKey (m' (NonEmpty k') v') ~ (NonEmpty k')
+--     , MapValue (m k v) ~ v, MapValue (m' k' v') ~ v'
+--     , MapValue (m (NonEmpty k) v) ~ v, MapValue (m' (NonEmpty k') v') ~ v'
+--     )
+--   => Iso (NestedMap m k v)
+--         (NestedMap m' k' v')
+--         (m (NonEmpty k) v)
+--         (m' (NonEmpty k') v')
+-- _NestedMap = iso undefined yon
+--   where
+--     hither (NestedMap m) = undefined . mapToList $ m
+--     yon mkv = undefined