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+module Language.Fiddle.Internal.Scopes where
+
+import Control.Monad (forM)
+import Data.List (inits, intercalate)
+import Data.List.NonEmpty (NonEmpty (..), prependList)
+import Data.Map (Map)
+import qualified Data.Map as Map
+import Data.Maybe (fromMaybe)
+
+-- | 'Scope' represents a hierarchical structure for storing key-value pairs.
+--   It can contain nested sub-scopes, which are stored in 'subScopes',
+--   and the values for a specific scope are stored in 'scopeValues'.
+data Scope k v = Scope
+  { subScopes :: Map k (Scope k v), -- Nested sub-scopes
+    scopeValues :: Map k v -- Values stored in the current scope
+  }
+
+-- | 'ScopePath' keeps track of the current scope path as a list of keys,
+--   and also includes any additional paths (like imported modules or
+--   using namespaces) that might be referenced for lookup.
+data ScopePath k = ScopePath
+  { currentScope :: [k], -- Current path within the scope hierarchy
+    usingPaths :: [[k]] -- Additional paths for resolving symbols
+  }
+
+-- | The 'Semigroup' instance for 'Scope' allows combining two scopes,
+--   where sub-scopes and values are merged together.
+instance (Ord k) => Semigroup (Scope k t) where
+  (Scope ss1 sv1) <> (Scope ss2 sv2) =
+    Scope (Map.unionWith (<>) ss1 ss2) (Map.union sv1 sv2)
+
+-- | The 'Monoid' instance for 'Scope' provides an empty scope with
+--   no sub-scopes or values.
+instance (Ord k) => Monoid (Scope k t) where
+  mempty = Scope mempty mempty
+
+-- | The 'Semigroup' instance for 'ScopePath' allows combining paths,
+--   appending the current scope path and using paths.
+instance Semigroup (ScopePath k) where
+  (ScopePath a1 b1) <> (ScopePath a2 b2) = ScopePath (a1 <> a2) (b1 <> b2)
+
+-- | The 'Monoid' instance for 'ScopePath' provides an empty path.
+instance Monoid (ScopePath k) where
+  mempty = ScopePath mempty mempty
+
+-- | 'insertScope' inserts a value 'v' into the scope at the specified
+--   key path ('NonEmpty k'). If the key path does not exist, it is created.
+insertScope :: (Ord k) => NonEmpty k -> t -> Scope k t -> Scope k t
+insertScope (s :| []) v (Scope ss sv) = Scope ss (Map.insert s v sv)
+insertScope (s :| (a : as)) v (Scope ss sv) =
+  Scope
+    ( Map.alter
+        ( \case
+            (fromMaybe mempty -> mp) -> Just (insertScope (a :| as) v mp)
+        )
+        s
+        ss
+    )
+    sv
+
+-- | 'lookupScope' performs a lookup of a value in the scope using a key path
+--   ('NonEmpty k'). It traverses through sub-scopes as defined by the path.
+lookupScope :: (Ord k) => NonEmpty k -> Scope k t -> Maybe t
+lookupScope (s :| []) (Scope _ sv) = Map.lookup s sv
+lookupScope (s :| (a : as)) (Scope ss _) = do
+  subscope <- Map.lookup s ss
+  lookupScope (a :| as) subscope
+
+-- | 'lookupScopeWithPath' searches for a key in the scope by trying all possible
+--   paths, including the current scope path and any additional 'using' paths.
+--   It returns a list of all valid matches, each paired with the corresponding
+--   full key path that resolved to a value. This is useful in cases where symbol
+--   resolution might be ambiguous or multiple valid resolutions exist.
+--
+--   The result is a list of tuples, where the first element of each tuple is the
+--   fully-resolved key path (as a 'NonEmpty' list) and the second element is the
+--   resolved value.
+--
+--   This function handles multiple levels of scope resolution, such as those
+--   introduced by 'using' directives, and concatenates the results from each
+--   possible path.
+--
+--   Parameters:
+--   * 'ScopePath k' - The current scope path and any additional 'using' paths.
+--   * 'NonEmpty k'  - The key path to resolve within the scope.
+--   * 'Scope k t'   - The scope containing sub-scopes and values.
+--
+--   Returns: A list of tuples where each tuple contains a fully-resolved key
+--   path (as a 'NonEmpty k') and the corresponding value ('t').
+lookupScopeWithPath ::
+  (Ord k) =>
+  ScopePath k ->
+  NonEmpty k ->
+  Scope k t ->
+  [(NonEmpty k, t)]
+lookupScopeWithPath (ScopePath current others) key scope =
+  let allPaths = reverse (inits current) ++ others
+   in flip concatMap allPaths $ \prefix -> do
+        case lookupScope (prependList prefix key) scope of
+          Just s -> [(prependList prefix key, s)]
+          Nothing -> []