Define an use an overwritingUnion for environments

Semigroup instances for Map is using `union`, which is left biased and not always the behavior we want when appending environments.

src/Control/Abstract/Value.hs

@@ -205,7 +205,7 @@ instance ( Monad m
   namespace n env = do
     maybeAddr <- lookupEnv n
     env' <- maybe (pure mempty) (asNamespaceEnv <=< deref) maybeAddr
-    pure (injValue (Namespace n (env' <> env)))
+    pure (injValue (Namespace n (Env.overwritingUnion env' env)))
     where asNamespaceEnv v
             | Just (Namespace _ env') <- prjValue v = pure env'
             | otherwise                             = fail ("expected " <> show v <> " to be a namespace")

src/Data/Abstract/Environment.hs

@@ -5,6 +5,7 @@ module Data.Abstract.Environment
   , bind
   , delete
   , head
+  , overwritingUnion
   , insert
   , lookup
   , names
@@ -73,6 +74,14 @@ pop (Environment (_ :| a : as)) = Environment (a :| as)
 head :: Environment l a -> Environment l a
 head (Environment (a :| _)) = Environment (a :| [])
 
+
+-- | Take the union of two environments. When duplicate keys are found in the
+--   name to address map, the second definition wins.
+overwritingUnion :: Environment l a -> Environment l a -> Environment l a
+overwritingUnion (Environment (a :| as)) (Environment (b :| bs)) =
+  Environment (combine a b :| alignWith (mergeThese combine) as bs)
+  where combine = Map.unionWith (flip const)
+
 -- | Extract an association list of bindings from an 'Environment'.
 --
 -- >>> pairs shadowed