Implement AddParent() method

container.go

@@ -122,6 +122,12 @@ func (c *Container) Provide(constructor Constructor, options ...ProvideOption) e
 	return nil
 }
 
+// AddParent adds a parent container. Types are resolved from the container,
+// it's parents, and ancestors.  An error is a cycle is detected in ancestry tree.
+func (c *Container) AddParent(parent *Container) error {
+	return c.schema.addParent(parent.schema)
+}
+
 // ProvideValue provides value as is.
 func (c *Container) ProvideValue(value Value, options ...ProvideOption) error {
 	if err := c.provideValue(value, options...); err != nil {

container_test.go

@@ -1306,3 +1306,47 @@ func TestContainer_Cleanup(t *testing.T) {
 		require.Equal(t, []string{"server", "mux"}, cleanupCalls)
 	})
 }
+
+func TestParentContainer(t *testing.T) {
+	t.Run("provide in parent and resolve in child", func(t *testing.T) {
+		parent, err := di.New()
+		require.NoError(t, err)
+		require.NotNil(t, parent)
+		mux := &http.ServeMux{}
+		err = parent.ProvideValue(mux, di.As(new(http.Handler)))
+		require.NoError(t, err)
+		child, err := di.New()
+		require.NoError(t, err)
+		require.NotNil(t, child)
+		err = child.Provide(func(handler http.Handler) *http.Server {
+			return &http.Server{
+				Handler: handler,
+			}
+		})
+		require.NoError(t, err)
+		err = child.AddParent(parent)
+		require.NoError(t, err)
+		var server *http.Server
+		err = child.Resolve(&server)
+		require.NoError(t, err)
+		require.Equal(t, fmt.Sprintf("%p", mux), fmt.Sprintf("%p", server.Handler))
+	})
+
+	t.Run("chain errors", func(t *testing.T) {
+		parent, err := di.New()
+		require.NoError(t, err)
+		require.NotNil(t, parent)
+		err = parent.AddParent(parent)
+		require.Contains(t, err.Error(), "self cycle detected")
+		child, err := di.New()
+		require.NoError(t, err)
+		require.NotNil(t, child)
+		err = child.AddParent(parent)
+		require.NoError(t, err)
+		err = parent.AddParent(child)
+		require.Contains(t, err.Error(), "cycle detected")
+		err = child.AddParent(parent)
+		require.Contains(t, err.Error(), "parent already chained")
+	})
+
+}

docs/advanced.md

@@ -204,3 +204,22 @@ if err != nil {
 container.Cleanup() // file was closed
 ```
 
+### Container Chains
+You can chain containers together so that values can be resolved from a parent container.  The child 
+
+```go
+parent, err := container.New(
+    di.Provide(NewServer),
+    di.Provide(NewServeMux),
+)
+
+child, err := container.New()
+
+err = child.AddParent(parent)
+if err != nil {
+// handle error
+}
+
+var server *http.Server
+err := child.Resolve(&server)
+```

schema.go

@@ -15,6 +15,7 @@ type schema interface {
 
 // schema is a dependency injection schema.
 type defaultSchema struct {
+	parents  []*defaultSchema
 	nodes    map[reflect.Type][]*node
 	cleanups []func()
 }
@@ -43,7 +44,7 @@ func (s *defaultSchema) register(n *node) {
 
 // find finds provideFunc by its reflect.Type and Tags.
 func (s *defaultSchema) find(t reflect.Type, tags Tags) (*node, error) {
-	nodes, ok := s.nodes[t]
+	nodes, ok := s.list(t)
 	// type found
 	if ok {
 		matched := matchTags(nodes, tags)
@@ -94,3 +95,42 @@ func (s *defaultSchema) group(t reflect.Type, tags Tags) (*node, error) {
 	}
 	return node, nil
 }
+
+// list lists all the nodes of its reflect.Type
+func (s *defaultSchema) list(t reflect.Type) ([]*node, bool) {
+	nodes, ok := s.nodes[t]
+	for _, parent := range s.parents {
+		if n, o := parent.list(t); o {
+			nodes = append(nodes, n...)
+			ok = true
+		}
+	}
+	return nodes, ok
+}
+
+// isAncestor returns true if a
+func (s *defaultSchema) isAncestor(a *defaultSchema) bool {
+	for _, parent := range s.parents {
+		if parent == a {
+			return true
+		}
+		if parent.isAncestor(a) {
+			return true
+		}
+	}
+	return false
+}
+
+func (s *defaultSchema) addParent(parent *defaultSchema) error {
+	if parent == s  {
+		return fmt.Errorf("self cycle detected")
+	}
+	if parent.isAncestor(s) {
+		return fmt.Errorf("cycle detected")
+	}
+	if s.isAncestor(parent) {
+		return fmt.Errorf("parent already chained")
+	}
+	s.parents = append(s.parents, parent)
+	return nil
+}