feat: add error handling

README.md

@@ -102,8 +102,9 @@ func (c *Container) MustResolve(target interface{})
 - [x] Topological sorting and circular dependency detection
 - [x] Thread-safe container operations
 - [x] Interface binding support
-- [ ] Error handling
+- [x] Error handling
 - [ ] Named dependency resolution
+- [ ] Dependency graph visualization
 
 ## 📊 Benefits
 

container.go

@@ -145,16 +145,25 @@ func (c *Container) analyzeFunction(fnType reflect.Type) (fnSignature, error) {
 	}
 
 	// Analyze return values
-	// todo: add here error handling
+	// Support either: (*T) or (*T, error)
+	if fnType.NumOut() == 0 || fnType.NumOut() > 2 {
+		return fnSignature{}, fmt.Errorf("constructor must return (*T) or (*T, error)")
+	}
 
-	if fnType.NumOut() != 1 {
-		return fnSignature{}, fmt.Errorf("constructor must have one return value")
+	firstOut := fnType.Out(0)
+	if firstOut.Kind() != reflect.Pointer {
+		return fnSignature{}, fmt.Errorf("constructor must return pointer value as first result")
 	}
-	if fnType.Out(0).Kind() != reflect.Pointer {
-		return fnSignature{}, fmt.Errorf("constructor must return pointer value")
+
+	if fnType.NumOut() == 2 {
+		secondOut := fnType.Out(1)
+		errorType := reflect.TypeOf((*error)(nil)).Elem()
+		if !secondOut.Implements(errorType) {
+			return fnSignature{}, fmt.Errorf("second return value must be error")
+		}
 	}
 
-	return fnSignature{args, fnType.Out(0)}, nil
+	return fnSignature{args, firstOut}, nil
 }
 
 // Resolve resolves and returns an instance of the requested type.
@@ -367,10 +376,11 @@ func (c *Container) resolveInstance(typ reflect.Type) error {
 	// Call constructor
 	results := constructorValue.Call(args)
 
-	// Handle error return
-	if len(results) > 0 {
+	// Handle optional error return (when present and non-nil)
+	if len(results) > 1 {
 		lastResult := results[len(results)-1]
-		if lastResult.Type().String() == "error" && !lastResult.IsNil() {
+		errorType := reflect.TypeOf((*error)(nil)).Elem()
+		if lastResult.Type().Implements(errorType) && !lastResult.IsNil() {
 			return lastResult.Interface().(error)
 		}
 	}

container_test.go

@@ -220,14 +220,14 @@ var _ = Describe("Container", func() {
 	})
 
 	Describe("Error Handling", func() {
-		PIt("should handle constructor errors", func() {
+		It("should handle constructor errors", func() {
 			Expect(container.Provide(NewErrorDatabase)).To(Succeed())
 
 			var db *Database
 			Expect(container.Resolve(&db)).To(MatchError(ContainSubstring("database connection failed")))
 		})
 
-		PIt("should handle successful constructors with error return", func() {
+		It("should handle successful constructors with error return", func() {
 			Expect(container.Provide(NewSuccessfulDatabase)).To(Succeed())
 
 			var db *Database