diff --git a/go/fn/object.go b/go/fn/object.go
index 9ef963e9e..46db6a6f4 100644
--- a/go/fn/object.go
+++ b/go/fn/object.go
@@ -431,7 +431,7 @@ func NewFromTypedObject(v interface{}) (*KubeObject, error) {
 }
 
 // String serializes the object in yaml format.
-func (o *KubeObject) String() string {
+func (o *SubObject) String() string {
 	doc := internal.NewDoc([]*yaml.Node{o.obj.Node()}...)
 	s, _ := doc.ToYAML()
 	return string(s)
diff --git a/go/fn/run.go b/go/fn/run.go
index a378648ac..d5d2d95d4 100644
--- a/go/fn/run.go
+++ b/go/fn/run.go
@@ -59,7 +59,15 @@ func AsMain(input interface{}) error {
 
 // Run evaluates the function. input must be a resourceList in yaml format. An
 // updated resourceList will be returned.
-func Run(p ResourceListProcessor, input []byte) (out []byte, err error) {
+func Run(p ResourceListProcessor, input []byte) ([]byte, error) {
+	switch input := p.(type) {
+	case runnerProcessor:
+		p = input
+	case ResourceListProcessorFunc:
+		p = input
+	default:
+		return nil, fmt.Errorf("unknown input type %T", input)
+	}
 	rl, err := ParseResourceList(input)
 	if err != nil {
 		return nil, err
diff --git a/go/fn/testhelpers/golden.go b/go/fn/testhelpers/golden.go
new file mode 100644
index 000000000..6456f25b3
--- /dev/null
+++ b/go/fn/testhelpers/golden.go
@@ -0,0 +1,244 @@
+package testhelpers
+
+import (
+	"bytes"
+	"fmt"
+	"io"
+	"os"
+	"path/filepath"
+	"sort"
+	"strings"
+	"testing"
+
+	"github.com/GoogleContainerTools/kpt-functions-sdk/go/fn"
+	"github.com/google/go-cmp/cmp"
+)
+
+// RunGoldenTests provides the test infra to run golden test.
+// - "basedir" should be the parent directory, under where the sub-directories contains test data.
+//   For example, the "testdata" is the basedir. It contains two cases "test1" and "test2"
+//   └── testdata
+//       └── test1
+//           ├── _expected.yaml
+//           ├── _fnconfig.yaml
+//           └── resources.yaml
+//       └── test2
+//           ├── _expected.yaml
+//           ├── _fnconfig.yaml
+//           └── resources.yaml
+// - "krmFunction" should be your ResourceListProcessor implementation.
+func RunGoldenTests(t *testing.T, basedir string, krmFunction fn.ResourceListProcessor) {
+	dirEntries, err := os.ReadDir(basedir)
+	if err != nil {
+		t.Fatalf("ReadDir(%q) failed: %v", basedir, err)
+	}
+
+	for _, dirEntry := range dirEntries {
+		dir := filepath.Join(basedir, dirEntry.Name())
+		if !dirEntry.IsDir() {
+			t.Errorf("expected directory, found %s", dir)
+			continue
+		}
+
+		t.Run(dir, func(t *testing.T) {
+			files, err := os.ReadDir(dir)
+			if err != nil {
+				t.Fatalf("failed to read directory %q: %v", basedir, err)
+			}
+			sort.Slice(files, func(i, j int) bool { return files[i].Name() < files[j].Name() })
+			var items []*fn.KubeObject
+			for _, f := range files {
+				if strings.HasPrefix(f.Name(), "_") {
+					continue
+				}
+				fileItems := mustParseFile(t, filepath.Join(dir, f.Name()))
+				items = append(items, fileItems...)
+			}
+
+			config := mustParseFile(t, filepath.Join(dir, "_fnconfig.yaml"))
+
+			var functionConfig *fn.KubeObject
+			if len(config) == 0 {
+				functionConfig = nil
+			} else if len(config) == 1 {
+				functionConfig = config[0]
+			} else {
+				t.Fatalf("found multiple config objects in %s", filepath.Join(dir, "_fnconfig.yaml"))
+			}
+
+			rl := &fn.ResourceList{Items: items, FunctionConfig: functionConfig}
+			success, err := krmFunction.Process(rl)
+			if err != nil {
+				t.Fatalf("run failed unexpectedly: %v", err)
+			}
+			if !success {
+				t.Fatalf("run did not succeed")
+			}
+
+			rlYAML, err := rl.ToYAML()
+			if err != nil {
+				t.Fatalf("failed to convert resource list to yaml: %v", err)
+			}
+
+			p := filepath.Join(dir, "_expected.yaml")
+			CompareGoldenFile(t, p, rlYAML)
+		})
+	}
+}
+
+// MustReadFile reads the data from "expectedPath"
+func MustReadFile(t *testing.T, expectedPath string) []byte {
+	b, err := os.ReadFile(expectedPath)
+	if err != nil {
+		t.Fatalf("failed to read file %q: %v", expectedPath, err)
+	}
+	return b
+}
+
+// CompareGoldenFile compares the "got" data with the data stored in a "expectedPath".
+func CompareGoldenFile(t *testing.T, expectedPath string, got []byte) {
+	if os.Getenv("WRITE_GOLDEN_OUTPUT") != "" {
+		// Short-circuit when the output is correct
+		b, err := os.ReadFile(expectedPath)
+		if err == nil && bytes.Equal(b, got) {
+			return
+		}
+
+		if err := os.WriteFile(expectedPath, got, 0600); err != nil {
+			t.Fatalf("failed to write golden output %s: %v", expectedPath, err)
+		}
+		t.Errorf("wrote output to %s", expectedPath)
+	} else {
+		want := MustReadFile(t, expectedPath)
+		if diff := cmp.Diff(string(want), string(got)); diff != "" {
+			t.Errorf("unexpected diff in %s: %s", expectedPath, diff)
+		}
+	}
+}
+
+// CopyDir copies an entire directory from "src" to "dest"
+func CopyDir(src, dest string) error {
+	srcFiles, err := os.ReadDir(src)
+	if err != nil {
+		return fmt.Errorf("ReadDir(%q) failed: %w", src, err)
+	}
+	for _, srcFile := range srcFiles {
+		srcPath := filepath.Join(src, srcFile.Name())
+		destPath := filepath.Join(dest, srcFile.Name())
+		if srcFile.IsDir() {
+			if err = CopyDir(srcPath, destPath); err != nil {
+				return err
+			}
+		} else {
+			if err = CopyFile(srcPath, destPath); err != nil {
+				return err
+			}
+		}
+	}
+	return nil
+}
+
+// CopyFile copies a single file from "src" to "dest"
+func CopyFile(src, dest string) error {
+	in, err := os.Open(src)
+	if err != nil {
+		return fmt.Errorf("OpenFile(%q) failed: %w", src, err)
+	}
+	defer in.Close()
+
+	out, err := os.Create(dest)
+	if err != nil {
+		return fmt.Errorf("create(%q) failed: %w", dest, err)
+	}
+
+	if _, err := io.Copy(out, in); err != nil {
+		out.Close()
+		return fmt.Errorf("byte copy from %s to %s failed: %w", src, dest, err)
+	}
+
+	if err := out.Close(); err != nil {
+		return fmt.Errorf("close(%q) failed: %w", dest, err)
+	}
+
+	return nil
+}
+
+// CompareDir compares the contents of two directories
+// only compare KRM YAML resources?
+func CompareDir(t *testing.T, expectDir, actualDir string) {
+	expectFiles, err := os.ReadDir(expectDir)
+	if err != nil {
+		t.Fatalf("failed to read expectation directory %q: %v", expectDir, err)
+	}
+	expectFileMap := make(map[string]os.DirEntry)
+	for _, expectFile := range expectFiles {
+		expectFileMap[expectFile.Name()] = expectFile
+	}
+
+	actualFiles, err := os.ReadDir(actualDir)
+	if err != nil {
+		t.Fatalf("failed to read actual directory %q: %v", actualDir, err)
+	}
+	actualFileMap := make(map[string]os.DirEntry)
+	for _, actualFile := range actualFiles {
+		actualFileMap[actualFile.Name()] = actualFile
+	}
+
+	for _, expectFile := range expectFiles {
+		name := expectFile.Name()
+		actualFile := actualFileMap[name]
+		if actualFile == nil {
+			t.Errorf("expected file %s not found", name)
+			continue
+		}
+
+		if expectFile.IsDir() {
+			if !actualFile.IsDir() {
+				t.Errorf("expected file %s was not a directory", name)
+				continue
+			}
+			CompareDir(t, filepath.Join(expectDir, name), filepath.Join(actualDir, name))
+		} else {
+			if actualFile.IsDir() {
+				t.Errorf("expected file %s was not a file", name)
+				continue
+			}
+			CompareFile(t, expectDir, actualDir, name)
+		}
+	}
+
+	for _, actualFile := range actualFiles {
+		name := actualFile.Name()
+		expectFile := expectFileMap[name]
+		if expectFile == nil {
+			t.Errorf("additional file %s found in output", name)
+			continue
+		}
+	}
+}
+
+// CompareFile compares a single file of the same relative path between "expectDir" and "actualDir"
+func CompareFile(t *testing.T, expectDir, actualDir string, relPath string) {
+	expectAbs := filepath.Join(expectDir, relPath)
+
+	actualAbs := filepath.Join(actualDir, relPath)
+	actualBytes, err := os.ReadFile(actualAbs)
+	if err != nil {
+		if os.IsNotExist(err) {
+			t.Errorf("expected file %s not found", relPath)
+		} else {
+			t.Fatalf("error reading file %s: %v", actualAbs, err)
+		}
+	}
+
+	CompareGoldenFile(t, expectAbs, actualBytes)
+}
+
+func mustParseFile(t *testing.T, path string) fn.KubeObjects {
+	b := MustReadFile(t, path)
+	objects, err := fn.ParseKubeObjects(b)
+	if err != nil {
+		t.Fatalf("failed to parse objects from file %q: %v", path, err)
+	}
+	return objects
+}