diff --git a/.gitignore b/.gitignore
index 8dfd59f..e18d195 100644
--- a/.gitignore
+++ b/.gitignore
@@ -12,3 +12,4 @@
 *.out
 
 go.work*
+.gop/
diff --git a/gopmod/classfile.go b/gopmod/classfile.go
index b132bf2..3bd6fca 100644
--- a/gopmod/classfile.go
+++ b/gopmod/classfile.go
@@ -33,7 +33,7 @@ var (
 	SpxProject = &Project{
 		Ext:      ".spx",
 		Class:    "Game",
-		Works:    []*Class{{Ext: ".spx", Class: "Sprite"}},
+		Works:    []*modfile.Class{{Ext: ".spx", Class: "Sprite"}},
 		PkgPaths: []string{"github.com/goplus/spx", "math"},
 	}
 	TestProject = &Project{
diff --git a/modload/module.go b/modload/module.go
index b58e93d..f20d6be 100644
--- a/modload/module.go
+++ b/modload/module.go
@@ -23,6 +23,7 @@ import (
 	"strings"
 
 	"github.com/goplus/mod"
+	"github.com/goplus/mod/env"
 	"github.com/goplus/mod/modfile"
 	"github.com/qiniu/x/errors"
 	"golang.org/x/mod/module"
@@ -54,6 +55,14 @@ func (p Module) Modfile() string {
 	return ""
 }
 
+func (p Module) workFile() string {
+	if syn := p.Syntax; syn != nil {
+		dir, _ := filepath.Split(syn.Name)
+		return dir + "go.work"
+	}
+	return ""
+}
+
 // Root returns absolute root path of this module.
 func (p Module) Root() string {
 	if syn := p.Syntax; syn != nil {
@@ -115,6 +124,12 @@ func Create(dir string, modPath, goVer, gopVer string) (p Module, err error) {
 		return Module{}, fmt.Errorf("gop: %s already exists", gopmod)
 	}
 
+	if goVer == "" {
+		goVer = defaultGoVer
+	}
+	if gopVer == "" {
+		gopVer = defaultGopVer
+	}
 	mod := newGoMod(gomod, modPath, goVer)
 	opt := newGopMod(gopmod, gopVer)
 	return Module{mod, opt}, nil
@@ -285,115 +300,47 @@ func (p Module) Save() (err error) {
 	return
 }
 
-/*
-const (
-	gopMod = "github.com/goplus/gop"
-)
-
-// UpdateGoMod updates the go.mod file.
-func (p Module) UpdateGoMod(env *env.Gop, checkChanged bool) error {
-	gopmod := p.Modfile()
-	dir, file := filepath.Split(gopmod)
-	if file == "go.mod" {
-		return nil
-	}
-	gomod := dir + "go.mod"
-	if checkChanged && notChanged(gomod, gopmod) {
-		return nil
-	}
-	return p.saveGoMod(gomod, env)
-}
-
-func (p Module) saveGoMod(gomod string, env *env.Gop) error {
-	gof := p.convToGoMod(env)
-	data, err := gof.Format()
-	if err == nil {
-		err = os.WriteFile(gomod, data, 0644)
-	}
-	return err
-}
-
-func (p Module) convToGoMod(env *env.Gop) *gomodfile.File {
-	copy := p.File.File
-	copy.Syntax = cloneGoFileSyntax(copy.Syntax)
-	addRequireIfNotExist(&copy, gopMod, env.Version)
-	addReplaceIfNotExist(&copy, gopMod, "", env.Root, "")
-	return &copy
-}
-
-func addRequireIfNotExist(f *gomodfile.File, path, vers string) {
-	for _, r := range f.Require {
-		if r.Mod.Path == path {
-			return
-		}
-	}
-	f.AddNewRequire(path, vers, false)
-}
-
-func addReplaceIfNotExist(f *gomodfile.File, oldPath, oldVers, newPath, newVers string) {
-	for _, r := range f.Replace {
-		if r.Old.Path == oldPath && (oldVers == "" || r.Old.Version == oldVers) {
-			return
-		}
+// SaveWithGopMod adds `require github.com/goplus/gop` and saves all
+// changes of this module.
+func (p Module) SaveWithGopMod(gop *env.Gop, flags int) (err error) {
+	gopVer := getGopVer(gop)
+	p.requireGop(gop, gopVer, flags)
+	if err = p.Save(); err != nil {
+		return
 	}
-	f.AddReplace(oldPath, oldVers, newPath, newVers)
-}
 
-func notChanged(target, src string) bool {
-	fiTarget, err := os.Stat(target)
+	var work *gomodfile.WorkFile
+	var workFile = p.workFile()
+	b, err := os.ReadFile(workFile)
 	if err != nil {
-		return false
-	}
-	fiSrc, err := os.Stat(src)
-	if err != nil {
-		return false
-	}
-	return fiTarget.ModTime().After(fiSrc.ModTime())
-}
-
-// -----------------------------------------------------------------------------
-
-func cloneGoFileSyntax(syn *modfile.FileSyntax) *modfile.FileSyntax {
-	stmt := make([]modfile.Expr, 0, len(syn.Stmt))
-	for _, e := range syn.Stmt {
-		if isGopOrDeletedExpr(e) {
-			continue
+		if os.IsNotExist(err) {
+			b = []byte(`go ` + p.Go.Version)
+		} else {
+			return
 		}
-		stmt = append(stmt, cloneExpr(e))
-	}
-	return &modfile.FileSyntax{
-		Name:     syn.Name,
-		Comments: syn.Comments,
-		Stmt:     stmt,
 	}
-}
-
-func cloneExpr(e modfile.Expr) modfile.Expr {
-	if v, ok := e.(*modfile.LineBlock); ok {
-		copy := *v
-		return &copy
+	var fixed bool
+	fix := fixVersion(&fixed)
+	if work, err = gomodfile.ParseWork(workFile, b, fix); err != nil {
+		return
 	}
-	return e
+	work.AddReplace("github.com/goplus/gop", gopVer, gop.Root, "")
+	return os.WriteFile(workFile, gomodfile.Format(work.Syntax), 0666)
 }
 
-func isGopOrDeletedExpr(e modfile.Expr) bool {
-	switch verb := getVerb(e); verb {
-	case "", "gop", "register", "project", "class":
-		return true
-	}
-	return false
+// requireGop adds require for the github.com/goplus/gop module.
+func (p Module) requireGop(gop *env.Gop, gopVer string, flags int) {
+	p.File.AddRequire("github.com/goplus/gop", gopVer)
+	// TODO: AddRequire "github.com/qiniu/x" if necessary
 }
 
-func getVerb(e modfile.Expr) string {
-	if line, ok := e.(*modfile.Line); ok {
-		if token := line.Token; len(token) > 0 {
-			return token[0]
-		}
-		return "" // deleted line
+func getGopVer(gop *env.Gop) string {
+	ver := gop.Version
+	if pos := strings.IndexByte(ver, ' '); pos > 0 { // v1.2.0 devel
+		ver = ver[:pos]
 	}
-	return e.(*modfile.LineBlock).Token[0]
+	return ver
 }
-*/
 
 // -----------------------------------------------------------------------------
 
diff --git a/modload/module_test.go b/modload/module_test.go
index 248d84e..40a0da7 100644
--- a/modload/module_test.go
+++ b/modload/module_test.go
@@ -18,9 +18,11 @@ package modload
 
 import (
 	"encoding/json"
+	"os"
 	"runtime"
 	"testing"
 
+	"github.com/goplus/mod/env"
 	"github.com/goplus/mod/modfile"
 	gomodfile "golang.org/x/mod/modfile"
 )
@@ -131,3 +133,76 @@ replace github.com/goplus/yap v0.7.2 => ../
 		}
 	}
 }
+
+func TestSaveDefault(t *testing.T) {
+	if v := Default.workFile(); v != "" {
+		t.Fatal("Default.workFile:", v)
+	}
+	if err := Default.Save(); err != ErrSaveDefault {
+		t.Fatal("Default.Save:", err)
+	}
+}
+
+func TestSave(t *testing.T) {
+	dir := ".gop/_tempdir"
+	os.RemoveAll(dir)
+	os.MkdirAll(dir, 0777)
+	mod, err := Create(dir, "github.com/foo/bar", "", "")
+	if err != nil {
+		t.Fatal("Create:", err)
+	}
+	if err = mod.AddRequire("github.com/goplus/yap", "v0.5.0", true); err != nil {
+		t.Fatal("mod.AddRequire:", err)
+	}
+	mod.Save()
+
+	mod, err = Load(dir)
+	if err != nil {
+		t.Fatal("Load:", err)
+	}
+	if err = mod.SaveWithGopMod(&env.Gop{Version: "v1.2.0 devel", Root: "/foo/bar/gop"}, 0); err != nil {
+		t.Fatal("mod.SaveWithGopMod:", err)
+	}
+	if b, err := mod.File.Format(); err != nil {
+		t.Fatal("Format:", err)
+	} else if v := string(b); v != `module github.com/foo/bar
+
+go 1.18
+
+require (
+	github.com/goplus/yap v0.5.0 //gop:class
+	github.com/goplus/gop v1.2.0
+)
+` {
+		t.Fatal("SaveWithGopMod:", v)
+	}
+	b, err := os.ReadFile(mod.workFile())
+	if err != nil {
+		t.Fatal("read workFile:", err)
+	}
+	if v := string(b); v != `go 1.18
+
+replace github.com/goplus/gop v1.2.0 => /foo/bar/gop
+` {
+		t.Fatal("workFile:", v)
+	}
+	mod.Opt.Projects = append(mod.Opt.Projects, spxProject)
+	mod.Save()
+	b, err = os.ReadFile(mod.Opt.Syntax.Name)
+	if err != nil {
+		t.Fatal("read gop.mod:", err)
+	}
+	if v := string(b); v != `gop 1.2
+` {
+		t.Fatal("gop.mod:", v)
+	}
+}
+
+var (
+	spxProject = &modfile.Project{
+		Ext:      ".spx",
+		Class:    "Game",
+		Works:    []*modfile.Class{{Ext: ".spx", Class: "Sprite"}},
+		PkgPaths: []string{"github.com/goplus/spx", "math"},
+	}
+)