text/template: new, simpler API

The Set type is gone. Instead, templates are automatically associated by
being parsed together; nested definitions implicitly create associations.
Only associated templates can invoke one another.

This approach dramatically reduces the breadth of the construction API.

For now, html/template is deleted from src/pkg/Makefile, so this can
be checked in. Nothing in the tree depends on it. It will be updated next.

R=dsymonds, adg, rsc, r, gri, mikesamuel, nigeltao
CC=golang-dev
https://golang.org/cl/5415060

doc/codelab/wiki/final-noclosure.go

@@ -68,7 +68,7 @@ func saveHandler(w http.ResponseWriter, r *http.Request) {
 }
 
 func renderTemplate(w http.ResponseWriter, tmpl string, p *Page) {
-	t, err := template.ParseFile(tmpl + ".html")
+	t, err := template.ParseFiles(tmpl + ".html")
 	if err != nil {
 		http.Error(w, err.Error(), http.StatusInternalServerError)
 		return

doc/codelab/wiki/final-noerror.go

@@ -33,14 +33,14 @@ func editHandler(w http.ResponseWriter, r *http.Request) {
 	if err != nil {
 		p = &Page{Title: title}
 	}
-	t, _ := template.ParseFile("edit.html")
+	t, _ := template.ParseFiles("edit.html")
 	t.Execute(w, p)
 }
 
 func viewHandler(w http.ResponseWriter, r *http.Request) {
 	title := r.URL.Path[lenPath:]
 	p, _ := loadPage(title)
-	t, _ := template.ParseFile("view.html")
+	t, _ := template.ParseFiles("view.html")
 	t.Execute(w, p)
 }
 

doc/codelab/wiki/final-parsetemplate.go

@@ -55,7 +55,7 @@ func saveHandler(w http.ResponseWriter, r *http.Request, title string) {
 }
 
 func renderTemplate(w http.ResponseWriter, tmpl string, p *Page) {
-	t, err := template.ParseFile(tmpl+".html", nil)
+	t, err := template.ParseFiles(tmpl+".html", nil)
 	if err != nil {
 		http.Error(w, err.Error(), http.StatusInternalServerError)
 		return

doc/codelab/wiki/final-template.go

@@ -51,7 +51,7 @@ func saveHandler(w http.ResponseWriter, r *http.Request) {
 }
 
 func renderTemplate(w http.ResponseWriter, tmpl string, p *Page) {
-	t, _ := template.ParseFile(tmpl+".html", nil)
+	t, _ := template.ParseFiles(tmpl+".html", nil)
 	t.Execute(w, p)
 }
 

doc/codelab/wiki/final.go

@@ -58,7 +58,7 @@ var templates = make(map[string]*template.Template)
 
 func init() {
 	for _, tmpl := range []string{"edit", "view"} {
-		t := template.Must(template.ParseFile(tmpl + ".html"))
+		t := template.Must(template.ParseFiles(tmpl + ".html"))
 		templates[tmpl] = t
 	}
 }

doc/codelab/wiki/index.html

@@ -476,7 +476,7 @@ func editHandler(w http.ResponseWriter, r *http.Request) {
 	if err != nil {
 		p = &Page{Title: title}
 	}
-	t, _ := template.ParseFile("edit.html")
+	t, _ := template.ParseFiles("edit.html")
 	t.Execute(w, p)
 }
 </pre>
@@ -530,7 +530,7 @@ Modify <code>viewHandler</code> accordingly:
 func viewHandler(w http.ResponseWriter, r *http.Request) {
 	title := r.URL.Path[lenPath:]
 	p, _ := loadPage(title)
-	t, _ := template.ParseFile(&#34;view.html&#34;)
+	t, _ := template.ParseFiles(&#34;view.html&#34;)
 	t.Execute(w, p)
 }
 </pre>
@@ -558,7 +558,7 @@ func editHandler(w http.ResponseWriter, r *http.Request) {
 }
 
 func renderTemplate(w http.ResponseWriter, tmpl string, p *Page) {
-	t, _ := template.ParseFile(tmpl+&#34;.html&#34;, nil)
+	t, _ := template.ParseFiles(tmpl+&#34;.html&#34;, nil)
 	t.Execute(w, p)
 }
 </pre>
@@ -643,7 +643,7 @@ First, let's handle the errors in <code>renderTemplate</code>:
 
 <pre>
 func renderTemplate(w http.ResponseWriter, tmpl string, p *Page) {
-	t, err := template.ParseFile(tmpl+&#34;.html&#34;, nil)
+	t, err := template.ParseFiles(tmpl+&#34;.html&#34;, nil)
 	if err != nil {
 		http.Error(w, err.Error(), http.StatusInternalServerError)
 		return
@@ -719,7 +719,7 @@ can't be loaded the only sensible thing to do is exit the program.
 <pre>
 func init() {
 	for _, tmpl := range []string{&#34;edit&#34;, &#34;view&#34;} {
-		t := template.Must(template.ParseFile(tmpl + &#34;.html&#34;))
+		t := template.Must(template.ParseFiles(tmpl + &#34;.html&#34;))
 		templates[tmpl] = t
 	}
 }

doc/tmpltohtml.go

@@ -45,7 +45,7 @@ func main() {
 	// Read and parse the input.
 	name := flag.Args()[0]
 	tmpl := template.New(name).Funcs(template.FuncMap{"code": code})
-	if _, err := tmpl.ParseFile(name); err != nil {
+	if _, err := tmpl.ParseFiles(name); err != nil {
 		log.Fatal(err)
 	}
 

src/pkg/Makefile

@@ -102,7 +102,6 @@ DIRS=\
 	hash/crc64\
 	hash/fnv\
 	html\
-	html/template\
 	image\
 	image/bmp\
 	image/color\

src/pkg/text/template/Makefile

@@ -10,7 +10,6 @@ GOFILES=\
 	exec.go\
 	funcs.go\
 	helper.go\
-	parse.go\
-	set.go\
+	template.go\
 
 include ../../../Make.pkg

src/pkg/text/template/doc.go

@@ -18,8 +18,7 @@ The input text for a template is UTF-8-encoded text in any format.
 "{{" and "}}"; all text outside actions is copied to the output unchanged.
 Actions may not span newlines, although comments can.
 
-Once constructed, templates and template sets can be executed safely in
-parallel.
+Once constructed, a template may be executed safely in parallel.
 
 Actions
 
@@ -221,10 +220,9 @@ All produce the quoted word "output":
 
 Functions
 
-During execution functions are found in three function maps: first in the
-template, then in the "template set" (described below), and finally in the
-global function map. By default, no functions are defined in the template or
-the set but the Funcs methods can be used to add them.
+During execution functions are found in two function maps: first in the
+template, then in the global function map. By default, no functions are defined
+in the template but the Funcs methods can be used to add them.
 
 Predefined global functions are named as follows.
 
@@ -265,49 +263,63 @@ Predefined global functions are named as follows.
 The boolean functions take any zero value to be false and a non-zero value to
 be true.
 
-Template sets
+Associated templates
 
-Each template is named by a string specified when it is created.  A template may
-use a template invocation to instantiate another template directly or by its
-name; see the explanation of the template action above. The name is looked up
-in the template set associated with the template.
+Each template is named by a string specified when it is created. Also, each
+template is associated with zero or more other templates that it may invoke by
+name; such associations are transitive and form a name space of templates.
 
-If no template invocation actions occur in the template, the issue of template
-sets can be ignored.  If it does contain invocations, though, the template
-containing the invocations must be part of a template set in which to look up
-the names.
+A template may use a template invocation to instantiate another associated
+template; see the explanation of the "template" action above. The name must be
+that of a template associated with the template that contains the invocation.
 
-There are two ways to construct template sets.
+Nested template definitions
 
-The first is to use a Set's Parse method to create a set of named templates from
-a single input defining multiple templates.  The syntax of the definitions is to
-surround each template declaration with a define and end action.
+When parsing a template, another template may be defined and associated with the
+template being parsed. Template definitions must appear at the top level of the
+template, much like global variables in a Go program.
+
+The syntax of such definitions is to surround each template declaration with a
+"define" and "end" action.
 
 The define action names the template being created by providing a string
-constant. Here is a simple example of input to Set.Parse:
+constant. Here is a simple example:
 
-	`{{define "T1"}} definition of template T1 {{end}}
-	{{define "T2"}} definition of template T2 {{end}}
-	{{define "T3"}} {{template "T1"}} {{template "T2"}} {{end}}`
+	`{{define "T1"}}ONE{{end}}
+	{{define "T2"}}TWO{{end}}
+	{{define "T3"}}{{template "T1"}} {{template "T2"}}{{end}}
+	{{template "T3"}}`
 
 This defines two templates, T1 and T2, and a third T3 that invokes the other two
-when it is executed.
+when it is executed. Finally it invokes T3. If executed this template will
+produce the text
+
+	ONE TWO
+
+By construction, a template may reside in only one association. If it's
+necessary to have a template addressable from multiple associations, the
+template definition must be parsed multiple times to create distinct *Template
+values.
+
+Parse may be called multiple times to assemble the various associated templates;
+see the ParseFiles and ParseGlob functions and methods for simple ways to parse
+related templates stored in files.
 
-The second way to build a template set is to use Set's Add method to add a
-parsed template to a set.  A template may be bound to at most one set.  If it's
-necessary to have a template in multiple sets, the template definition must be
-parsed multiple times to create distinct *Template values.
+A template may be executed directly or through ExecuteTemplate, which executes
+an associated template identified by name. To invoke our example above, we
+might write,
 
-Set.Parse may be called multiple times on different inputs to construct the set.
-Two sets may therefore be constructed with a common base set of templates plus,
-through a second Parse call each, specializations for some elements.
+	err := tmpl.Execute(os.Stdout, "no data needed")
+	if err != nil {
+		log.Fatalf("execution failed: %s", err)
+	}
 
-A template may be executed directly or through Set.Execute, which executes a
-named template from the set.  To invoke our example above, we might write,
+or to invoke a particular template explicitly by name,
 
-	err := set.Execute(os.Stdout, "T3", "no data needed")
+	err := tmpl.ExecuteTemplate(os.Stdout, "T2", "no data needed")
 	if err != nil {
 		log.Fatalf("execution failed: %s", err)
 	}
+
 */
 package template

src/pkg/text/template/exec.go

@@ -85,8 +85,18 @@ func errRecover(errp *error) {
 	}
 }
 
+// ExecuteTemplate applies the template associated with t that has the given name
+// to the specified data object and writes the output to wr.
+func (t *Template) ExecuteTemplate(wr io.Writer, name string, data interface{}) error {
+	tmpl := t.tmpl[name]
+	if tmpl == nil {
+		return fmt.Errorf("template: no template %q associated with template %q", name, t.name)
+	}
+	return tmpl.Execute(wr, data)
+}
+
 // Execute applies a parsed template to the specified data object,
-// writing the output to wr.
+// and writes the output to wr.
 func (t *Template) Execute(wr io.Writer, data interface{}) (err error) {
 	defer errRecover(&err)
 	value := reflect.ValueOf(data)
@@ -251,13 +261,9 @@ func (s *state) walkRange(dot reflect.Value, r *parse.RangeNode) {
 }
 
 func (s *state) walkTemplate(dot reflect.Value, t *parse.TemplateNode) {
-	set := s.tmpl.set
-	if set == nil {
-		s.errorf("no set defined in which to invoke template named %q", t.Name)
-	}
-	tmpl := set.tmpl[t.Name]
+	tmpl := s.tmpl.tmpl[t.Name]
 	if tmpl == nil {
-		s.errorf("template %q not in set", t.Name)
+		s.errorf("template %q not defined", t.Name)
 	}
 	// Variables declared by the pipeline persist.
 	dot = s.evalPipeline(dot, t.Pipe)
@@ -376,7 +382,7 @@ func (s *state) evalFieldChain(dot, receiver reflect.Value, ident []string, args
 }
 
 func (s *state) evalFunction(dot reflect.Value, name string, args []parse.Node, final reflect.Value) reflect.Value {
-	function, ok := findFunction(name, s.tmpl, s.tmpl.set)
+	function, ok := findFunction(name, s.tmpl)
 	if !ok {
 		s.errorf("%q is not a defined function", name)
 	}
@@ -398,7 +404,7 @@ func (s *state) evalField(dot reflect.Value, fieldName string, args []parse.Node
 	if ptr.Kind() != reflect.Interface && ptr.CanAddr() {
 		ptr = ptr.Addr()
 	}
-	if method, ok := methodByName(ptr, fieldName); ok {
+	if method := ptr.MethodByName(fieldName); method.IsValid() {
 		return s.evalCall(dot, method, fieldName, args, final)
 	}
 	hasArgs := len(args) > 1 || final.IsValid()
@@ -433,17 +439,6 @@ func (s *state) evalField(dot reflect.Value, fieldName string, args []parse.Node
 	panic("not reached")
 }
 
-// TODO: delete when reflect's own MethodByName is released.
-func methodByName(receiver reflect.Value, name string) (reflect.Value, bool) {
-	typ := receiver.Type()
-	for i := 0; i < typ.NumMethod(); i++ {
-		if typ.Method(i).Name == name {
-			return receiver.Method(i), true // This value includes the receiver.
-		}
-	}
-	return zero, false
-}
-
 var (
 	errorType       = reflect.TypeOf((*error)(nil)).Elem()
 	fmtStringerType = reflect.TypeOf((*fmt.Stringer)(nil)).Elem()

src/pkg/text/template/exec_test.go

@@ -476,7 +476,7 @@ func vfunc(V, *V) string {
 	return "vfunc"
 }
 
-func testExecute(execTests []execTest, set *Set, t *testing.T) {
+func testExecute(execTests []execTest, template *Template, t *testing.T) {
 	b := new(bytes.Buffer)
 	funcs := FuncMap{
 		"count":    count,
@@ -486,12 +486,13 @@ func testExecute(execTests []execTest, set *Set, t *testing.T) {
 		"zeroArgs": zeroArgs,
 	}
 	for _, test := range execTests {
-		tmpl := New(test.name).Funcs(funcs)
-		theSet := set
-		if theSet == nil {
-			theSet = new(Set)
+		var tmpl *Template
+		var err error
+		if template == nil {
+			tmpl, err = New(test.name).Funcs(funcs).Parse(test.input)
+		} else {
+			tmpl, err = template.New(test.name).Funcs(funcs).Parse(test.input)
 		}
-		_, err := tmpl.ParseInSet(test.input, theSet)
 		if err != nil {
 			t.Errorf("%s: parse error: %s", test.name, err)
 			continue
@@ -663,24 +664,34 @@ func TestTree(t *testing.T) {
 			},
 		},
 	}
-	set := new(Set)
-	_, err := set.Delims("(", ")").Parse(treeTemplate)
+	tmpl, err := New("root").Delims("(", ")").Parse(treeTemplate)
 	if err != nil {
 		t.Fatal("parse error:", err)
 	}
 	var b bytes.Buffer
-	err = set.Execute(&b, "tree", tree)
-	if err != nil {
-		t.Fatal("exec error:", err)
-	}
 	stripSpace := func(r rune) rune {
 		if r == '\t' || r == '\n' {
 			return -1
 		}
 		return r
 	}
-	result := strings.Map(stripSpace, b.String())
 	const expect = "[1[2[3[4]][5[6]]][7[8[9]][10[11]]]]"
+	// First by looking up the template.
+	err = tmpl.Template("tree").Execute(&b, tree)
+	if err != nil {
+		t.Fatal("exec error:", err)
+	}
+	result := strings.Map(stripSpace, b.String())
+	if result != expect {
+		t.Errorf("expected %q got %q", expect, result)
+	}
+	// Then direct to execution.
+	b.Reset()
+	err = tmpl.ExecuteTemplate(&b, "tree", tree)
+	if err != nil {
+		t.Fatal("exec error:", err)
+	}
+	result = strings.Map(stripSpace, b.String())
 	if result != expect {
 		t.Errorf("expected %q got %q", expect, result)
 	}

src/pkg/text/template/funcs.go

@@ -17,8 +17,9 @@ import (
 
 // FuncMap is the type of the map defining the mapping from names to functions.
 // Each function must have either a single return value, or two return values of
-// which the second has type error. If the second argument evaluates to non-nil
-// during execution, execution terminates and Execute returns an error.
+// which the second has type error. In that case, if the second (error)
+// argument evaluates to non-nil during execution, execution terminates and
+// Execute returns that error.
 type FuncMap map[string]interface{}
 
 var builtins = FuncMap{
@@ -78,18 +79,13 @@ func goodFunc(typ reflect.Type) bool {
 	return false
 }
 
-// findFunction looks for a function in the template, set, and global map.
-func findFunction(name string, tmpl *Template, set *Set) (reflect.Value, bool) {
-	if tmpl != nil {
+// findFunction looks for a function in the template, and global map.
+func findFunction(name string, tmpl *Template) (reflect.Value, bool) {
+	if tmpl != nil && tmpl.common != nil {
 		if fn := tmpl.execFuncs[name]; fn.IsValid() {
 			return fn, true
 		}
 	}
-	if set != nil {
-		if fn := set.execFuncs[name]; fn.IsValid() {
-			return fn, true
-		}
-	}
 	if fn := builtinFuncs[name]; fn.IsValid() {
 		return fn, true
 	}
@@ -310,7 +306,6 @@ func JSEscape(w io.Writer, b []byte) {
 			if unicode.IsPrint(r) {
 				w.Write(b[i : i+size])
 			} else {
-				// TODO(dsymonds): Do this without fmt?
 				fmt.Fprintf(w, "\\u%04X", r)
 			}
 			i += size - 1

src/pkg/text/template/helper.go

@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// Helper functions to make constructing templates and sets easier.
+// Helper functions to make constructing templates easier.
 
 package template
 
@@ -12,11 +12,11 @@ import (
 	"path/filepath"
 )
 
-// Functions and methods to parse a single template.
+// Functions and methods to parse templates.
 
 // Must is a helper that wraps a call to a function returning (*Template, error)
-// and panics if the error is non-nil. It is intended for use in variable initializations
-// such as
+// and panics if the error is non-nil. It is intended for use in variable
+// initializations such as
 //	var t = template.Must(template.New("name").Parse("text"))
 func Must(t *Template, err error) *Template {
 	if err != nil {
@@ -25,217 +25,84 @@ func Must(t *Template, err error) *Template {
 	return t
 }
 
-// ParseFile creates a new Template and parses the template definition from
-// the named file.  The template name is the base name of the file.
-func ParseFile(filename string) (*Template, error) {
-	t := New(filepath.Base(filename))
-	return t.ParseFile(filename)
+// ParseFiles creates a new Template and parses the template definitions from
+// the named files. The returned template's name will have the (base) name and
+// (parsed) contents of the first file. There must be at least one file.
+// If an error occurs, parsing stops and the returned *Template is nil.
+func ParseFiles(filenames ...string) (*Template, error) {
+	return parseFiles(nil, filenames...)
 }
 
-// parseFileInSet creates a new Template and parses the template
-// definition from the named file. The template name is the base name
-// of the file. It also adds the template to the set. Function bindings are
-// checked against those in the set.
-func parseFileInSet(filename string, set *Set) (*Template, error) {
-	t := New(filepath.Base(filename))
-	return t.parseFileInSet(filename, set)
+// ParseFiles parses the named files and associates the resulting templates with
+// t. If an error occurs, parsing stops and the returned template is nil;
+// otherwise it is t. There must be at least one file.
+func (t *Template) ParseFiles(filenames ...string) (*Template, error) {
+	return parseFiles(t, filenames...)
 }
 
-// ParseFile reads the template definition from a file and parses it to
-// construct an internal representation of the template for execution.
-// The returned template will be nil if an error occurs.
-func (t *Template) ParseFile(filename string) (*Template, error) {
-	b, err := ioutil.ReadFile(filename)
-	if err != nil {
-		return nil, err
-	}
-	return t.Parse(string(b))
-}
-
-// parseFileInSet is the same as ParseFile except that function bindings
-// are checked against those in the set and the template is added
-// to the set.
-// The returned template will be nil if an error occurs.
-func (t *Template) parseFileInSet(filename string, set *Set) (*Template, error) {
-	b, err := ioutil.ReadFile(filename)
-	if err != nil {
-		return nil, err
-	}
-	return t.ParseInSet(string(b), set)
-}
-
-// Functions and methods to parse a set.
-
-// SetMust is a helper that wraps a call to a function returning (*Set, error)
-// and panics if the error is non-nil. It is intended for use in variable initializations
-// such as
-//	var s = template.SetMust(template.ParseSetFiles("file"))
-func SetMust(s *Set, err error) *Set {
-	if err != nil {
-		panic(err)
+// parseFiles is the helper for the method and function. If the argument
+// template is nil, it is created from the first file.
+func parseFiles(t *Template, filenames ...string) (*Template, error) {
+	if len(filenames) == 0 {
+		// Not really a problem, but be consistent.
+		return nil, fmt.Errorf("template: no files named in call to ParseFiles")
 	}
-	return s
-}
-
-// ParseFiles parses the named files into a set of named templates.
-// Each file must be parseable by itself.
-// If an error occurs, parsing stops and the returned set is nil.
-func (s *Set) ParseFiles(filenames ...string) (*Set, error) {
 	for _, filename := range filenames {
 		b, err := ioutil.ReadFile(filename)
 		if err != nil {
 			return nil, err
 		}
-		_, err = s.Parse(string(b))
-		if err != nil {
-			return nil, err
-		}
+		s := string(b)
+		name := filepath.Base(filename)
+		// First template becomes return value if not already defined,
+		// and we use that one for subsequent New calls to associate
+		// all the templates together. Also, if this file has the same name
+		// as t, this file becomes the contents of t, so
+		//  t, err := New(name).Funcs(xxx).ParseFiles(name)
+		// works. Otherwise we create a new template associated with t.
+		var tmpl *Template
+		if t == nil {
+			t = New(name)
 		}
-	return s, nil
-}
-
-// ParseSetFiles creates a new Set and parses the set definition from the
-// named files. Each file must be individually parseable.
-func ParseSetFiles(filenames ...string) (*Set, error) {
-	s := new(Set)
-	for _, filename := range filenames {
-		b, err := ioutil.ReadFile(filename)
-		if err != nil {
-			return nil, err
+		if name == t.Name() {
+			tmpl = t
+		} else {
+			tmpl = t.New(name)
 		}
-		_, err = s.Parse(string(b))
+		_, err = tmpl.Parse(s)
 		if err != nil {
 			return nil, err
 		}
 	}
-	return s, nil
+	return t, nil
 }
 
-// ParseGlob parses the set definition from the files identified by the
-// pattern.  The pattern is processed by filepath.Glob and must match at
-// least one file.
-// If an error occurs, parsing stops and the returned set is nil.
-func (s *Set) ParseGlob(pattern string) (*Set, error) {
-	filenames, err := filepath.Glob(pattern)
-	if err != nil {
-		return nil, err
-	}
-	if len(filenames) == 0 {
-		return nil, fmt.Errorf("pattern matches no files: %#q", pattern)
-	}
-	return s.ParseFiles(filenames...)
+// ParseGlob creates a new Template and parses the template definitions from the
+// files identified by the pattern, which must match at least one file. The
+// returned template will have the (base) name and (parsed) contents of the
+// first file matched by the pattern. ParseGlob is equivalent to calling
+// ParseFiles with the list of files matched by the pattern.
+func ParseGlob(pattern string) (*Template, error) {
+	return parseGlob(nil, pattern)
 }
 
-// ParseSetGlob creates a new Set and parses the set definition from the
-// files identified by the pattern. The pattern is processed by filepath.Glob
-// and must match at least one file.
-func ParseSetGlob(pattern string) (*Set, error) {
-	set, err := new(Set).ParseGlob(pattern)
-	if err != nil {
-		return nil, err
-	}
-	return set, nil
+// ParseGlob parses the template definitions in the files identified by the
+// pattern and associates the resulting templates with t. The pattern is
+// processed by filepath.Glob and must match at least one file. ParseGlob is
+// equivalent to calling t.ParseFiles with the list of files matched by the
+// pattern.
+func (t *Template) ParseGlob(pattern string) (*Template, error) {
+	return parseGlob(t, pattern)
 }
 
-// Functions and methods to parse stand-alone template files into a set.
-
-// ParseTemplateFiles parses the named template files and adds
-// them to the set. Each template will be named the base name of
-// its file.
-// Unlike with ParseFiles, each file should be a stand-alone template
-// definition suitable for Template.Parse (not Set.Parse); that is, the
-// file does not contain {{define}} clauses. ParseTemplateFiles is
-// therefore equivalent to calling the ParseFile function to create
-// individual templates, which are then added to the set.
-// Each file must be parseable by itself.
-// If an error occurs, parsing stops and the returned set is nil.
-func (s *Set) ParseTemplateFiles(filenames ...string) (*Set, error) {
-	for _, filename := range filenames {
-		_, err := parseFileInSet(filename, s)
-		if err != nil {
-			return nil, err
-		}
-	}
-	return s, nil
-}
-
-// ParseTemplateGlob parses the template files matched by the
-// patern and adds them to the set. Each template will be named
-// the base name of its file.
-// Unlike with ParseGlob, each file should be a stand-alone template
-// definition suitable for Template.Parse (not Set.Parse); that is, the
-// file does not contain {{define}} clauses. ParseTemplateGlob is
-// therefore equivalent to calling the ParseFile function to create
-// individual templates, which are then added to the set.
-// Each file must be parseable by itself.
-// If an error occurs, parsing stops and the returned set is nil.
-func (s *Set) ParseTemplateGlob(pattern string) (*Set, error) {
-	filenames, err := filepath.Glob(pattern)
-	if err != nil {
-		return nil, err
-	}
-	for _, filename := range filenames {
-		_, err := parseFileInSet(filename, s)
-		if err != nil {
-			return nil, err
-		}
-	}
-	return s, nil
-}
-
-// ParseTemplateFiles creates a set by parsing the named files,
-// each of which defines a single template. Each template will be
-// named the base name of its file.
-// Unlike with ParseFiles, each file should be a stand-alone template
-// definition suitable for Template.Parse (not Set.Parse); that is, the
-// file does not contain {{define}} clauses. ParseTemplateFiles is
-// therefore equivalent to calling the ParseFile function to create
-// individual templates, which are then added to the set.
-// Each file must be parseable by itself. Parsing stops if an error is
-// encountered.
-func ParseTemplateFiles(filenames ...string) (*Set, error) {
-	set := new(Set)
-	set.init()
-	for _, filename := range filenames {
-		t, err := ParseFile(filename)
-		if err != nil {
-			return nil, err
-		}
-		if err := set.add(t); err != nil {
-			return nil, err
-		}
-	}
-	return set, nil
-}
-
-// ParseTemplateGlob creates a set by parsing the files matched
-// by the pattern, each of which defines a single template. The pattern
-// is processed by filepath.Glob and must match at least one file. Each
-// template will be named the base name of its file.
-// Unlike with ParseGlob, each file should be a stand-alone template
-// definition suitable for Template.Parse (not Set.Parse); that is, the
-// file does not contain {{define}} clauses. ParseTemplateGlob is
-// therefore equivalent to calling the ParseFile function to create
-// individual templates, which are then added to the set.
-// Each file must be parseable by itself. Parsing stops if an error is
-// encountered.
-func ParseTemplateGlob(pattern string) (*Set, error) {
-	set := new(Set)
+// parseGlob is the implementation of the function and method ParseGlob.
+func parseGlob(t *Template, pattern string) (*Template, error) {
 	filenames, err := filepath.Glob(pattern)
 	if err != nil {
 		return nil, err
 	}
 	if len(filenames) == 0 {
-		return nil, fmt.Errorf("pattern matches no files: %#q", pattern)
-	}
-	for _, filename := range filenames {
-		t, err := ParseFile(filename)
-		if err != nil {
-			return nil, err
-		}
-		if err := set.add(t); err != nil {
-			return nil, err
-		}
+		return nil, fmt.Errorf("template: pattern matches no files: %#q", pattern)
 	}
-	return set, nil
+	return parseFiles(t, filenames...)
 }

src/pkg/text/template/set_test.go → src/pkg/text/template/multi_test.go

@@ -4,17 +4,49 @@
 
 package template
 
+// Tests for mulitple-template parsing and execution.
+
 import (
+	"bytes"
 	"fmt"
 	"testing"
 )
 
+type isEmptyTest struct {
+	name  string
+	input string
+	empty bool
+}
+
+var isEmptyTests = []isEmptyTest{
+	{"empty", ``, true},
+	{"nonempty", `hello`, false},
+	{"spaces only", " \t\n \t\n", true},
+	{"definition", `{{define "x"}}something{{end}}`, true},
+	{"definitions and space", "{{define `x`}}something{{end}}\n\n{{define `y`}}something{{end}}\n\n", true},
+	{"definitions and text", "{{define `x`}}something{{end}}\nx\n{{define `y`}}something{{end}}\ny\n}}", false},
+	{"definition and action", "{{define `x`}}something{{end}}{{if 3}}foo{{end}}", false},
+}
+
+func TestIsEmpty(t *testing.T) {
+	for _, test := range isEmptyTests {
+		template, err := New("root").Parse(test.input)
+		if err != nil {
+			t.Errorf("%q: unexpected error: %v", test.name, err)
+			continue
+		}
+		if empty := isEmpty(template.Root); empty != test.empty {
+			t.Errorf("%q: expected %t got %t", test.name, test.empty, empty)
+		}
+	}
+}
+
 const (
 	noError  = true
 	hasError = false
 )
 
-type setParseTest struct {
+type multiParseTest struct {
 	name    string
 	input   string
 	ok      bool
@@ -22,7 +54,7 @@ type setParseTest struct {
 	results []string
 }
 
-var setParseTests = []setParseTest{
+var multiParseTests = []multiParseTest{
 	{"empty", "", noError,
 		nil,
 		nil},
@@ -41,9 +73,9 @@ var setParseTests = []setParseTest{
 		nil},
 }
 
-func TestSetParse(t *testing.T) {
-	for _, test := range setParseTests {
-		set, err := new(Set).Parse(test.input)
+func TestMultiParse(t *testing.T) {
+	for _, test := range multiParseTests {
+		template, err := New("root").Parse(test.input)
 		switch {
 		case err == nil && !test.ok:
 			t.Errorf("%q: expected error; got none", test.name)
@@ -58,15 +90,15 @@ func TestSetParse(t *testing.T) {
 			}
 			continue
 		}
-		if set == nil {
+		if template == nil {
 			continue
 		}
-		if len(set.tmpl) != len(test.names) {
-			t.Errorf("%s: wrong number of templates; wanted %d got %d", test.name, len(test.names), len(set.tmpl))
+		if len(template.tmpl) != len(test.names)+1 { // +1 for root
+			t.Errorf("%s: wrong number of templates; wanted %d got %d", test.name, len(test.names), len(template.tmpl))
 			continue
 		}
 		for i, name := range test.names {
-			tmpl, ok := set.tmpl[name]
+			tmpl, ok := template.tmpl[name]
 			if !ok {
 				t.Errorf("%s: can't find template %q", test.name, name)
 				continue
@@ -79,7 +111,7 @@ func TestSetParse(t *testing.T) {
 	}
 }
 
-var setExecTests = []execTest{
+var multiExecTests = []execTest{
 	{"empty", "", "", nil, true},
 	{"text", "some text", "some text", nil, true},
 	{"invoke x", `{{template "x" .SI}}`, "TEXT", tVal, true},
@@ -96,144 +128,124 @@ var setExecTests = []execTest{
 }
 
 // These strings are also in testdata/*.
-const setText1 = `
+const multiText1 = `
 	{{define "x"}}TEXT{{end}}
 	{{define "dotV"}}{{.V}}{{end}}
 `
 
-const setText2 = `
+const multiText2 = `
 	{{define "dot"}}{{.}}{{end}}
 	{{define "nested"}}{{template "dot" .}}{{end}}
 `
 
-func TestSetExecute(t *testing.T) {
-	// Declare a set with a couple of templates first.
-	set := new(Set)
-	_, err := set.Parse(setText1)
+func TestMultiExecute(t *testing.T) {
+	// Declare a couple of templates first.
+	template, err := New("root").Parse(multiText1)
 	if err != nil {
-		t.Fatalf("error parsing set: %s", err)
+		t.Fatalf("parse error for 1: %s", err)
 	}
-	_, err = set.Parse(setText2)
+	_, err = template.Parse(multiText2)
 	if err != nil {
-		t.Fatalf("error parsing set: %s", err)
+		t.Fatalf("parse error for 2: %s", err)
 	}
-	testExecute(setExecTests, set, t)
+	testExecute(multiExecTests, template, t)
 }
 
-func TestSetParseFiles(t *testing.T) {
-	set := new(Set)
-	_, err := set.ParseFiles("DOES NOT EXIST")
+func TestParseFiles(t *testing.T) {
+	_, err := ParseFiles("DOES NOT EXIST")
 	if err == nil {
 		t.Error("expected error for non-existent file; got none")
 	}
-	_, err = set.ParseFiles("testdata/file1.tmpl", "testdata/file2.tmpl")
+	template := New("root")
+	_, err = template.ParseFiles("testdata/file1.tmpl", "testdata/file2.tmpl")
 	if err != nil {
 		t.Fatalf("error parsing files: %v", err)
 	}
-	testExecute(setExecTests, set, t)
+	testExecute(multiExecTests, template, t)
 }
 
-func TestParseSetFiles(t *testing.T) {
-	set := new(Set)
-	_, err := ParseSetFiles("DOES NOT EXIST")
+func TestParseGlob(t *testing.T) {
+	_, err := ParseGlob("DOES NOT EXIST")
 	if err == nil {
 		t.Error("expected error for non-existent file; got none")
 	}
-	set, err = ParseSetFiles("testdata/file1.tmpl", "testdata/file2.tmpl")
-	if err != nil {
-		t.Fatalf("error parsing files: %v", err)
-	}
-	testExecute(setExecTests, set, t)
-}
-
-func TestSetParseGlob(t *testing.T) {
-	_, err := new(Set).ParseGlob("DOES NOT EXIST")
-	if err == nil {
-		t.Error("expected error for non-existent file; got none")
-	}
-	_, err = new(Set).ParseGlob("[x")
+	_, err = New("error").ParseGlob("[x")
 	if err == nil {
 		t.Error("expected error for bad pattern; got none")
 	}
-	set, err := new(Set).ParseGlob("testdata/file*.tmpl")
+	template := New("root")
+	_, err = template.ParseGlob("testdata/file*.tmpl")
 	if err != nil {
 		t.Fatalf("error parsing files: %v", err)
 	}
-	testExecute(setExecTests, set, t)
+	testExecute(multiExecTests, template, t)
 }
 
-func TestParseSetGlob(t *testing.T) {
-	_, err := ParseSetGlob("DOES NOT EXIST")
-	if err == nil {
-		t.Error("expected error for non-existent file; got none")
-	}
-	_, err = ParseSetGlob("[x")
-	if err == nil {
-		t.Error("expected error for bad pattern; got none")
-	}
-	set, err := ParseSetGlob("testdata/file*.tmpl")
-	if err != nil {
-		t.Fatalf("error parsing files: %v", err)
-	}
-	testExecute(setExecTests, set, t)
-}
+// In these tests, actual content (not just template definitions) comes from the parsed files.
 
 var templateFileExecTests = []execTest{
-	{"test", `{{template "tmpl1.tmpl"}}{{template "tmpl2.tmpl"}}`, "template1\ntemplate2\n", 0, true},
+	{"test", `{{template "tmpl1.tmpl"}}{{template "tmpl2.tmpl"}}`, "template1\n\ny\ntemplate2\n\nx\n", 0, true},
 }
 
-func TestSetParseTemplateFiles(t *testing.T) {
-	_, err := ParseTemplateFiles("DOES NOT EXIST")
-	if err == nil {
-		t.Error("expected error for non-existent file; got none")
-	}
-	set, err := new(Set).ParseTemplateFiles("testdata/tmpl1.tmpl", "testdata/tmpl2.tmpl")
+func TestParseFilesWithData(t *testing.T) {
+	template, err := New("root").ParseFiles("testdata/tmpl1.tmpl", "testdata/tmpl2.tmpl")
 	if err != nil {
 		t.Fatalf("error parsing files: %v", err)
 	}
-	testExecute(templateFileExecTests, set, t)
+	testExecute(templateFileExecTests, template, t)
 }
 
-func TestParseTemplateFiles(t *testing.T) {
-	_, err := ParseTemplateFiles("DOES NOT EXIST")
-	if err == nil {
-		t.Error("expected error for non-existent file; got none")
-	}
-	set, err := new(Set).ParseTemplateFiles("testdata/tmpl1.tmpl", "testdata/tmpl2.tmpl")
+func TestParseGlobWithData(t *testing.T) {
+	template, err := New("root").ParseGlob("testdata/tmpl*.tmpl")
 	if err != nil {
 		t.Fatalf("error parsing files: %v", err)
 	}
-	testExecute(templateFileExecTests, set, t)
+	testExecute(templateFileExecTests, template, t)
 }
 
-func TestSetParseTemplateGlob(t *testing.T) {
-	_, err := ParseTemplateGlob("DOES NOT EXIST")
-	if err == nil {
-		t.Error("expected error for non-existent file; got none")
+const (
+	cloneText1 = `{{define "a"}}{{template "b"}}{{template "c"}}{{end}}`
+	cloneText2 = `{{define "b"}}b{{end}}`
+	cloneText3 = `{{define "c"}}root{{end}}`
+	cloneText4 = `{{define "c"}}clone{{end}}`
+)
+
+func TestClone(t *testing.T) {
+	// Create some templates and clone the root.
+	root, err := New("root").Parse(cloneText1)
+	if err != nil {
+		t.Fatal(err)
 	}
-	_, err = new(Set).ParseTemplateGlob("[x")
-	if err == nil {
-		t.Error("expected error for bad pattern; got none")
+	_, err = root.Parse(cloneText2)
+	if err != nil {
+		t.Fatal(err)
 	}
-	set, err := new(Set).ParseTemplateGlob("testdata/tmpl*.tmpl")
+	clone := root.Clone()
+	// Add variants to both.
+	_, err = root.Parse(cloneText3)
 	if err != nil {
-		t.Fatalf("error parsing files: %v", err)
+		t.Fatal(err)
 	}
-	testExecute(templateFileExecTests, set, t)
-}
-
-func TestParseTemplateGlob(t *testing.T) {
-	_, err := ParseTemplateGlob("DOES NOT EXIST")
-	if err == nil {
-		t.Error("expected error for non-existent file; got none")
+	_, err = clone.Parse(cloneText4)
+	if err != nil {
+		t.Fatal(err)
 	}
-	_, err = ParseTemplateGlob("[x")
-	if err == nil {
-		t.Error("expected error for bad pattern; got none")
+	// Execute root.
+	var b bytes.Buffer
+	err = root.ExecuteTemplate(&b, "a", 0)
+	if err != nil {
+		t.Fatal(err)
 	}
-	set, err := ParseTemplateGlob("testdata/tmpl*.tmpl")
+	if b.String() != "broot" {
+		t.Errorf("expected %q got %q", "broot", b.String())
+	}
+	// Execute copy.
+	b.Reset()
+	err = clone.ExecuteTemplate(&b, "a", 0)
 	if err != nil {
-		t.Fatalf("error parsing files: %v", err)
+		t.Fatal(err)
+	}
+	if b.String() != "bclone" {
+		t.Errorf("expected %q got %q", "bclone", b.String())
 	}
-	testExecute(templateFileExecTests, set, t)
 }

src/pkg/text/template/parse.go

-// Copyright 2011 The Go Authors. All rights reserved.
-// Use of this source code is governed by a BSD-style
-// license that can be found in the LICENSE file.
-
-package template
-
-import (
-	"reflect"
-	"text/template/parse"
-)
-
-// Template is the representation of a parsed template.
-type Template struct {
-	name string
-	*parse.Tree
-	leftDelim  string
-	rightDelim string
-	// We use two maps, one for parsing and one for execution.
-	// This separation makes the API cleaner since it doesn't
-	// expose reflection to the client.
-	parseFuncs FuncMap
-	execFuncs  map[string]reflect.Value
-	set        *Set // can be nil.
-}
-
-// Name returns the name of the template.
-func (t *Template) Name() string {
-	return t.name
-}
-
-// Parsing.
-
-// New allocates a new template with the given name.
-func New(name string) *Template {
-	return &Template{
-		name:       name,
-		parseFuncs: make(FuncMap),
-		execFuncs:  make(map[string]reflect.Value),
-	}
-}
-
-// Delims sets the action delimiters, to be used in a subsequent
-// parse, to the specified strings.
-// An empty delimiter stands for the corresponding default: {{ or }}.
-// The return value is the template, so calls can be chained.
-func (t *Template) Delims(left, right string) *Template {
-	t.leftDelim = left
-	t.rightDelim = right
-	return t
-}
-
-// Funcs adds the elements of the argument map to the template's function
-// map.  It panics if a value in the map is not a function with appropriate
-// return type.
-// The return value is the template, so calls can be chained.
-func (t *Template) Funcs(funcMap FuncMap) *Template {
-	addValueFuncs(t.execFuncs, funcMap)
-	addFuncs(t.parseFuncs, funcMap)
-	return t
-}
-
-// Parse parses the template definition string to construct an internal
-// representation of the template for execution.
-func (t *Template) Parse(s string) (tmpl *Template, err error) {
-	t.Tree, err = parse.New(t.name).Parse(s, t.leftDelim, t.rightDelim, nil, t.parseFuncs, builtins)
-	if err != nil {
-		return nil, err
-	}
-	return t, nil
-}
-
-// ParseInSet parses the template definition string to construct an internal
-// representation of the template for execution. It also adds the template
-// to the set, which must not be nil. It is an error if s is already defined in the set.
-// Function bindings are checked against those in the set.
-func (t *Template) ParseInSet(s string, set *Set) (tmpl *Template, err error) {
-	t.Tree, err = parse.New(t.name).Parse(s, t.leftDelim, t.rightDelim, set.trees, t.parseFuncs, set.parseFuncs, builtins)
-	if err != nil {
-		return nil, err
-	}
-	err = set.add(t)
-	return t, err
-}

src/pkg/text/template/parse/parse.go

@@ -97,6 +97,15 @@ func (t *Tree) expect(expected itemType, context string) item {
 	return token
 }
 
+// expectEither consumes the next token and guarantees it has one of the required types.
+func (t *Tree) expectOneOf(expected1, expected2 itemType, context string) item {
+	token := t.next()
+	if token.typ != expected1 && token.typ != expected2 {
+		t.errorf("expected %s or %s in %s; got %s", expected1, expected2, context, token)
+	}
+	return token
+}
+
 // unexpected complains about the token and terminates processing.
 func (t *Tree) unexpected(token item, context string) {
 	t.errorf("unexpected %s in %s", token, context)
@@ -162,9 +171,18 @@ func (t *Tree) Parse(s, leftDelim, rightDelim string, treeSet map[string]*Tree,
 	t.startParse(funcs, lex(t.Name, s, leftDelim, rightDelim))
 	t.parse(treeSet)
 	t.stopParse()
+	t.add(treeSet)
 	return t, nil
 }
 
+// add adds tree to the treeSet.
+func (t *Tree) add(treeSet map[string]*Tree) {
+	if _, present := treeSet[t.Name]; present {
+		t.errorf("template: multiple definition of template %q", t.Name)
+	}
+	treeSet[t.Name] = t
+}
+
 // parse is the top-level parser for a template, essentially the same
 // as itemList except it also parses {{define}} actions.
 // It runs to EOF.
@@ -174,7 +192,7 @@ func (t *Tree) parse(treeSet map[string]*Tree) (next Node) {
 		if t.peek().typ == itemLeftDelim {
 			delim := t.next()
 			if t.next().typ == itemDefine {
-				newT := New("new definition") // name will be updated once we know it.
+				newT := New("definition") // name will be updated once we know it.
 				newT.startParse(t.funcs, t.lex)
 				newT.parseDefinition(treeSet)
 				continue
@@ -194,11 +212,8 @@ func (t *Tree) parse(treeSet map[string]*Tree) (next Node) {
 // installs the definition in the treeSet map.  The "define" keyword has already
 // been scanned.
 func (t *Tree) parseDefinition(treeSet map[string]*Tree) {
-	if treeSet == nil {
-		t.errorf("no set specified for template definition")
-	}
 	const context = "define clause"
-	name := t.expect(itemString, context)
+	name := t.expectOneOf(itemString, itemRawString, context)
 	var err error
 	t.Name, err = strconv.Unquote(name.val)
 	if err != nil {
@@ -211,10 +226,7 @@ func (t *Tree) parseDefinition(treeSet map[string]*Tree) {
 		t.errorf("unexpected %s in %s", end, context)
 	}
 	t.stopParse()
-	if _, present := treeSet[t.Name]; present {
-		t.errorf("template: %q multiply defined", name)
-	}
-	treeSet[t.Name] = t
+	t.add(treeSet)
 }
 
 // itemList:

src/pkg/text/template/parse/parse_test.go

@@ -236,7 +236,7 @@ var builtins = map[string]interface{}{
 
 func TestParse(t *testing.T) {
 	for _, test := range parseTests {
-		tmpl, err := New(test.name).Parse(test.input, "", "", nil, builtins)
+		tmpl, err := New(test.name).Parse(test.input, "", "", make(map[string]*Tree), builtins)
 		switch {
 		case err == nil && !test.ok:
 			t.Errorf("%q: expected error; got none", test.name)

src/pkg/text/template/set.go

-// Copyright 2011 The Go Authors. All rights reserved.
-// Use of this source code is governed by a BSD-style
-// license that can be found in the LICENSE file.
-
-package template
-
-import (
-	"fmt"
-	"io"
-	"reflect"
-	"text/template/parse"
-)
-
-// Set holds a set of related templates that can refer to one another by name.
-// The zero value represents an empty set.
-// A template may be a member of multiple sets.
-type Set struct {
-	tmpl       map[string]*Template
-	trees      map[string]*parse.Tree // maintained by parse package
-	leftDelim  string
-	rightDelim string
-	parseFuncs FuncMap
-	execFuncs  map[string]reflect.Value
-}
-
-func (s *Set) init() {
-	if s.tmpl == nil {
-		s.tmpl = make(map[string]*Template)
-		s.parseFuncs = make(FuncMap)
-		s.execFuncs = make(map[string]reflect.Value)
-	}
-}
-
-// Delims sets the action delimiters, to be used in a subsequent
-// parse, to the specified strings.
-// An empty delimiter stands for the corresponding default: {{ or }}.
-// The return value is the set, so calls can be chained.
-func (s *Set) Delims(left, right string) *Set {
-	s.leftDelim = left
-	s.rightDelim = right
-	return s
-}
-
-// Funcs adds the elements of the argument map to the set's function map.  It
-// panics if a value in the map is not a function with appropriate return
-// type.
-// The return value is the set, so calls can be chained.
-func (s *Set) Funcs(funcMap FuncMap) *Set {
-	s.init()
-	addValueFuncs(s.execFuncs, funcMap)
-	addFuncs(s.parseFuncs, funcMap)
-	return s
-}
-
-// Add adds the argument templates to the set. It panics if two templates
-// with the same name are added or if a template is already a member of
-// a set.
-// The return value is the set, so calls can be chained.
-func (s *Set) Add(templates ...*Template) *Set {
-	for _, t := range templates {
-		if err := s.add(t); err != nil {
-			panic(err)
-		}
-	}
-	return s
-}
-
-// add adds the argument template to the set.
-func (s *Set) add(t *Template) error {
-	s.init()
-	if t.set != nil {
-		return fmt.Errorf("template: %q already in a set", t.name)
-	}
-	if _, ok := s.tmpl[t.name]; ok {
-		return fmt.Errorf("template: %q already defined in set", t.name)
-	}
-	s.tmpl[t.name] = t
-	t.set = s
-	return nil
-}
-
-// Template returns the template with the given name in the set,
-// or nil if there is no such template.
-func (s *Set) Template(name string) *Template {
-	return s.tmpl[name]
-}
-
-// FuncMap returns the set's function map.
-func (s *Set) FuncMap() FuncMap {
-	return s.parseFuncs
-}
-
-// Execute applies the named template to the specified data object, writing
-// the output to wr.
-func (s *Set) Execute(wr io.Writer, name string, data interface{}) error {
-	tmpl := s.tmpl[name]
-	if tmpl == nil {
-		return fmt.Errorf("template: no template %q in set", name)
-	}
-	return tmpl.Execute(wr, data)
-}
-
-// Parse parses a string into a set of named templates.  Parse may be called
-// multiple times for a given set, adding the templates defined in the string
-// to the set.  It is an error if a template has a name already defined in the set.
-func (s *Set) Parse(text string) (*Set, error) {
-	// TODO: "ROOT" is just a placeholder while we rejig the API.
-	trees, err := parse.Parse("ROOT", text, s.leftDelim, s.rightDelim, s.parseFuncs, builtins)
-	if err != nil {
-		return nil, err
-	}
-	s.init()
-	for name, tree := range trees {
-		tmpl := New(name)
-		tmpl.Tree = tree
-		err = s.add(tmpl)
-		if err != nil {
-			return s, err
-		}
-	}
-	return s, nil
-}

src/pkg/text/template/template.go

+// Copyright 2011 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package template
+
+import (
+	"bytes"
+	"fmt"
+	"reflect"
+	"text/template/parse"
+)
+
+// common holds the information shared by related templates.
+type common struct {
+	tmpl map[string]*Template
+	// We use two maps, one for parsing and one for execution.
+	// This separation makes the API cleaner since it doesn't
+	// expose reflection to the client.
+	parseFuncs FuncMap
+	execFuncs  map[string]reflect.Value
+}
+
+// Template is the representation of a parsed template. The *parse.Tree
+// field is exported only for use by html/template and should be treated
+// as unexported by all other clients.
+type Template struct {
+	name string
+	*parse.Tree
+	*common
+	leftDelim  string
+	rightDelim string
+}
+
+// New allocates a new template with the given name.
+func New(name string) *Template {
+	return &Template{
+		name: name,
+	}
+}
+
+// Name returns the name of the template.
+func (t *Template) Name() string {
+	return t.name
+}
+
+// New allocates a new template associated with the given one and with the same
+// delimiters. The association, which is transitive, allows one template to
+// invoke another with a {{template}} action.
+func (t *Template) New(name string) *Template {
+	t.init()
+	return &Template{
+		name:       name,
+		common:     t.common,
+		leftDelim:  t.leftDelim,
+		rightDelim: t.rightDelim,
+	}
+}
+
+func (t *Template) init() {
+	if t.common == nil {
+		t.common = new(common)
+		t.tmpl = make(map[string]*Template)
+		t.parseFuncs = make(FuncMap)
+		t.execFuncs = make(map[string]reflect.Value)
+	}
+}
+
+// Clone returns a duplicate of the template, including all associated
+// templates. The actual representation is not copied, but the name space of
+// associated templates is, so further calls to Parse in the copy will add
+// templates to the copy but not to the original. Clone can be used to prepare
+// common templates and use them with variant definitions for other templates by
+// adding the variants after the clone is made.
+func (t *Template) Clone() *Template {
+	nt := t.copy()
+	nt.init()
+	for k, v := range t.tmpl {
+		// The associated templates share nt's common structure.
+		tmpl := v.copy()
+		tmpl.common = nt.common
+		nt.tmpl[k] = tmpl
+	}
+	for k, v := range t.parseFuncs {
+		nt.parseFuncs[k] = v
+	}
+	for k, v := range t.execFuncs {
+		nt.execFuncs[k] = v
+	}
+	return nt
+}
+
+// copy returns a shallow copy of t, with common set to nil.
+func (t *Template) copy() *Template {
+	nt := New(t.name)
+	nt.Tree = t.Tree
+	nt.leftDelim = t.leftDelim
+	nt.rightDelim = t.rightDelim
+	return nt
+}
+
+// Templates returns a slice of the templates associated with t, including t
+// itself.
+func (t *Template) Templates() []*Template {
+	// Return a slice so we don't expose the map.
+	m := make([]*Template, 0, len(t.tmpl))
+	for _, v := range t.tmpl {
+		m = append(m, v)
+	}
+	return m
+}
+
+// Delims sets the action delimiters to the specified strings, to be used in
+// subsequent calls to Parse, ParseFiles, or ParseGlob. Nested template
+// definitions will inherit the settings. An empty delimiter stands for the
+// corresponding default: {{ or }}.
+// The return value is the template, so calls can be chained.
+func (t *Template) Delims(left, right string) *Template {
+	t.leftDelim = left
+	t.rightDelim = right
+	return t
+}
+
+// Funcs adds the elements of the argument map to the template's function map.
+// It panics if a value in the map is not a function with appropriate return
+// type. However, it is legal to overwrite elements of the map. The return
+// value is the template, so calls can be chained.
+func (t *Template) Funcs(funcMap FuncMap) *Template {
+	t.init()
+	addValueFuncs(t.execFuncs, funcMap)
+	addFuncs(t.parseFuncs, funcMap)
+	return t
+}
+
+// Template returns the template with the given name that is associated with t,
+// or nil if there is no such template.
+func (t *Template) Template(name string) *Template {
+	return t.tmpl[name]
+}
+
+// Parse parses a string into a template. Nested template definitions will be
+// associated with the top-level template t. Parse may be called multiple times
+// to parse definitions of templates to associate with t. It is an error if a
+// resulting template is non-empty (contains content other than template
+// definitions) and would replace a non-empty template with the same name.
+// (In multiple calls to Parse with the same receiver template, only one call
+// can contain text other than space, comments, and template definitions.)
+func (t *Template) Parse(text string) (*Template, error) {
+	t.init()
+	trees, err := parse.Parse(t.name, text, t.leftDelim, t.rightDelim, t.parseFuncs, builtins)
+	if err != nil {
+		return nil, err
+	}
+	// Add the newly parsed trees, including the one for t, into our common structure.
+	for name, tree := range trees {
+		// If the name we parsed is the name of this template, overwrite this template.
+		// The associate method checks it's not a redefinition.
+		tmpl := t
+		if name != t.name {
+			tmpl = t.New(name)
+		}
+		// Even if t == tmpl, we need to install it in the common.tmpl map.
+		if err := t.associate(tmpl); err != nil {
+			return nil, err
+		}
+		tmpl.Tree = tree
+		tmpl.leftDelim = t.leftDelim
+		tmpl.rightDelim = t.rightDelim
+	}
+	return t, nil
+}
+
+// associate installs the new template into the group of templates associated
+// with t. It is an error to reuse a name except to overwrite an empty
+// template. The two are already known to share the common structure.
+func (t *Template) associate(new *Template) error {
+	if new.common != t.common {
+		panic("internal error: associate not common")
+	}
+	name := new.name
+	if old := t.tmpl[name]; old != nil {
+		oldIsEmpty := isEmpty(old.Root)
+		newIsEmpty := isEmpty(new.Root)
+		if !oldIsEmpty && !newIsEmpty {
+			return fmt.Errorf("template: redefinition of template %q", name)
+		}
+		if newIsEmpty {
+			// Whether old is empty or not, new is empty; no reason to replace old.
+			return nil
+		}
+	}
+	t.tmpl[name] = new
+	return nil
+}
+
+// isEmpty reports whether this tree (node) is empty of everything but space.
+func isEmpty(n parse.Node) bool {
+	switch n := n.(type) {
+	case *parse.ActionNode:
+	case *parse.IfNode:
+	case *parse.ListNode:
+		for _, node := range n.Nodes {
+			if !isEmpty(node) {
+				return false
+			}
+		}
+		return true
+	case *parse.RangeNode:
+	case *parse.TemplateNode:
+	case *parse.TextNode:
+		return len(bytes.TrimSpace(n.Text)) == 0
+	case *parse.WithNode:
+	default:
+		panic("unknown node: " + n.String())
+	}
+	return false
+}

src/pkg/text/template/testdata/tmpl1.tmpl

 template1
+{{define "x"}}x{{end}}
+{{template "y"}}

src/pkg/text/template/testdata/tmpl2.tmpl

 template2
+{{define "y"}}y{{end}}
+{{template "x"}}