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// 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 (
// common holds the information shared by related templates.
type common struct {
tmpl map[string]*Template // Map from name to defined templates.
option option
// 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.
muFuncs sync.RWMutex // protects parseFuncs and execFuncs
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
leftDelim string
rightDelim string
// New allocates a new, undefined template with the given name.
func New(name string) *Template {
t := &Template{
name: name,
return t
// Name returns the name of the template.
func (t *Template) Name() string {
// New allocates a new, undefined 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.
// Because associated templates share underlying data, template construction
// cannot be done safely in parallel. Once the templates are constructed, they
// can be executed in parallel.
func (t *Template) New(name string) *Template {
nt := &Template{
name: name,
common: t.common,
leftDelim: t.leftDelim,
rightDelim: t.rightDelim,
return nt
// init guarantees that t has a valid common structure.
func (t *Template) init() {
if t.common == nil {
c := new(common)
c.tmpl = make(map[string]*Template)
c.parseFuncs = make(FuncMap)
c.execFuncs = make(map[string]reflect.Value)
t.common = c
// 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, error) {
nt := t.copy(nil)
if t.common == nil {
return nt, nil
for k, v := range t.tmpl {
if k == {
nt.tmpl[] = nt
// The associated templates share nt's common structure.
tmpl := v.copy(nt.common)
nt.tmpl[k] = tmpl
defer t.muFuncs.RUnlock()
for k, v := range t.parseFuncs {
nt.parseFuncs[k] = v
for k, v := range t.execFuncs {
nt.execFuncs[k] = v
return nt, nil
// copy returns a shallow copy of t, with common set to the argument.
func (t *Template) copy(c *common) *Template {
nt := New(
nt.Tree = t.Tree
nt.common = c
nt.leftDelim = t.leftDelim
nt.rightDelim = t.rightDelim
return nt
// AddParseTree adds parse tree for template with given name and associates it with t.
// If the template does not already exist, it will create a new one.
// If the template does exist, it will be replaced.
func (t *Template) AddParseTree(name string, tree *parse.Tree) (*Template, error) {
// If the name is the name of this template, overwrite this template.
nt := t
if name != {
nt = t.New(name)
// Even if nt == t, we need to install it in the common.tmpl map.
if t.associate(nt, tree) || nt.Tree == nil {
nt.Tree = tree
return nt, nil
// Templates returns a slice of defined templates associated with t.
func (t *Template) Templates() []*Template {
if t.common == nil {
return nil
// 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 must be called before the template is parsed.
// It panics if a value in the map is not a function with appropriate return
// type or if the name cannot be used syntactically as a function in a template.
// 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 {
defer t.muFuncs.Unlock()
addValueFuncs(t.execFuncs, funcMap)
addFuncs(t.parseFuncs, funcMap)
return t
// Lookup returns the template with the given name that is associated with t.
// It returns nil if there is no such template or the template has no definition.
func (t *Template) Lookup(name string) *Template {
if t.common == nil {
return nil
return t.tmpl[name]
// Parse parses text as a template body for t.
// Named template definitions ({{define ...}} or {{block ...}} statements) in text
// define additional templates associated with t and are removed from the
// definition of t itself.
// Templates can be redefined in successive calls to Parse.
// A template definition with a body containing only white space and comments
// is considered empty and will not replace an existing template's body.
// This allows using Parse to add new named template definitions without
// overwriting the main template body.
func (t *Template) Parse(text string) (*Template, error) {
trees, err := parse.Parse(, 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 _, err := t.AddParseTree(name, tree); err != nil {
return nil, err
return t, nil
// associate installs the new template into the group of templates associated
// with t. The two are already known to share the common structure.
// The boolean return value reports whether to store this tree as t.Tree.
func (t *Template) associate(new *Template, tree *parse.Tree) bool {
if new.common != t.common {
panic("internal error: associate not common")
if old := t.tmpl[]; old != nil && parse.IsEmptyTree(tree.Root) && old.Tree != nil {
// If a template by that name exists,
// don't replace it with an empty template.
return false
t.tmpl[] = new
return true