package template

import "text/template"

Package template implements data-driven templates for generating textual output.

To generate HTML output, see html/template, which has the same interface as this package but automatically secures HTML output against certain attacks.

Templates are executed by applying them to a data structure. Annotations in the template refer to elements of the data structure (typically a field of a struct or a key in a map) to control execution and derive values to be displayed. Execution of the template walks the structure and sets the cursor, represented by a period '.' and called "dot", to the value at the current location in the structure as execution proceeds.

The input text for a template is UTF-8-encoded text in any format. "Actions"--data evaluations or control structures--are delimited by "{{" and "}}"; all text outside actions is copied to the output unchanged.

Once parsed, a template may be executed safely in parallel, although if parallel executions share a Writer the output may be interleaved.

Here is a trivial example that prints "17 items are made of wool". 

type Inventory struct {
	Material string
	Count    uint
}
sweaters := Inventory{"wool", 17}
tmpl, err := template.New("test").Parse("{{.Count}} items are made of {{.Material}}")
if err != nil { panic(err) }
err = tmpl.Execute(os.Stdout, sweaters)
if err != nil { panic(err) }

More intricate examples appear below.

Text and spaces

By default, all text between actions is copied verbatim when the template is executed. For example, the string " items are made of " in the example above appears on standard output when the program is run.

However, to aid in formatting template source code, if an action's left delimiter (by default "{{") is followed immediately by a minus sign and white space, all trailing white space is trimmed from the immediately preceding text. Similarly, if the right delimiter ("}}") is preceded by white space and a minus sign, all leading white space is trimmed from the immediately following text. In these trim markers, the white space must be present: "{{- 3}}" is like "{{3}}" but trims the immediately preceding text, while "{{-3}}" parses as an action containing the number -3.

For instance, when executing the template whose source is 

"{{23 -}} < {{- 45}}"

the generated output would be 

"23<45"

For this trimming, the definition of white space characters is the same as in Go: space, horizontal tab, carriage return, and newline.

Actions

Here is the list of actions. "Arguments" and "pipelines" are evaluations of data, defined in detail in the corresponding sections that follow. 

{{/* a comment */}}
{{- /* a comment with white space trimmed from preceding and following text */ -}}
	A comment; discarded. May contain newlines.
	Comments do not nest and must start and end at the
	delimiters, as shown here.

{{pipeline}}
	The default textual representation (the same as would be
	printed by fmt.Print) of the value of the pipeline is copied
	to the output.

{{if pipeline}} T1 {{end}}
	If the value of the pipeline is empty, no output is generated;
	otherwise, T1 is executed. The empty values are false, 0, any
	nil pointer or interface value, and any array, slice, map, or
	string of length zero.
	Dot is unaffected.

{{if pipeline}} T1 {{else}} T0 {{end}}
	If the value of the pipeline is empty, T0 is executed;
	otherwise, T1 is executed. Dot is unaffected.

{{if pipeline}} T1 {{else if pipeline}} T0 {{end}}
	To simplify the appearance of if-else chains, the else action
	of an if may include another if directly; the effect is exactly
	the same as writing
		{{if pipeline}} T1 {{else}}{{if pipeline}} T0 {{end}}{{end}}

{{range pipeline}} T1 {{end}}
	The value of the pipeline must be an array, slice, map, or channel.
	If the value of the pipeline has length zero, nothing is output;
	otherwise, dot is set to the successive elements of the array,
	slice, or map and T1 is executed. If the value is a map and the
	keys are of basic type with a defined order, the elements will be
	visited in sorted key order.

{{range pipeline}} T1 {{else}} T0 {{end}}
	The value of the pipeline must be an array, slice, map, or channel.
	If the value of the pipeline has length zero, dot is unaffected and
	T0 is executed; otherwise, dot is set to the successive elements
	of the array, slice, or map and T1 is executed.

{{break}}
	The innermost {{range pipeline}} loop is ended early, stopping the
	current iteration and bypassing all remaining iterations.

{{continue}}
	The current iteration of the innermost {{range pipeline}} loop is
	stopped, and the loop starts the next iteration.

{{template "name"}}
	The template with the specified name is executed with nil data.

{{template "name" pipeline}}
	The template with the specified name is executed with dot set
	to the value of the pipeline.

{{block "name" pipeline}} T1 {{end}}
	A block is shorthand for defining a template
		{{define "name"}} T1 {{end}}
	and then executing it in place
		{{template "name" pipeline}}
	The typical use is to define a set of root templates that are
	then customized by redefining the block templates within.

{{with pipeline}} T1 {{end}}
	If the value of the pipeline is empty, no output is generated;
	otherwise, dot is set to the value of the pipeline and T1 is
	executed.

{{with pipeline}} T1 {{else}} T0 {{end}}
	If the value of the pipeline is empty, dot is unaffected and T0
	is executed; otherwise, dot is set to the value of the pipeline
	and T1 is executed.

Arguments

An argument is a simple value, denoted by one of the following.

Arguments may evaluate to any type; if they are pointers the implementation automatically indirects to the base type when required. If an evaluation yields a function value, such as a function-valued field of a struct, the function is not invoked automatically, but it can be used as a truth value for an if action and the like. To invoke it, use the call function, defined below.

Pipelines

A pipeline is a possibly chained sequence of "commands". A command is a simple value (argument) or a function or method call, possibly with multiple arguments: 

Argument
	The result is the value of evaluating the argument.
.Method [Argument...]
	The method can be alone or the last element of a chain but,
	unlike methods in the middle of a chain, it can take arguments.
	The result is the value of calling the method with the
	arguments:
		dot.Method(Argument1, etc.)
functionName [Argument...]
	The result is the value of calling the function associated
	with the name:
		function(Argument1, etc.)
	Functions and function names are described below.

A pipeline may be "chained" by separating a sequence of commands with pipeline characters '|'. In a chained pipeline, the result of each command is passed as the last argument of the following command. The output of the final command in the pipeline is the value of the pipeline.

The output of a command will be either one value or two values, the second of which has type error. If that second value is present and evaluates to non-nil, execution terminates and the error is returned to the caller of Execute.

Variables

A pipeline inside an action may initialize a variable to capture the result. The initialization has syntax 

$variable := pipeline

where $variable is the name of the variable. An action that declares a variable produces no output.

Variables previously declared can also be assigned, using the syntax 

$variable = pipeline

If a "range" action initializes a variable, the variable is set to the successive elements of the iteration. Also, a "range" may declare two variables, separated by a comma: 

range $index, $element := pipeline

in which case $index and $element are set to the successive values of the array/slice index or map key and element, respectively. Note that if there is only one variable, it is assigned the element; this is opposite to the convention in Go range clauses.

A variable's scope extends to the "end" action of the control structure ("if", "with", or "range") in which it is declared, or to the end of the template if there is no such control structure. A template invocation does not inherit variables from the point of its invocation.

When execution begins, $ is set to the data argument passed to Execute, that is, to the starting value of dot.

Examples

Here are some example one-line templates demonstrating pipelines and variables. All produce the quoted word "output": 

{{"\"output\""}}
	A string constant.
{{`"output"`}}
	A raw string constant.
{{printf "%q" "output"}}
	A function call.
{{"output" | printf "%q"}}
	A function call whose final argument comes from the previous
	command.
{{printf "%q" (print "out" "put")}}
	A parenthesized argument.
{{"put" | printf "%s%s" "out" | printf "%q"}}
	A more elaborate call.
{{"output" | printf "%s" | printf "%q"}}
	A longer chain.
{{with "output"}}{{printf "%q" .}}{{end}}
	A with action using dot.
{{with $x := "output" | printf "%q"}}{{$x}}{{end}}
	A with action that creates and uses a variable.
{{with $x := "output"}}{{printf "%q" $x}}{{end}}
	A with action that uses the variable in another action.
{{with $x := "output"}}{{$x | printf "%q"}}{{end}}
	The same, but pipelined.

Functions

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 method can be used to add them.

Predefined global functions are named as follows. 

and
	Returns the boolean AND of its arguments by returning the
	first empty argument or the last argument. That is,
	"and x y" behaves as "if x then y else x."
	Evaluation proceeds through the arguments left to right
	and returns when the result is determined.
call
	Returns the result of calling the first argument, which
	must be a function, with the remaining arguments as parameters.
	Thus "call .X.Y 1 2" is, in Go notation, dot.X.Y(1, 2) where
	Y is a func-valued field, map entry, or the like.
	The first argument must be the result of an evaluation
	that yields a value of function type (as distinct from
	a predefined function such as print). The function must
	return either one or two result values, the second of which
	is of type error. If the arguments don't match the function
	or the returned error value is non-nil, execution stops.
html
	Returns the escaped HTML equivalent of the textual
	representation of its arguments. This function is unavailable
	in html/template, with a few exceptions.
index
	Returns the result of indexing its first argument by the
	following arguments. Thus "index x 1 2 3" is, in Go syntax,
	x[1][2][3]. Each indexed item must be a map, slice, or array.
slice
	slice returns the result of slicing its first argument by the
	remaining arguments. Thus "slice x 1 2" is, in Go syntax, x[1:2],
	while "slice x" is x[:], "slice x 1" is x[1:], and "slice x 1 2 3"
	is x[1:2:3]. The first argument must be a string, slice, or array.
js
	Returns the escaped JavaScript equivalent of the textual
	representation of its arguments.
len
	Returns the integer length of its argument.
not
	Returns the boolean negation of its single argument.
or
	Returns the boolean OR of its arguments by returning the
	first non-empty argument or the last argument, that is,
	"or x y" behaves as "if x then x else y".
	Evaluation proceeds through the arguments left to right
	and returns when the result is determined.
print
	An alias for fmt.Sprint
printf
	An alias for fmt.Sprintf
println
	An alias for fmt.Sprintln
urlquery
	Returns the escaped value of the textual representation of
	its arguments in a form suitable for embedding in a URL query.
	This function is unavailable in html/template, with a few
	exceptions.

The boolean functions take any zero value to be false and a non-zero value to be true.

There is also a set of binary comparison operators defined as functions: 

eq
	Returns the boolean truth of arg1 == arg2
ne
	Returns the boolean truth of arg1 != arg2
lt
	Returns the boolean truth of arg1 < arg2
le
	Returns the boolean truth of arg1 <= arg2
gt
	Returns the boolean truth of arg1 > arg2
ge
	Returns the boolean truth of arg1 >= arg2

For simpler multi-way equality tests, eq (only) accepts two or more arguments and compares the second and subsequent to the first, returning in effect 

arg1==arg2 || arg1==arg3 || arg1==arg4 ...

(Unlike with || in Go, however, eq is a function call and all the arguments will be evaluated.)

The comparison functions work on any values whose type Go defines as comparable. For basic types such as integers, the rules are relaxed: size and exact type are ignored, so any integer value, signed or unsigned, may be compared with any other integer value. (The arithmetic value is compared, not the bit pattern, so all negative integers are less than all unsigned integers.) However, as usual, one may not compare an int with a float32 and so on.

Associated templates

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.

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.

Nested template definitions

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: 

{{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. 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, or must be copied with Template.Clone or Template.AddParseTree.

Parse may be called multiple times to assemble the various associated templates; see ParseFiles, ParseGlob, Template.ParseFiles and Template.ParseGlob for simple ways to parse related templates stored in files.

A template may be executed directly or through Template.ExecuteTemplate, which executes an associated template identified by name. To invoke our example above, we might write, 

err := tmpl.Execute(os.Stdout, "no data needed")
if err != nil {
	log.Fatalf("execution failed: %s", err)
}

or to invoke a particular template explicitly by name, 

err := tmpl.ExecuteTemplate(os.Stdout, "T2", "no data needed")
if err != nil {
	log.Fatalf("execution failed: %s", err)
}

Index

Examples

Functions

func HTMLEscape

func HTMLEscape(w io.Writer, b []byte)

HTMLEscape writes to w the escaped HTML equivalent of the plain text data b.

func HTMLEscapeString

func HTMLEscapeString(s string) string

HTMLEscapeString returns the escaped HTML equivalent of the plain text data s.

func HTMLEscaper

func HTMLEscaper(args ...any) string

HTMLEscaper returns the escaped HTML equivalent of the textual representation of its arguments.

func IsTrue

func IsTrue(val any) (truth, ok bool)

IsTrue reports whether the value is 'true', in the sense of not the zero of its type, and whether the value has a meaningful truth value. This is the definition of truth used by if and other such actions.

func JSEscape

func JSEscape(w io.Writer, b []byte)

JSEscape writes to w the escaped JavaScript equivalent of the plain text data b.

func JSEscapeString

func JSEscapeString(s string) string

JSEscapeString returns the escaped JavaScript equivalent of the plain text data s.

func JSEscaper

func JSEscaper(args ...any) string

JSEscaper returns the escaped JavaScript equivalent of the textual representation of its arguments.

func URLQueryEscaper

func URLQueryEscaper(args ...any) string

URLQueryEscaper returns the escaped value of the textual representation of its arguments in a form suitable for embedding in a URL query.

Types

type ExecError

type ExecError struct {
	Name string // Name of template.
	Err  error  // Pre-formatted error.
}

ExecError is the custom error type returned when Execute has an error evaluating its template. (If a write error occurs, the actual error is returned; it will not be of type ExecError.)

func (ExecError) Error

func (e ExecError) Error() string

func (ExecError) Unwrap

func (e ExecError) Unwrap() error

type FuncMap

type FuncMap map[string]any

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. In that case, if the second (error) return value evaluates to non-nil during execution, execution terminates and Execute returns that error.

Errors returned by Execute wrap the underlying error; call errors.As to unwrap them.

When template execution invokes a function with an argument list, that list must be assignable to the function's parameter types. Functions meant to apply to arguments of arbitrary type can use parameters of type interface{} or of type reflect.Value. Similarly, functions meant to return a result of arbitrary type can return interface{} or reflect.Value.

type Template

type Template struct {
	*parse.Tree
	// contains filtered or unexported fields
}

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.

Example 
package main

import (
	"log"
	"os"
	"text/template"
)

func main() {
	// Define a template.
	const letter = `
Dear {{.Name}},
{{if .Attended}}
It was a pleasure to see you at the wedding.
{{- else}}
It is a shame you couldn't make it to the wedding.
{{- end}}
{{with .Gift -}}
Thank you for the lovely {{.}}.
{{end}}
Best wishes,
Josie
`

	// Prepare some data to insert into the template.
	type Recipient struct {
		Name, Gift string
		Attended   bool
	}
	var recipients = []Recipient{
		{"Aunt Mildred", "bone china tea set", true},
		{"Uncle John", "moleskin pants", false},
		{"Cousin Rodney", "", false},
	}

	// Create a new template and parse the letter into it.
	t := template.Must(template.New("letter").Parse(letter))

	// Execute the template for each recipient.
	for _, r := range recipients {
		err := t.Execute(os.Stdout, r)
		if err != nil {
			log.Println("executing template:", err)
		}
	}

}

Output:

Dear Aunt Mildred,

It was a pleasure to see you at the wedding.
Thank you for the lovely bone china tea set.

Best wishes,
Josie

Dear Uncle John,

It is a shame you couldn't make it to the wedding.
Thank you for the lovely moleskin pants.

Best wishes,
Josie

Dear Cousin Rodney,

It is a shame you couldn't make it to the wedding.

Best wishes,
Josie
Example (Block) 
package main

import (
	"log"
	"os"
	"strings"
	"text/template"
)

func main() {
	const (
		master  = `Names:{{block "list" .}}{{"\n"}}{{range .}}{{println "-" .}}{{end}}{{end}}`
		overlay = `{{define "list"}} {{join . ", "}}{{end}} `
	)
	var (
		funcs     = template.FuncMap{"join": strings.Join}
		guardians = []string{"Gamora", "Groot", "Nebula", "Rocket", "Star-Lord"}
	)
	masterTmpl, err := template.New("master").Funcs(funcs).Parse(master)
	if err != nil {
		log.Fatal(err)
	}
	overlayTmpl, err := template.Must(masterTmpl.Clone()).Parse(overlay)
	if err != nil {
		log.Fatal(err)
	}
	if err := masterTmpl.Execute(os.Stdout, guardians); err != nil {
		log.Fatal(err)
	}
	if err := overlayTmpl.Execute(os.Stdout, guardians); err != nil {
		log.Fatal(err)
	}
}

Output:

Names:
- Gamora
- Groot
- Nebula
- Rocket
- Star-Lord
Names: Gamora, Groot, Nebula, Rocket, Star-Lord
Example (Func)

This example demonstrates a custom function to process template text. It installs the strings.Title function and uses it to Make Title Text Look Good In Our Template's Output. 

package main

import (
	"log"
	"os"
	"strings"
	"text/template"
)

func main() {
	// First we create a FuncMap with which to register the function.
	funcMap := template.FuncMap{
		// The name "title" is what the function will be called in the template text.
		"title": strings.Title,
	}

	// A simple template definition to test our function.
	// We print the input text several ways:
	// - the original
	// - title-cased
	// - title-cased and then printed with %q
	// - printed with %q and then title-cased.
	const templateText = `
Input: {{printf "%q" .}}
Output 0: {{title .}}
Output 1: {{title . | printf "%q"}}
Output 2: {{printf "%q" . | title}}
`

	// Create a template, add the function map, and parse the text.
	tmpl, err := template.New("titleTest").Funcs(funcMap).Parse(templateText)
	if err != nil {
		log.Fatalf("parsing: %s", err)
	}

	// Run the template to verify the output.
	err = tmpl.Execute(os.Stdout, "the go programming language")
	if err != nil {
		log.Fatalf("execution: %s", err)
	}

}

Output:

Input: "the go programming language"
Output 0: The Go Programming Language
Output 1: "The Go Programming Language"
Output 2: "The Go Programming Language"
Example (Glob)

Here we demonstrate loading a set of templates from a directory. 

package main

import (
	"io"
	"log"
	"os"
	"path/filepath"
	"text/template"
)

// templateFile defines the contents of a template to be stored in a file, for testing.
type templateFile struct {
	name     string
	contents string
}

func createTestDir(files []templateFile) string {
	dir, err := os.MkdirTemp("", "template")
	if err != nil {
		log.Fatal(err)
	}
	for _, file := range files {
		f, err := os.Create(filepath.Join(dir, file.name))
		if err != nil {
			log.Fatal(err)
		}
		defer f.Close()
		_, err = io.WriteString(f, file.contents)
		if err != nil {
			log.Fatal(err)
		}
	}
	return dir
}

func main() {
	// Here we create a temporary directory and populate it with our sample
	// template definition files; usually the template files would already
	// exist in some location known to the program.
	dir := createTestDir([]templateFile{
		// T0.tmpl is a plain template file that just invokes T1.
		{"T0.tmpl", `T0 invokes T1: ({{template "T1"}})`},
		// T1.tmpl defines a template, T1 that invokes T2.
		{"T1.tmpl", `{{define "T1"}}T1 invokes T2: ({{template "T2"}}){{end}}`},
		// T2.tmpl defines a template T2.
		{"T2.tmpl", `{{define "T2"}}This is T2{{end}}`},
	})
	// Clean up after the test; another quirk of running as an example.
	defer os.RemoveAll(dir)

	// pattern is the glob pattern used to find all the template files.
	pattern := filepath.Join(dir, "*.tmpl")

	// Here starts the example proper.
	// T0.tmpl is the first name matched, so it becomes the starting template,
	// the value returned by ParseGlob.
	tmpl := template.Must(template.ParseGlob(pattern))

	err := tmpl.Execute(os.Stdout, nil)
	if err != nil {
		log.Fatalf("template execution: %s", err)
	}
}

Output:

T0 invokes T1: (T1 invokes T2: (This is T2))
Example (Helpers)

This example demonstrates one way to share some templates and use them in different contexts. In this variant we add multiple driver templates by hand to an existing bundle of templates. 

package main

import (
	"io"
	"log"
	"os"
	"path/filepath"
	"text/template"
)

// templateFile defines the contents of a template to be stored in a file, for testing.
type templateFile struct {
	name     string
	contents string
}

func createTestDir(files []templateFile) string {
	dir, err := os.MkdirTemp("", "template")
	if err != nil {
		log.Fatal(err)
	}
	for _, file := range files {
		f, err := os.Create(filepath.Join(dir, file.name))
		if err != nil {
			log.Fatal(err)
		}
		defer f.Close()
		_, err = io.WriteString(f, file.contents)
		if err != nil {
			log.Fatal(err)
		}
	}
	return dir
}

func main() {
	// Here we create a temporary directory and populate it with our sample
	// template definition files; usually the template files would already
	// exist in some location known to the program.
	dir := createTestDir([]templateFile{
		// T1.tmpl defines a template, T1 that invokes T2.
		{"T1.tmpl", `{{define "T1"}}T1 invokes T2: ({{template "T2"}}){{end}}`},
		// T2.tmpl defines a template T2.
		{"T2.tmpl", `{{define "T2"}}This is T2{{end}}`},
	})
	// Clean up after the test; another quirk of running as an example.
	defer os.RemoveAll(dir)

	// pattern is the glob pattern used to find all the template files.
	pattern := filepath.Join(dir, "*.tmpl")

	// Here starts the example proper.
	// Load the helpers.
	templates := template.Must(template.ParseGlob(pattern))
	// Add one driver template to the bunch; we do this with an explicit template definition.
	_, err := templates.Parse("{{define `driver1`}}Driver 1 calls T1: ({{template `T1`}})\n{{end}}")
	if err != nil {
		log.Fatal("parsing driver1: ", err)
	}
	// Add another driver template.
	_, err = templates.Parse("{{define `driver2`}}Driver 2 calls T2: ({{template `T2`}})\n{{end}}")
	if err != nil {
		log.Fatal("parsing driver2: ", err)
	}
	// We load all the templates before execution. This package does not require
	// that behavior but html/template's escaping does, so it's a good habit.
	err = templates.ExecuteTemplate(os.Stdout, "driver1", nil)
	if err != nil {
		log.Fatalf("driver1 execution: %s", err)
	}
	err = templates.ExecuteTemplate(os.Stdout, "driver2", nil)
	if err != nil {
		log.Fatalf("driver2 execution: %s", err)
	}
}

Output:

Driver 1 calls T1: (T1 invokes T2: (This is T2))
Driver 2 calls T2: (This is T2)
Example (Share)

This example demonstrates how to use one group of driver templates with distinct sets of helper templates. 

package main

import (
	"io"
	"log"
	"os"
	"path/filepath"
	"text/template"
)

// templateFile defines the contents of a template to be stored in a file, for testing.
type templateFile struct {
	name     string
	contents string
}

func createTestDir(files []templateFile) string {
	dir, err := os.MkdirTemp("", "template")
	if err != nil {
		log.Fatal(err)
	}
	for _, file := range files {
		f, err := os.Create(filepath.Join(dir, file.name))
		if err != nil {
			log.Fatal(err)
		}
		defer f.Close()
		_, err = io.WriteString(f, file.contents)
		if err != nil {
			log.Fatal(err)
		}
	}
	return dir
}

func main() {
	// Here we create a temporary directory and populate it with our sample
	// template definition files; usually the template files would already
	// exist in some location known to the program.
	dir := createTestDir([]templateFile{
		// T0.tmpl is a plain template file that just invokes T1.
		{"T0.tmpl", "T0 ({{.}} version) invokes T1: ({{template `T1`}})\n"},
		// T1.tmpl defines a template, T1 that invokes T2. Note T2 is not defined
		{"T1.tmpl", `{{define "T1"}}T1 invokes T2: ({{template "T2"}}){{end}}`},
	})
	// Clean up after the test; another quirk of running as an example.
	defer os.RemoveAll(dir)

	// pattern is the glob pattern used to find all the template files.
	pattern := filepath.Join(dir, "*.tmpl")

	// Here starts the example proper.
	// Load the drivers.
	drivers := template.Must(template.ParseGlob(pattern))

	// We must define an implementation of the T2 template. First we clone
	// the drivers, then add a definition of T2 to the template name space.

	// 1. Clone the helper set to create a new name space from which to run them.
	first, err := drivers.Clone()
	if err != nil {
		log.Fatal("cloning helpers: ", err)
	}
	// 2. Define T2, version A, and parse it.
	_, err = first.Parse("{{define `T2`}}T2, version A{{end}}")
	if err != nil {
		log.Fatal("parsing T2: ", err)
	}

	// Now repeat the whole thing, using a different version of T2.
	// 1. Clone the drivers.
	second, err := drivers.Clone()
	if err != nil {
		log.Fatal("cloning drivers: ", err)
	}
	// 2. Define T2, version B, and parse it.
	_, err = second.Parse("{{define `T2`}}T2, version B{{end}}")
	if err != nil {
		log.Fatal("parsing T2: ", err)
	}

	// Execute the templates in the reverse order to verify the
	// first is unaffected by the second.
	err = second.ExecuteTemplate(os.Stdout, "T0.tmpl", "second")
	if err != nil {
		log.Fatalf("second execution: %s", err)
	}
	err = first.ExecuteTemplate(os.Stdout, "T0.tmpl", "first")
	if err != nil {
		log.Fatalf("first: execution: %s", err)
	}

}

Output:

T0 (second version) invokes T1: (T1 invokes T2: (T2, version B))
T0 (first version) invokes T1: (T1 invokes T2: (T2, version A))

func Must

func Must(t *Template, err error) *Template

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 

var t = template.Must(template.New("name").Parse("text"))

func New

func New(name string) *Template

New allocates a new, undefined template with the given name.

func ParseFS

func ParseFS(fsys fs.FS, patterns ...string) (*Template, error)

ParseFS is like ParseFiles or ParseGlob but reads from the file system fsys instead of the host operating system's file system. It accepts a list of glob patterns. (Note that most file names serve as glob patterns matching only themselves.)

func ParseFiles

func ParseFiles(filenames ...string) (*Template, error)

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.

When parsing multiple files with the same name in different directories, the last one mentioned will be the one that results. For instance, ParseFiles("a/foo", "b/foo") stores "b/foo" as the template named "foo", while "a/foo" is unavailable.

func ParseGlob

func ParseGlob(pattern string) (*Template, error)

ParseGlob creates a new Template and parses the template definitions from the files identified by the pattern. The files are matched according to the semantics of filepath.Match, and the pattern 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.

When parsing multiple files with the same name in different directories, the last one mentioned will be the one that results.

func (*Template) AddParseTree

func (t *Template) AddParseTree(name string, tree *parse.Tree) (*Template, error)

AddParseTree associates the argument parse tree with the template t, giving it the specified name. If the template has not been defined, this tree becomes its definition. If it has been defined and already has that name, the existing definition is replaced; otherwise a new template is created, defined, and returned.

func (*Template) Clone

func (t *Template) Clone() (*Template, error)

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 (*Template) DefinedTemplates

func (t *Template) DefinedTemplates() string

DefinedTemplates returns a string listing the defined templates, prefixed by the string "; defined templates are: ". If there are none, it returns the empty string. For generating an error message here and in html/template.

func (*Template) Delims

func (t *Template) Delims(left, right string) *Template

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 (*Template) Execute

func (t *Template) Execute(wr io.Writer, data any) error

Execute applies a parsed template to the specified data object, and writes the output to wr. If an error occurs executing the template or writing its output, execution stops, but partial results may already have been written to the output writer. A template may be executed safely in parallel, although if parallel executions share a Writer the output may be interleaved.

If data is a reflect.Value, the template applies to the concrete value that the reflect.Value holds, as in fmt.Print.

func (*Template) ExecuteTemplate

func (t *Template) ExecuteTemplate(wr io.Writer, name string, data any) error

ExecuteTemplate applies the template associated with t that has the given name to the specified data object and writes the output to wr. If an error occurs executing the template or writing its output, execution stops, but partial results may already have been written to the output writer. A template may be executed safely in parallel, although if parallel executions share a Writer the output may be interleaved.

func (*Template) Funcs

func (t *Template) Funcs(funcMap FuncMap) *Template

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 (*Template) Lookup

func (t *Template) Lookup(name string) *Template

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 (*Template) Name

func (t *Template) Name() string

Name returns the name of the template.

func (*Template) New

func (t *Template) New(name string) *Template

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 (*Template) Option

func (t *Template) Option(opt ...string) *Template

Option sets options for the template. Options are described by strings, either a simple string or "key=value". There can be at most one equals sign in an option string. If the option string is unrecognized or otherwise invalid, Option panics.

Known options:

missingkey: Control the behavior during execution if a map is indexed with a key that is not present in the map. 

"missingkey=default" or "missingkey=invalid"
	The default behavior: Do nothing and continue execution.
	If printed, the result of the index operation is the string
	"<no value>".
"missingkey=zero"
	The operation returns the zero value for the map type's element.
"missingkey=error"
	Execution stops immediately with an error.

func (*Template) Parse

func (t *Template) Parse(text string) (*Template, error)

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 (*Template) ParseFS

func (t *Template) ParseFS(fsys fs.FS, patterns ...string) (*Template, error)

ParseFS is like ParseFiles or ParseGlob but reads from the file system fsys instead of the host operating system's file system. It accepts a list of glob patterns. (Note that most file names serve as glob patterns matching only themselves.)

func (*Template) ParseFiles

func (t *Template) ParseFiles(filenames ...string) (*Template, error)

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. Since the templates created by ParseFiles are named by the base names of the argument files, t should usually have the name of one of the (base) names of the files. If it does not, depending on t's contents before calling ParseFiles, t.Execute may fail. In that case use t.ExecuteTemplate to execute a valid template.

When parsing multiple files with the same name in different directories, the last one mentioned will be the one that results.

func (*Template) ParseGlob

func (t *Template) ParseGlob(pattern string) (*Template, error)

ParseGlob parses the template definitions in the files identified by the pattern and associates the resulting templates with t. The files are matched according to the semantics of filepath.Match, and the pattern must match at least one file. ParseGlob is equivalent to calling t.ParseFiles with the list of files matched by the pattern.

When parsing multiple files with the same name in different directories, the last one mentioned will be the one that results.

func (*Template) Templates

func (t *Template) Templates() []*Template

Templates returns a slice of defined templates associated with t.

Directories

parse Package parse builds parse trees for templates as defined by text/template and html/template.