Overview

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

To generate HTML output, see package 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. Except for raw
strings, actions may not span newlines, although comments can.

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”.

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 ASCII
space character (“{{- “), all trailing white space is trimmed from the
immediately preceding text. Similarly, if the right delimiter (“}}”) is preceded
by a space and minus sign (“ -}}”), all leading white space is trimmed from the
immediately following text. In these trim markers, the ASCII space must be
present; “{{-3}}” parses as an action containing the number -3.

For instance, when executing the template whose source is

  1. "{{23 -}} < {{- 45}}"

the generated output would be

  1. "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.

  1. {{/* a comment */}}
  2.     A comment; discarded. May contain newlines.
  3.     Comments do not nest and must start and end at the
  4.     delimiters, as shown here.
  6. {{pipeline}}
  7.     The default textual representation (the same as would be
  8.     printed by fmt.Print) of the value of the pipeline is copied
  9.     to the output.
  11. {{if pipeline}} T1 {{end}}
  12.     If the value of the pipeline is empty, no output is generated;
  13.     otherwise, T1 is executed. The empty values are false, 0, any
  14.     nil pointer or interface value, and any array, slice, map, or
  15.     string of length zero.
  16.     Dot is unaffected.
  18. {{if pipeline}} T1 {{else}} T0 {{end}}
  19.     If the value of the pipeline is empty, T0 is executed;
  20.     otherwise, T1 is executed. Dot is unaffected.
  22. {{if pipeline}} T1 {{else if pipeline}} T0 {{end}}
  23.     To simplify the appearance of if-else chains, the else action
  24.     of an if may include another if directly; the effect is exactly
  25.     the same as writing
  26.         {{if pipeline}} T1 {{else}}{{if pipeline}} T0 {{end}}{{end}}
  28. {{range pipeline}} T1 {{end}}
  29.     The value of the pipeline must be an array, slice, map, or channel.
  30.     If the value of the pipeline has length zero, nothing is output;
  31.     otherwise, dot is set to the successive elements of the array,
  32.     slice, or map and T1 is executed. If the value is a map and the
  33.     keys are of basic type with a defined order ("comparable"), the
  34.     elements will be visited in sorted key order.
  36. {{range pipeline}} T1 {{else}} T0 {{end}}
  37.     The value of the pipeline must be an array, slice, map, or channel.
  38.     If the value of the pipeline has length zero, dot is unaffected and
  39.     T0 is executed; otherwise, dot is set to the successive elements
  40.     of the array, slice, or map and T1 is executed.
  42. {{template "name"}}
  43.     The template with the specified name is executed with nil data.
  45. {{template "name" pipeline}}
  46.     The template with the specified name is executed with dot set
  47.     to the value of the pipeline.
  49. {{block "name" pipeline}} T1 {{end}}
  50.     A block is shorthand for defining a template
  51.         {{define "name"}} T1 {{end}}
  52.     and then executing it in place
  53.         {{template "name" .}}
  54.     The typical use is to define a set of root templates that are
  55.     then customized by redefining the block templates within.
  57. {{with pipeline}} T1 {{end}}
  58.     If the value of the pipeline is empty, no output is generated;
  59.     otherwise, dot is set to the value of the pipeline and T1 is
  60.     executed.
  62. {{with pipeline}} T1 {{else}} T0 {{end}}
  63.     If the value of the pipeline is empty, dot is unaffected and T0
  64.     is executed; otherwise, dot is set to the value of the pipeline
  65.     and T1 is executed.

Arguments

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

  1. - A boolean, string, character, integer, floating-point, imaginary
  2.   or complex constant in Go syntax. These behave like Go's untyped
  3.   constants.
  4. - The keyword nil, representing an untyped Go nil.
  5. - The character '.' (period):
  6.     .
  7.   The result is the value of dot.
  8. - A variable name, which is a (possibly empty) alphanumeric string
  9.   preceded by a dollar sign, such as
  10.     $piOver2
  11.   or
  12.     $
  13.   The result is the value of the variable.
  14.   Variables are described below.
  15. - The name of a field of the data, which must be a struct, preceded
  16.   by a period, such as
  17.     .Field
  18.   The result is the value of the field. Field invocations may be
  19.   chained:
  20.     .Field1.Field2
  21.   Fields can also be evaluated on variables, including chaining:
  22.     $x.Field1.Field2
  23. - The name of a key of the data, which must be a map, preceded
  24.   by a period, such as
  25.     .Key
  26.   The result is the map element value indexed by the key.
  27.   Key invocations may be chained and combined with fields to any
  28.   depth:
  29.     .Field1.Key1.Field2.Key2
  30.   Although the key must be an alphanumeric identifier, unlike with
  31.   field names they do not need to start with an upper case letter.
  32.   Keys can also be evaluated on variables, including chaining:
  33.     $x.key1.key2
  34. - The name of a niladic method of the data, preceded by a period,
  35.   such as
  36.     .Method
  37.   The result is the value of invoking the method with dot as the
  38.   receiver, dot.Method(). Such a method must have one return value (of
  39.   any type) or two return values, the second of which is an error.
  40.   If it has two and the returned error is non-nil, execution terminates
  41.   and an error is returned to the caller as the value of Execute.
  42.   Method invocations may be chained and combined with fields and keys
  43.   to any depth:
  44.     .Field1.Key1.Method1.Field2.Key2.Method2
  45.   Methods can also be evaluated on variables, including chaining:
  46.     $x.Method1.Field
  47. - The name of a niladic function, such as
  48.     fun
  49.   The result is the value of invoking the function, fun(). The return
  50.   types and values behave as in methods. Functions and function
  51.   names are described below.
  52. - A parenthesized instance of one the above, for grouping. The result
  53.   may be accessed by a field or map key invocation.
  54.     print (.F1 arg1) (.F2 arg2)
  55.     (.StructValuedMethod "arg").Field

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:

  1. Argument
  2.     The result is the value of evaluating the argument.
  3. .Method [Argument...]
  4.     The method can be alone or the last element of a chain but,
  5.     unlike methods in the middle of a chain, it can take arguments.
  6.     The result is the value of calling the method with the
  7.     arguments:
  8.         dot.Method(Argument1, etc.)
  9. functionName [Argument...]
  10.     The result is the value of calling the function associated
  11.     with the name:
  12.         function(Argument1, etc.)
  13.     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

  1. $variable := pipeline

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

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:

  1. 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”:

  1. {{"\"output\""}}
  2.     A string constant.
  3. {{`"output"`}}
  4.     A raw string constant.
  5. {{printf "%q" "output"}}
  6.     A function call.
  7. {{"output" | printf "%q"}}
  8.     A function call whose final argument comes from the previous
  9.     command.
  10. {{printf "%q" (print "out" "put")}}
  11.     A parenthesized argument.
  12. {{"put" | printf "%s%s" "out" | printf "%q"}}
  13.     A more elaborate call.
  14. {{"output" | printf "%s" | printf "%q"}}
  15.     A longer chain.
  16. {{with "output"}}{{printf "%q" .}}{{end}}
  17.     A with action using dot.
  18. {{with $x := "output" | printf "%q"}}{{$x}}{{end}}
  19.     A with action that creates and uses a variable.
  20. {{with $x := "output"}}{{printf "%q" $x}}{{end}}
  21.     A with action that uses the variable in another action.
  22. {{with $x := "output"}}{{$x | printf "%q"}}{{end}}
  23.     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.

  1. and
  2.     Returns the boolean AND of its arguments by returning the
  3.     first empty argument or the last argument, that is,
  4.     "and x y" behaves as "if x then y else x". All the
  6. call
  7.     Returns the result of calling the first argument, which
  8.     must be a function, with the remaining arguments as parameters.
  9.     Thus "call .X.Y 1 2" is, in Go notation, dot.X.Y(1, 2) where
  10.     Y is a func-valued field, map entry, or the like.
  11.     The first argument must be the result of an evaluation
  12.     that yields a value of function type (as distinct from
  13.     a predefined function such as print). The function must
  14.     return either one or two result values, the second of which
  15.     is of type error. If the arguments don't match the function
  16.     or the returned error value is non-nil, execution stops.
  17. html
  18.     Returns the escaped HTML equivalent of the textual
  19.     representation of its arguments. This function is unavailable
  20.     in html/template, with a few exceptions.
  21. index
  22.     Returns the result of indexing its first argument by the
  23.     following arguments. Thus "index x 1 2 3" is, in Go syntax,
  24.     x[1][2][3]. Each indexed item must be a map, slice, or array.
  25. js
  26.     representation of its arguments.
  27. len
  28.     Returns the integer length of its argument.
  29. not
  30.     Returns the boolean negation of its single argument.
  31. or
  32.     Returns the boolean OR of its arguments by returning the
  33.     first non-empty argument or the last argument, that is,
  34.     "or x y" behaves as "if x then x else y". All the
  35.     arguments are evaluated.
  36. print
  37.     An alias for fmt.Sprint
  38. printf
  39.     An alias for fmt.Sprintf
  40. println
  41.     An alias for fmt.Sprintln
  42. urlquery
  43.     Returns the escaped value of the textual representation of
  44.     its arguments in a form suitable for embedding in a URL query.
  45.     This function is unavailable in html/template, with a few
  46.     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:

  1. eq
  2.     Returns the boolean truth of arg1 == arg2
  3. ne
  4.     Returns the boolean truth of arg1 != arg2
  5. lt
  6.     Returns the boolean truth of arg1 < arg2
  7. le
  8.     Returns the boolean truth of arg1 <= arg2
  9. gt
  10.     Returns the boolean truth of arg1 > arg2
  11. ge
  12.     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

  1. 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 basic types only (or named basic types, such as
“type Celsius float32”). They implement the Go rules for comparison of values,
except that 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:

  1. `{{define "T1"}}ONE{{end}}
  2. {{define "T2"}}TWO{{end}}
  3. {{define "T3"}}{{template "T1"}} {{template "T2"}}{{end}}
  4. {{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

  1. 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 the Clone or AddParseTree method.

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.

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,

  1. err := tmpl.Execute(os.Stdout, "no data needed")
  2. if err != nil {
  3.     log.Fatalf("execution failed: %s", err)
  4. }

or to invoke a particular template explicitly by name,

  1. err := tmpl.ExecuteTemplate(os.Stdout, "T2", "no data needed")
  2. if err != nil {
  3.     log.Fatalf("execution failed: %s", err)
  4. }

Index

Package files

func HTMLEscape

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

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

func 

  1. func HTMLEscapeString(s ) string

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

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

func 

  1. func IsTrue(val interface{}) (truth, ok )

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

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

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

func 

  1. func JSEscapeString(s ) string

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

func 

  1. func JSEscaper(args ...interface{})

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

  1. func URLQueryEscaper(args ...interface{}) string

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

type 

  1. type ExecError struct {
  2.     Name  // Name of template.
  3.     Err  error  // Pre-formatted error.
  4. }

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) 

  1. func (e ) Error() string

type 

  1. type FuncMap map[]interface{}

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.

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

  1. type Template struct {
  2.     *parse.
  3.     // contains filtered or unexported fields
  4. }

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:

  1. // Define a template.
  2. const letter = `
  3. Dear {{.Name}},
  4. {{if .Attended}}
  5. It was a pleasure to see you at the wedding.
  6. {{- else}}
  7. It is a shame you couldn't make it to the wedding.
  8. {{- end}}
  9. {{with .Gift -}}
  10. Thank you for the lovely {{.}}.
  11. {{end}}
  12. Best wishes,
  13. Josie
  14. `
  16. // Prepare some data to insert into the template.
  17. type Recipient struct {
  18.     Name, Gift string
  19.     Attended   bool
  20. }
  21. var recipients = []Recipient{
  22.     {"Aunt Mildred", "bone china tea set", true},
  23.     {"Uncle John", "moleskin pants", false},
  24.     {"Cousin Rodney", "", false},
  25. }
  27. // Create a new template and parse the letter into it.
  28. t := template.Must(template.New("letter").Parse(letter))
  30. // Execute the template for each recipient.
  31. for _, r := range recipients {
  32.     err := t.Execute(os.Stdout, r)
  33.     if err != nil {
  34.         log.Println("executing template:", err)
  35.     }
  36. }
  38. // Output:
  39. // Dear Aunt Mildred,
  40. //
  41. // It was a pleasure to see you at the wedding.
  42. // Thank you for the lovely bone china tea set.
  43. //
  44. // Best wishes,
  45. // Josie
  46. //
  47. // Dear Uncle John,
  48. //
  49. // It is a shame you couldn't make it to the wedding.
  50. // Thank you for the lovely moleskin pants.
  51. //
  52. // Best wishes,
  53. // Josie
  54. //
  55. // Dear Cousin Rodney,
  56. //
  57. // It is a shame you couldn't make it to the wedding.
  58. //
  59. // Best wishes,
  60. // Josie


Example:

  1. const (
  2.     master  = `Names:{{block "list" .}}{{"\n"}}{{range .}}{{println "-" .}}{{end}}{{end}}`
  3.     overlay = `{{define "list"}} {{join . ", "}}{{end}} `
  4. )
  5. var (
  6.     funcs     = template.FuncMap{"join": strings.Join}
  7.     guardians = []string{"Gamora", "Groot", "Nebula", "Rocket", "Star-Lord"}
  8. )
  9. masterTmpl, err := template.New("master").Funcs(funcs).Parse(master)
  10. if err != nil {
  11.     log.Fatal(err)
  12. }
  13. overlayTmpl, err := template.Must(masterTmpl.Clone()).Parse(overlay)
  14. if err != nil {
  15.     log.Fatal(err)
  16. }
  17. if err := masterTmpl.Execute(os.Stdout, guardians); err != nil {
  18.     log.Fatal(err)
  19. }
  20. if err := overlayTmpl.Execute(os.Stdout, guardians); err != nil {
  21.     log.Fatal(err)
  22. }
  23. // Output:
  24. // Names:
  25. // - Gamora
  26. // - Groot
  27. // - Nebula
  28. // - Rocket
  29. // - Star-Lord


Example:

  1. // First we create a FuncMap with which to register the function.
  2. funcMap := template.FuncMap{
  3.     // The name "title" is what the function will be called in the template text.
  4.     "title": strings.Title,
  5. }
  8. // We print the input text several ways:
  9. // - the original
  10. // - title-cased
  11. // - title-cased and then printed with %q
  12. // - printed with %q and then title-cased.
  13. const templateText = `
  14. Input: {{printf "%q" .}}
  15. Output 0: {{title .}}
  16. Output 1: {{title . | printf "%q"}}
  17. Output 2: {{printf "%q" . | title}}
  18. `
  20. // Create a template, add the function map, and parse the text.
  21. tmpl, err := template.New("titleTest").Funcs(funcMap).Parse(templateText)
  22. if err != nil {
  23.     log.Fatalf("parsing: %s", err)
  24. }
  26. // Run the template to verify the output.
  27. err = tmpl.Execute(os.Stdout, "the go programming language")
  28. if err != nil {
  29.     log.Fatalf("execution: %s", err)
  30. }
  32. // Output:
  33. // Input: "the go programming language"
  34. // Output 0: The Go Programming Language
  35. // Output 1: "The Go Programming Language"
  36. // Output 2: "The Go Programming Language"


Example:

  1. // Here we create a temporary directory and populate it with our sample
  2. // template definition files; usually the template files would already
  3. // exist in some location known to the program.
  4. dir := createTestDir([]templateFile{
  5.     // T0.tmpl is a plain template file that just invokes T1.
  6.     {"T0.tmpl", `T0 invokes T1: ({{template "T1"}})`},
  7.     // T1.tmpl defines a template, T1 that invokes T2.
  8.     {"T1.tmpl", `{{define "T1"}}T1 invokes T2: ({{template "T2"}}){{end}}`},
  9.     // T2.tmpl defines a template T2.
  10.     {"T2.tmpl", `{{define "T2"}}This is T2{{end}}`},
  11. })
  12. // Clean up after the test; another quirk of running as an example.
  13. defer os.RemoveAll(dir)
  15. // pattern is the glob pattern used to find all the template files.
  16. pattern := filepath.Join(dir, "*.tmpl")
  18. // Here starts the example proper.
  19. // T0.tmpl is the first name matched, so it becomes the starting template,
  20. // the value returned by ParseGlob.
  21. tmpl := template.Must(template.ParseGlob(pattern))
  23. err := tmpl.Execute(os.Stdout, nil)
  24. if err != nil {
  25.     log.Fatalf("template execution: %s", err)
  26. }
  27. // Output:
  28. // T0 invokes T1: (T1 invokes T2: (This is T2))


Example:

  1. // Here we create a temporary directory and populate it with our sample
  2. // template definition files; usually the template files would already
  3. // exist in some location known to the program.
  4. dir := createTestDir([]templateFile{
  5.     // T1.tmpl defines a template, T1 that invokes T2.
  6.     {"T1.tmpl", `{{define "T1"}}T1 invokes T2: ({{template "T2"}}){{end}}`},
  7.     // T2.tmpl defines a template T2.
  8.     {"T2.tmpl", `{{define "T2"}}This is T2{{end}}`},
  9. })
  10. // Clean up after the test; another quirk of running as an example.
  11. defer os.RemoveAll(dir)
  13. // pattern is the glob pattern used to find all the template files.
  14. pattern := filepath.Join(dir, "*.tmpl")
  16. // Here starts the example proper.
  17. // Load the helpers.
  18. templates := template.Must(template.ParseGlob(pattern))
  19. // Add one driver template to the bunch; we do this with an explicit template definition.
  20. _, err := templates.Parse("{{define `driver1`}}Driver 1 calls T1: ({{template `T1`}})\n{{end}}")
  21. if err != nil {
  22.     log.Fatal("parsing driver1: ", err)
  23. }
  24. // Add another driver template.
  25. _, err = templates.Parse("{{define `driver2`}}Driver 2 calls T2: ({{template `T2`}})\n{{end}}")
  26. if err != nil {
  27.     log.Fatal("parsing driver2: ", err)
  28. }
  29. // We load all the templates before execution. This package does not require
  30. // that behavior but html/template's escaping does, so it's a good habit.
  31. err = templates.ExecuteTemplate(os.Stdout, "driver1", nil)
  32. if err != nil {
  33.     log.Fatalf("driver1 execution: %s", err)
  34. }
  35. err = templates.ExecuteTemplate(os.Stdout, "driver2", nil)
  36. if err != nil {
  37.     log.Fatalf("driver2 execution: %s", err)
  38. }
  39. // Output:
  40. // Driver 1 calls T1: (T1 invokes T2: (This is T2))
  41. // Driver 2 calls T2: (This is T2)


Example:

  1. // Here we create a temporary directory and populate it with our sample
  2. // template definition files; usually the template files would already
  3. // exist in some location known to the program.
  4. dir := createTestDir([]templateFile{
  5.     // T0.tmpl is a plain template file that just invokes T1.
  6.     {"T0.tmpl", "T0 ({{.}} version) invokes T1: ({{template `T1`}})\n"},
  7.     // T1.tmpl defines a template, T1 that invokes T2. Note T2 is not defined
  8.     {"T1.tmpl", `{{define "T1"}}T1 invokes T2: ({{template "T2"}}){{end}}`},
  9. })
  10. // Clean up after the test; another quirk of running as an example.
  11. defer os.RemoveAll(dir)
  13. // pattern is the glob pattern used to find all the template files.
  14. pattern := filepath.Join(dir, "*.tmpl")
  16. // Here starts the example proper.
  17. // Load the drivers.
  18. drivers := template.Must(template.ParseGlob(pattern))
  20. // We must define an implementation of the T2 template. First we clone
  21. // the drivers, then add a definition of T2 to the template name space.
  23. // 1. Clone the helper set to create a new name space from which to run them.
  24. first, err := drivers.Clone()
  25. if err != nil {
  26.     log.Fatal("cloning helpers: ", err)
  27. }
  28. // 2. Define T2, version A, and parse it.
  29. _, err = first.Parse("{{define `T2`}}T2, version A{{end}}")
  30. if err != nil {
  31.     log.Fatal("parsing T2: ", err)
  32. }
  34. // Now repeat the whole thing, using a different version of T2.
  35. // 1. Clone the drivers.
  36. second, err := drivers.Clone()
  37. if err != nil {
  38.     log.Fatal("cloning drivers: ", err)
  39. }
  40. // 2. Define T2, version B, and parse it.
  41. _, err = second.Parse("{{define `T2`}}T2, version B{{end}}")
  42. if err != nil {
  43.     log.Fatal("parsing T2: ", err)
  44. }
  46. // Execute the templates in the reverse order to verify the
  47. // first is unaffected by the second.
  48. err = second.ExecuteTemplate(os.Stdout, "T0.tmpl", "second")
  49. if err != nil {
  50.     log.Fatalf("second execution: %s", err)
  51. }
  52. err = first.ExecuteTemplate(os.Stdout, "T0.tmpl", "first")
  53. if err != nil {
  54.     log.Fatalf("first: execution: %s", err)
  55. }
  57. // Output:
  58. // T0 (second version) invokes T1: (T1 invokes T2: (T2, version B))
  59. // T0 (first version) invokes T1: (T1 invokes T2: (T2, version A))

func Must

  1. func Must(t *Template, err ) *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

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

func

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

func 

  1. func ParseFiles(filenames ...) (*Template, )

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

  1. func ParseGlob(pattern string) (*, error)

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.

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

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

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 (*Template) 

  1. func (t *) Clone() (*Template, )

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

  1. func (t *Template) DefinedTemplates()

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

  1. func (t *Template) Delims(left, right ) *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) 

  1. func (t *) Execute(wr io., data interface{}) 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) 

  1. func (t *) ExecuteTemplate(wr io., name string, data interface{})

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

  1. func (t *Template) Funcs(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.

  1. func (t *) Lookup(name string) *

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

  1. func (t *Template) Name()

Name returns the name of the template.

func (*Template) New

  1. func (t *Template) New(name ) *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.

func (*Template) 

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

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.

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

func (*Template) Parse

  1. func (t *Template) Parse(text ) (*Template, )

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) ParseFiles

  1. func (t *Template) ParseFiles(filenames ...) (*Template, )

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

  1. func (t *Template) ParseGlob(pattern ) (*Template, )

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.

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

Templates returns a slice of defined templates associated with t.

Subdirectories