src/pkg/text/template/doc.go - go - Git at Google

 // 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 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.
 Actions may not span newlines, although comments can.

 Once constructed, a template may be executed safely in parallel.

 Actions

 Here is the list of actions. "Arguments" and "pipelines" are evaluations of
 data, defined in detail below.

 */
 //	{{/* a comment */}}
 //		A comment; discarded. May contain newlines.
 //		Comments do not nest.
 /*

 	{{pipeline}}
 		The default textual representation 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.

 	{{range pipeline}} T1 {{end}}
 		The value of the pipeline must be an array, slice, or map. 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 ("comparable"), 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, or map. 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.

 	{{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.

 	{{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.

 	- A boolean, string, character, integer, floating-point, imaginary
 	  or complex constant in Go syntax. These behave like Go's untyped
 	  constants, although raw strings may not span newlines.
 	- The character '.' (period):
 		.
 	  The result is the value of dot.
 	- A variable name, which is a (possibly empty) alphanumeric string
 	  preceded by a dollar sign, such as
 		$piOver2
 	  or
 		$
 	  The result is the value of the variable.
 	  Variables are described below.
 	- The name of a field of the data, which must be a struct, preceded
 	  by a period, such as
 		.Field
 	  The result is the value of the field. Field invocations may be
 	  chained:
 	    .Field1.Field2
 	  Fields can also be evaluated on variables, including chaining:
 	    $x.Field1.Field2
 	- The name of a key of the data, which must be a map, preceded
 	  by a period, such as
 		.Key
 	  The result is the map element value indexed by the key.
 	  Key invocations may be chained and combined with fields to any
 	  depth:
 	    .Field1.Key1.Field2.Key2
 	  Although the key must be an alphanumeric identifier, unlike with
 	  field names they do not need to start with an upper case letter.
 	  Keys can also be evaluated on variables, including chaining:
 	    $x.key1.key2
 	- The name of a niladic method of the data, preceded by a period,
 	  such as
 		.Method
 	  The result is the value of invoking the method with dot as the
 	  receiver, dot.Method(). Such a method must have one return value (of
 	  any type) or two return values, the second of which is an error.
 	  If it has two and the returned error is non-nil, execution terminates
 	  and an error is returned to the caller as the value of Execute.
 	  Method invocations may be chained and combined with fields and keys
 	  to any depth:
 	    .Field1.Key1.Method1.Field2.Key2.Method2
 	  Methods can also be evaluated on variables, including chaining:
 	    $x.Method1.Field
 	- The name of a niladic function, such as
 		fun
 	  The result is the value of invoking the function, fun(). The return
 	  types and values behave as in methods. Functions and function
 	  names are described below.

 Arguments may evaluate to any type; if they are pointers the implementation
 automatically indirects to the base type when required.

 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.

 Pipelines

 A pipeline may be "chained" by separating a sequence of commands with pipeline
 characters '|'. In a chained pipeline, the result of the 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.

 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:

 	$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.
 	{{"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 methods 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". All the
 		arguments are evaluated.
 	html
 		Returns the escaped HTML equivalent of the textual
 		representation of its arguments.
 	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.
 	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". All the
 		arguments are evaluated.
 	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.

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

 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.

 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,

 	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)
 	}

 */
 package template
	// 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 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.
	Actions may not span newlines, although comments can.

	Once constructed, a template may be executed safely in parallel.

	Actions

	Here is the list of actions. "Arguments" and "pipelines" are evaluations of
	data, defined in detail below.

	*/
	// {{/* a comment */}}
	// A comment; discarded. May contain newlines.
	// Comments do not nest.
	/*

	{{pipeline}}
	The default textual representation 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.

	{{range pipeline}} T1 {{end}}
	The value of the pipeline must be an array, slice, or map. 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 ("comparable"), 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, or map. 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.

	{{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.

	{{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.

	- A boolean, string, character, integer, floating-point, imaginary
	or complex constant in Go syntax. These behave like Go's untyped
	constants, although raw strings may not span newlines.
	- The character '.' (period):
	.
	The result is the value of dot.
	- A variable name, which is a (possibly empty) alphanumeric string
	preceded by a dollar sign, such as
	$piOver2
	or
	$
	The result is the value of the variable.
	Variables are described below.
	- The name of a field of the data, which must be a struct, preceded
	by a period, such as
	.Field
	The result is the value of the field. Field invocations may be
	chained:
	.Field1.Field2
	Fields can also be evaluated on variables, including chaining:
	$x.Field1.Field2
	- The name of a key of the data, which must be a map, preceded
	by a period, such as
	.Key
	The result is the map element value indexed by the key.
	Key invocations may be chained and combined with fields to any
	depth:
	.Field1.Key1.Field2.Key2
	Although the key must be an alphanumeric identifier, unlike with
	field names they do not need to start with an upper case letter.
	Keys can also be evaluated on variables, including chaining:
	$x.key1.key2
	- The name of a niladic method of the data, preceded by a period,
	such as
	.Method
	The result is the value of invoking the method with dot as the
	receiver, dot.Method(). Such a method must have one return value (of
	any type) or two return values, the second of which is an error.
	If it has two and the returned error is non-nil, execution terminates
	and an error is returned to the caller as the value of Execute.
	Method invocations may be chained and combined with fields and keys
	to any depth:
	.Field1.Key1.Method1.Field2.Key2.Method2
	Methods can also be evaluated on variables, including chaining:
	$x.Method1.Field
	- The name of a niladic function, such as
	fun
	The result is the value of invoking the function, fun(). The return
	types and values behave as in methods. Functions and function
	names are described below.

	Arguments may evaluate to any type; if they are pointers the implementation
	automatically indirects to the base type when required.

	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.

	Pipelines

	A pipeline may be "chained" by separating a sequence of commands with pipeline
	characters '\|'. In a chained pipeline, the result of the 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.

	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:

	$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.
	{{"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 methods 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". All the
	arguments are evaluated.
	html
	Returns the escaped HTML equivalent of the textual
	representation of its arguments.
	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.
	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". All the
	arguments are evaluated.
	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.

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

	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.

	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,

	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)
	}

	*/
	package template