src/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.
 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".

 	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.

 	- A boolean, string, character, integer, floating-point, imaginary
 	  or complex constant in Go syntax. These behave like Go's untyped
 	  constants. Note that, as in Go, whether a large integer constant
 	  overflows when assigned or passed to a function can depend on whether
 	  the host machine's ints are 32 or 64 bits.
 	- The keyword nil, representing an untyped Go nil.
 	- 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.
 	- A parenthesized instance of one the above, for grouping. The result
 	  may be accessed by a field or map key invocation.
 		print (.F1 arg1) (.F2 arg2)
 		(.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:

 	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 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,

 	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.
	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".

	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.

	- A boolean, string, character, integer, floating-point, imaginary
	or complex constant in Go syntax. These behave like Go's untyped
	constants. Note that, as in Go, whether a large integer constant
	overflows when assigned or passed to a function can depend on whether
	the host machine's ints are 32 or 64 bits.
	- The keyword nil, representing an untyped Go nil.
	- 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.
	- A parenthesized instance of one the above, for grouping. The result
	may be accessed by a field or map key invocation.
	print (.F1 arg1) (.F2 arg2)
	(.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:

	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 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,

	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