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Source file src/text/template/doc.go

     1	// Copyright 2011 The Go Authors. All rights reserved.
     2	// Use of this source code is governed by a BSD-style
     3	// license that can be found in the LICENSE file.
     4	
     5	/*
     6	Package template implements data-driven templates for generating textual output.
     7	
     8	To generate HTML output, see package html/template, which has the same interface
     9	as this package but automatically secures HTML output against certain attacks.
    10	
    11	Templates are executed by applying them to a data structure. Annotations in the
    12	template refer to elements of the data structure (typically a field of a struct
    13	or a key in a map) to control execution and derive values to be displayed.
    14	Execution of the template walks the structure and sets the cursor, represented
    15	by a period '.' and called "dot", to the value at the current location in the
    16	structure as execution proceeds.
    17	
    18	The input text for a template is UTF-8-encoded text in any format.
    19	"Actions"--data evaluations or control structures--are delimited by
    20	"{{" and "}}"; all text outside actions is copied to the output unchanged.
    21	Except for raw strings, actions may not span newlines, although comments can.
    22	
    23	Once parsed, a template may be executed safely in parallel.
    24	
    25	Here is a trivial example that prints "17 items are made of wool".
    26	
    27		type Inventory struct {
    28			Material string
    29			Count    uint
    30		}
    31		sweaters := Inventory{"wool", 17}
    32		tmpl, err := template.New("test").Parse("{{.Count}} items are made of {{.Material}}")
    33		if err != nil { panic(err) }
    34		err = tmpl.Execute(os.Stdout, sweaters)
    35		if err != nil { panic(err) }
    36	
    37	More intricate examples appear below.
    38	
    39	Text and spaces
    40	
    41	By default, all text between actions is copied verbatim when the template is
    42	executed. For example, the string " items are made of " in the example above appears
    43	on standard output when the program is run.
    44	
    45	However, to aid in formatting template source code, if an action's left delimiter
    46	(by default "{{") is followed immediately by a minus sign and ASCII space character
    47	("{{- "), all trailing white space is trimmed from the immediately preceding text.
    48	Similarly, if the right delimiter ("}}") is preceded by a space and minus sign
    49	(" -}}"), all leading white space is trimmed from the immediately following text.
    50	In these trim markers, the ASCII space must be present; "{{-3}}" parses as an
    51	action containing the number -3.
    52	
    53	For instance, when executing the template whose source is
    54	
    55		"{{23 -}} < {{- 45}}"
    56	
    57	the generated output would be
    58	
    59		"23<45"
    60	
    61	For this trimming, the definition of white space characters is the same as in Go:
    62	space, horizontal tab, carriage return, and newline.
    63	
    64	Actions
    65	
    66	Here is the list of actions. "Arguments" and "pipelines" are evaluations of
    67	data, defined in detail in the corresponding sections that follow.
    68	
    69	*/
    70	//	{{/* a comment */}}
    71	//		A comment; discarded. May contain newlines.
    72	//		Comments do not nest and must start and end at the
    73	//		delimiters, as shown here.
    74	/*
    75	
    76		{{pipeline}}
    77			The default textual representation (the same as would be
    78			printed by fmt.Print) of the value of the pipeline is copied
    79			to the output.
    80	
    81		{{if pipeline}} T1 {{end}}
    82			If the value of the pipeline is empty, no output is generated;
    83			otherwise, T1 is executed.  The empty values are false, 0, any
    84			nil pointer or interface value, and any array, slice, map, or
    85			string of length zero.
    86			Dot is unaffected.
    87	
    88		{{if pipeline}} T1 {{else}} T0 {{end}}
    89			If the value of the pipeline is empty, T0 is executed;
    90			otherwise, T1 is executed.  Dot is unaffected.
    91	
    92		{{if pipeline}} T1 {{else if pipeline}} T0 {{end}}
    93			To simplify the appearance of if-else chains, the else action
    94			of an if may include another if directly; the effect is exactly
    95			the same as writing
    96				{{if pipeline}} T1 {{else}}{{if pipeline}} T0 {{end}}{{end}}
    97	
    98		{{range pipeline}} T1 {{end}}
    99			The value of the pipeline must be an array, slice, map, or channel.
   100			If the value of the pipeline has length zero, nothing is output;
   101			otherwise, dot is set to the successive elements of the array,
   102			slice, or map and T1 is executed. If the value is a map and the
   103			keys are of basic type with a defined order ("comparable"), the
   104			elements will be visited in sorted key order.
   105	
   106		{{range pipeline}} T1 {{else}} T0 {{end}}
   107			The value of the pipeline must be an array, slice, map, or channel.
   108			If the value of the pipeline has length zero, dot is unaffected and
   109			T0 is executed; otherwise, dot is set to the successive elements
   110			of the array, slice, or map and T1 is executed.
   111	
   112		{{template "name"}}
   113			The template with the specified name is executed with nil data.
   114	
   115		{{template "name" pipeline}}
   116			The template with the specified name is executed with dot set
   117			to the value of the pipeline.
   118	
   119		{{block "name" pipeline}} T1 {{end}}
   120			A block is shorthand for defining a template
   121				{{define "name"}} T1 {{end}}
   122			and then executing it in place
   123				{{template "name" .}}
   124			The typical use is to define a set of root templates that are
   125			then customized by redefining the block templates within.
   126	
   127		{{with pipeline}} T1 {{end}}
   128			If the value of the pipeline is empty, no output is generated;
   129			otherwise, dot is set to the value of the pipeline and T1 is
   130			executed.
   131	
   132		{{with pipeline}} T1 {{else}} T0 {{end}}
   133			If the value of the pipeline is empty, dot is unaffected and T0
   134			is executed; otherwise, dot is set to the value of the pipeline
   135			and T1 is executed.
   136	
   137	Arguments
   138	
   139	An argument is a simple value, denoted by one of the following.
   140	
   141		- A boolean, string, character, integer, floating-point, imaginary
   142		  or complex constant in Go syntax. These behave like Go's untyped
   143		  constants.
   144		- The keyword nil, representing an untyped Go nil.
   145		- The character '.' (period):
   146			.
   147		  The result is the value of dot.
   148		- A variable name, which is a (possibly empty) alphanumeric string
   149		  preceded by a dollar sign, such as
   150			$piOver2
   151		  or
   152			$
   153		  The result is the value of the variable.
   154		  Variables are described below.
   155		- The name of a field of the data, which must be a struct, preceded
   156		  by a period, such as
   157			.Field
   158		  The result is the value of the field. Field invocations may be
   159		  chained:
   160		    .Field1.Field2
   161		  Fields can also be evaluated on variables, including chaining:
   162		    $x.Field1.Field2
   163		- The name of a key of the data, which must be a map, preceded
   164		  by a period, such as
   165			.Key
   166		  The result is the map element value indexed by the key.
   167		  Key invocations may be chained and combined with fields to any
   168		  depth:
   169		    .Field1.Key1.Field2.Key2
   170		  Although the key must be an alphanumeric identifier, unlike with
   171		  field names they do not need to start with an upper case letter.
   172		  Keys can also be evaluated on variables, including chaining:
   173		    $x.key1.key2
   174		- The name of a niladic method of the data, preceded by a period,
   175		  such as
   176			.Method
   177		  The result is the value of invoking the method with dot as the
   178		  receiver, dot.Method(). Such a method must have one return value (of
   179		  any type) or two return values, the second of which is an error.
   180		  If it has two and the returned error is non-nil, execution terminates
   181		  and an error is returned to the caller as the value of Execute.
   182		  Method invocations may be chained and combined with fields and keys
   183		  to any depth:
   184		    .Field1.Key1.Method1.Field2.Key2.Method2
   185		  Methods can also be evaluated on variables, including chaining:
   186		    $x.Method1.Field
   187		- The name of a niladic function, such as
   188			fun
   189		  The result is the value of invoking the function, fun(). The return
   190		  types and values behave as in methods. Functions and function
   191		  names are described below.
   192		- A parenthesized instance of one the above, for grouping. The result
   193		  may be accessed by a field or map key invocation.
   194			print (.F1 arg1) (.F2 arg2)
   195			(.StructValuedMethod "arg").Field
   196	
   197	Arguments may evaluate to any type; if they are pointers the implementation
   198	automatically indirects to the base type when required.
   199	If an evaluation yields a function value, such as a function-valued
   200	field of a struct, the function is not invoked automatically, but it
   201	can be used as a truth value for an if action and the like. To invoke
   202	it, use the call function, defined below.
   203	
   204	Pipelines
   205	
   206	A pipeline is a possibly chained sequence of "commands". A command is a simple
   207	value (argument) or a function or method call, possibly with multiple arguments:
   208	
   209		Argument
   210			The result is the value of evaluating the argument.
   211		.Method [Argument...]
   212			The method can be alone or the last element of a chain but,
   213			unlike methods in the middle of a chain, it can take arguments.
   214			The result is the value of calling the method with the
   215			arguments:
   216				dot.Method(Argument1, etc.)
   217		functionName [Argument...]
   218			The result is the value of calling the function associated
   219			with the name:
   220				function(Argument1, etc.)
   221			Functions and function names are described below.
   222	
   223	A pipeline may be "chained" by separating a sequence of commands with pipeline
   224	characters '|'. In a chained pipeline, the result of each command is
   225	passed as the last argument of the following command. The output of the final
   226	command in the pipeline is the value of the pipeline.
   227	
   228	The output of a command will be either one value or two values, the second of
   229	which has type error. If that second value is present and evaluates to
   230	non-nil, execution terminates and the error is returned to the caller of
   231	Execute.
   232	
   233	Variables
   234	
   235	A pipeline inside an action may initialize a variable to capture the result.
   236	The initialization has syntax
   237	
   238		$variable := pipeline
   239	
   240	where $variable is the name of the variable. An action that declares a
   241	variable produces no output.
   242	
   243	If a "range" action initializes a variable, the variable is set to the
   244	successive elements of the iteration.  Also, a "range" may declare two
   245	variables, separated by a comma:
   246	
   247		range $index, $element := pipeline
   248	
   249	in which case $index and $element are set to the successive values of the
   250	array/slice index or map key and element, respectively.  Note that if there is
   251	only one variable, it is assigned the element; this is opposite to the
   252	convention in Go range clauses.
   253	
   254	A variable's scope extends to the "end" action of the control structure ("if",
   255	"with", or "range") in which it is declared, or to the end of the template if
   256	there is no such control structure.  A template invocation does not inherit
   257	variables from the point of its invocation.
   258	
   259	When execution begins, $ is set to the data argument passed to Execute, that is,
   260	to the starting value of dot.
   261	
   262	Examples
   263	
   264	Here are some example one-line templates demonstrating pipelines and variables.
   265	All produce the quoted word "output":
   266	
   267		{{"\"output\""}}
   268			A string constant.
   269		{{`"output"`}}
   270			A raw string constant.
   271		{{printf "%q" "output"}}
   272			A function call.
   273		{{"output" | printf "%q"}}
   274			A function call whose final argument comes from the previous
   275			command.
   276		{{printf "%q" (print "out" "put")}}
   277			A parenthesized argument.
   278		{{"put" | printf "%s%s" "out" | printf "%q"}}
   279			A more elaborate call.
   280		{{"output" | printf "%s" | printf "%q"}}
   281			A longer chain.
   282		{{with "output"}}{{printf "%q" .}}{{end}}
   283			A with action using dot.
   284		{{with $x := "output" | printf "%q"}}{{$x}}{{end}}
   285			A with action that creates and uses a variable.
   286		{{with $x := "output"}}{{printf "%q" $x}}{{end}}
   287			A with action that uses the variable in another action.
   288		{{with $x := "output"}}{{$x | printf "%q"}}{{end}}
   289			The same, but pipelined.
   290	
   291	Functions
   292	
   293	During execution functions are found in two function maps: first in the
   294	template, then in the global function map. By default, no functions are defined
   295	in the template but the Funcs method can be used to add them.
   296	
   297	Predefined global functions are named as follows.
   298	
   299		and
   300			Returns the boolean AND of its arguments by returning the
   301			first empty argument or the last argument, that is,
   302			"and x y" behaves as "if x then y else x". All the
   303			arguments are evaluated.
   304		call
   305			Returns the result of calling the first argument, which
   306			must be a function, with the remaining arguments as parameters.
   307			Thus "call .X.Y 1 2" is, in Go notation, dot.X.Y(1, 2) where
   308			Y is a func-valued field, map entry, or the like.
   309			The first argument must be the result of an evaluation
   310			that yields a value of function type (as distinct from
   311			a predefined function such as print). The function must
   312			return either one or two result values, the second of which
   313			is of type error. If the arguments don't match the function
   314			or the returned error value is non-nil, execution stops.
   315		html
   316			Returns the escaped HTML equivalent of the textual
   317			representation of its arguments.
   318		index
   319			Returns the result of indexing its first argument by the
   320			following arguments. Thus "index x 1 2 3" is, in Go syntax,
   321			x[1][2][3]. Each indexed item must be a map, slice, or array.
   322		js
   323			Returns the escaped JavaScript equivalent of the textual
   324			representation of its arguments.
   325		len
   326			Returns the integer length of its argument.
   327		not
   328			Returns the boolean negation of its single argument.
   329		or
   330			Returns the boolean OR of its arguments by returning the
   331			first non-empty argument or the last argument, that is,
   332			"or x y" behaves as "if x then x else y". All the
   333			arguments are evaluated.
   334		print
   335			An alias for fmt.Sprint
   336		printf
   337			An alias for fmt.Sprintf
   338		println
   339			An alias for fmt.Sprintln
   340		urlquery
   341			Returns the escaped value of the textual representation of
   342			its arguments in a form suitable for embedding in a URL query.
   343	
   344	The boolean functions take any zero value to be false and a non-zero
   345	value to be true.
   346	
   347	There is also a set of binary comparison operators defined as
   348	functions:
   349	
   350		eq
   351			Returns the boolean truth of arg1 == arg2
   352		ne
   353			Returns the boolean truth of arg1 != arg2
   354		lt
   355			Returns the boolean truth of arg1 < arg2
   356		le
   357			Returns the boolean truth of arg1 <= arg2
   358		gt
   359			Returns the boolean truth of arg1 > arg2
   360		ge
   361			Returns the boolean truth of arg1 >= arg2
   362	
   363	For simpler multi-way equality tests, eq (only) accepts two or more
   364	arguments and compares the second and subsequent to the first,
   365	returning in effect
   366	
   367		arg1==arg2 || arg1==arg3 || arg1==arg4 ...
   368	
   369	(Unlike with || in Go, however, eq is a function call and all the
   370	arguments will be evaluated.)
   371	
   372	The comparison functions work on basic types only (or named basic
   373	types, such as "type Celsius float32"). They implement the Go rules
   374	for comparison of values, except that size and exact type are
   375	ignored, so any integer value, signed or unsigned, may be compared
   376	with any other integer value. (The arithmetic value is compared,
   377	not the bit pattern, so all negative integers are less than all
   378	unsigned integers.) However, as usual, one may not compare an int
   379	with a float32 and so on.
   380	
   381	Associated templates
   382	
   383	Each template is named by a string specified when it is created. Also, each
   384	template is associated with zero or more other templates that it may invoke by
   385	name; such associations are transitive and form a name space of templates.
   386	
   387	A template may use a template invocation to instantiate another associated
   388	template; see the explanation of the "template" action above. The name must be
   389	that of a template associated with the template that contains the invocation.
   390	
   391	Nested template definitions
   392	
   393	When parsing a template, another template may be defined and associated with the
   394	template being parsed. Template definitions must appear at the top level of the
   395	template, much like global variables in a Go program.
   396	
   397	The syntax of such definitions is to surround each template declaration with a
   398	"define" and "end" action.
   399	
   400	The define action names the template being created by providing a string
   401	constant. Here is a simple example:
   402	
   403		`{{define "T1"}}ONE{{end}}
   404		{{define "T2"}}TWO{{end}}
   405		{{define "T3"}}{{template "T1"}} {{template "T2"}}{{end}}
   406		{{template "T3"}}`
   407	
   408	This defines two templates, T1 and T2, and a third T3 that invokes the other two
   409	when it is executed. Finally it invokes T3. If executed this template will
   410	produce the text
   411	
   412		ONE TWO
   413	
   414	By construction, a template may reside in only one association. If it's
   415	necessary to have a template addressable from multiple associations, the
   416	template definition must be parsed multiple times to create distinct *Template
   417	values, or must be copied with the Clone or AddParseTree method.
   418	
   419	Parse may be called multiple times to assemble the various associated templates;
   420	see the ParseFiles and ParseGlob functions and methods for simple ways to parse
   421	related templates stored in files.
   422	
   423	A template may be executed directly or through ExecuteTemplate, which executes
   424	an associated template identified by name. To invoke our example above, we
   425	might write,
   426	
   427		err := tmpl.Execute(os.Stdout, "no data needed")
   428		if err != nil {
   429			log.Fatalf("execution failed: %s", err)
   430		}
   431	
   432	or to invoke a particular template explicitly by name,
   433	
   434		err := tmpl.ExecuteTemplate(os.Stdout, "T2", "no data needed")
   435		if err != nil {
   436			log.Fatalf("execution failed: %s", err)
   437		}
   438	
   439	*/
   440	package template
   441	

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