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.
     5  /*
     6  Package template implements data-driven templates for generating textual output.
     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.
    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.
    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.
    22  Once parsed, a template may be executed safely in parallel, although if parallel
    23  executions share a Writer the output may be interleaved.
    25  Here is a trivial example that prints "17 items are made of wool".
    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) }
    37  More intricate examples appear below.
    39  Text and spaces
    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
    43  appears on standard output when the program is run.
    45  However, to aid in formatting template source code, if an action's left
    46  delimiter (by default "{{") is followed immediately by a minus sign and white
    47  space, all trailing white space is trimmed from the immediately preceding text.
    48  Similarly, if the right delimiter ("}}") is preceded by white space and a minus
    49  sign, all leading white space is trimmed from the immediately following text.
    50  In these trim markers, the white space must be present:
    51  "{{- 3}}" is like "{{3}}" but trims the immediately preceding text, while
    52  "{{-3}}" parses as an action containing the number -3.
    54  For instance, when executing the template whose source is
    56  	"{{23 -}} < {{- 45}}"
    58  the generated output would be
    60  	"23<45"
    62  For this trimming, the definition of white space characters is the same as in Go:
    63  space, horizontal tab, carriage return, and newline.
    65  Actions
    67  Here is the list of actions. "Arguments" and "pipelines" are evaluations of
    68  data, defined in detail in the corresponding sections that follow.
    70  */
    71  //	{{/* a comment */}}
    72  //	{{- /* a comment with white space trimmed from preceding and following text */ -}}
    73  //		A comment; discarded. May contain newlines.
    74  //		Comments do not nest and must start and end at the
    75  //		delimiters, as shown here.
    76  /*
    78  	{{pipeline}}
    79  		The default textual representation (the same as would be
    80  		printed by fmt.Print) of the value of the pipeline is copied
    81  		to the output.
    83  	{{if pipeline}} T1 {{end}}
    84  		If the value of the pipeline is empty, no output is generated;
    85  		otherwise, T1 is executed. The empty values are false, 0, any
    86  		nil pointer or interface value, and any array, slice, map, or
    87  		string of length zero.
    88  		Dot is unaffected.
    90  	{{if pipeline}} T1 {{else}} T0 {{end}}
    91  		If the value of the pipeline is empty, T0 is executed;
    92  		otherwise, T1 is executed. Dot is unaffected.
    94  	{{if pipeline}} T1 {{else if pipeline}} T0 {{end}}
    95  		To simplify the appearance of if-else chains, the else action
    96  		of an if may include another if directly; the effect is exactly
    97  		the same as writing
    98  			{{if pipeline}} T1 {{else}}{{if pipeline}} T0 {{end}}{{end}}
   100  	{{range pipeline}} T1 {{end}}
   101  		The value of the pipeline must be an array, slice, map, or channel.
   102  		If the value of the pipeline has length zero, nothing is output;
   103  		otherwise, dot is set to the successive elements of the array,
   104  		slice, or map and T1 is executed. If the value is a map and the
   105  		keys are of basic type with a defined order, the elements will be
   106  		visited in sorted key order.
   108  	{{range pipeline}} T1 {{else}} T0 {{end}}
   109  		The value of the pipeline must be an array, slice, map, or channel.
   110  		If the value of the pipeline has length zero, dot is unaffected and
   111  		T0 is executed; otherwise, dot is set to the successive elements
   112  		of the array, slice, or map and T1 is executed.
   114  	{{break}}
   115  		The innermost {{range pipeline}} loop is ended early, stopping the
   116  		current iteration and bypassing all remaining iterations.
   118  	{{continue}}
   119  		The current iteration of the innermost {{range pipeline}} loop is
   120  		stopped, and the loop starts the next iteration.
   122  	{{template "name"}}
   123  		The template with the specified name is executed with nil data.
   125  	{{template "name" pipeline}}
   126  		The template with the specified name is executed with dot set
   127  		to the value of the pipeline.
   129  	{{block "name" pipeline}} T1 {{end}}
   130  		A block is shorthand for defining a template
   131  			{{define "name"}} T1 {{end}}
   132  		and then executing it in place
   133  			{{template "name" pipeline}}
   134  		The typical use is to define a set of root templates that are
   135  		then customized by redefining the block templates within.
   137  	{{with pipeline}} T1 {{end}}
   138  		If the value of the pipeline is empty, no output is generated;
   139  		otherwise, dot is set to the value of the pipeline and T1 is
   140  		executed.
   142  	{{with pipeline}} T1 {{else}} T0 {{end}}
   143  		If the value of the pipeline is empty, dot is unaffected and T0
   144  		is executed; otherwise, dot is set to the value of the pipeline
   145  		and T1 is executed.
   147  Arguments
   149  An argument is a simple value, denoted by one of the following.
   151  	- A boolean, string, character, integer, floating-point, imaginary
   152  	  or complex constant in Go syntax. These behave like Go's untyped
   153  	  constants. Note that, as in Go, whether a large integer constant
   154  	  overflows when assigned or passed to a function can depend on whether
   155  	  the host machine's ints are 32 or 64 bits.
   156  	- The keyword nil, representing an untyped Go nil.
   157  	- The character '.' (period):
   158  		.
   159  	  The result is the value of dot.
   160  	- A variable name, which is a (possibly empty) alphanumeric string
   161  	  preceded by a dollar sign, such as
   162  		$piOver2
   163  	  or
   164  		$
   165  	  The result is the value of the variable.
   166  	  Variables are described below.
   167  	- The name of a field of the data, which must be a struct, preceded
   168  	  by a period, such as
   169  		.Field
   170  	  The result is the value of the field. Field invocations may be
   171  	  chained:
   172  	    .Field1.Field2
   173  	  Fields can also be evaluated on variables, including chaining:
   174  	    $x.Field1.Field2
   175  	- The name of a key of the data, which must be a map, preceded
   176  	  by a period, such as
   177  		.Key
   178  	  The result is the map element value indexed by the key.
   179  	  Key invocations may be chained and combined with fields to any
   180  	  depth:
   181  	    .Field1.Key1.Field2.Key2
   182  	  Although the key must be an alphanumeric identifier, unlike with
   183  	  field names they do not need to start with an upper case letter.
   184  	  Keys can also be evaluated on variables, including chaining:
   185  	    $x.key1.key2
   186  	- The name of a niladic method of the data, preceded by a period,
   187  	  such as
   188  		.Method
   189  	  The result is the value of invoking the method with dot as the
   190  	  receiver, dot.Method(). Such a method must have one return value (of
   191  	  any type) or two return values, the second of which is an error.
   192  	  If it has two and the returned error is non-nil, execution terminates
   193  	  and an error is returned to the caller as the value of Execute.
   194  	  Method invocations may be chained and combined with fields and keys
   195  	  to any depth:
   196  	    .Field1.Key1.Method1.Field2.Key2.Method2
   197  	  Methods can also be evaluated on variables, including chaining:
   198  	    $x.Method1.Field
   199  	- The name of a niladic function, such as
   200  		fun
   201  	  The result is the value of invoking the function, fun(). The return
   202  	  types and values behave as in methods. Functions and function
   203  	  names are described below.
   204  	- A parenthesized instance of one the above, for grouping. The result
   205  	  may be accessed by a field or map key invocation.
   206  		print (.F1 arg1) (.F2 arg2)
   207  		(.StructValuedMethod "arg").Field
   209  Arguments may evaluate to any type; if they are pointers the implementation
   210  automatically indirects to the base type when required.
   211  If an evaluation yields a function value, such as a function-valued
   212  field of a struct, the function is not invoked automatically, but it
   213  can be used as a truth value for an if action and the like. To invoke
   214  it, use the call function, defined below.
   216  Pipelines
   218  A pipeline is a possibly chained sequence of "commands". A command is a simple
   219  value (argument) or a function or method call, possibly with multiple arguments:
   221  	Argument
   222  		The result is the value of evaluating the argument.
   223  	.Method [Argument...]
   224  		The method can be alone or the last element of a chain but,
   225  		unlike methods in the middle of a chain, it can take arguments.
   226  		The result is the value of calling the method with the
   227  		arguments:
   228  			dot.Method(Argument1, etc.)
   229  	functionName [Argument...]
   230  		The result is the value of calling the function associated
   231  		with the name:
   232  			function(Argument1, etc.)
   233  		Functions and function names are described below.
   235  A pipeline may be "chained" by separating a sequence of commands with pipeline
   236  characters '|'. In a chained pipeline, the result of each command is
   237  passed as the last argument of the following command. The output of the final
   238  command in the pipeline is the value of the pipeline.
   240  The output of a command will be either one value or two values, the second of
   241  which has type error. If that second value is present and evaluates to
   242  non-nil, execution terminates and the error is returned to the caller of
   243  Execute.
   245  Variables
   247  A pipeline inside an action may initialize a variable to capture the result.
   248  The initialization has syntax
   250  	$variable := pipeline
   252  where $variable is the name of the variable. An action that declares a
   253  variable produces no output.
   255  Variables previously declared can also be assigned, using the syntax
   257  	$variable = pipeline
   259  If a "range" action initializes a variable, the variable is set to the
   260  successive elements of the iteration. Also, a "range" may declare two
   261  variables, separated by a comma:
   263  	range $index, $element := pipeline
   265  in which case $index and $element are set to the successive values of the
   266  array/slice index or map key and element, respectively. Note that if there is
   267  only one variable, it is assigned the element; this is opposite to the
   268  convention in Go range clauses.
   270  A variable's scope extends to the "end" action of the control structure ("if",
   271  "with", or "range") in which it is declared, or to the end of the template if
   272  there is no such control structure. A template invocation does not inherit
   273  variables from the point of its invocation.
   275  When execution begins, $ is set to the data argument passed to Execute, that is,
   276  to the starting value of dot.
   278  Examples
   280  Here are some example one-line templates demonstrating pipelines and variables.
   281  All produce the quoted word "output":
   283  	{{"\"output\""}}
   284  		A string constant.
   285  	{{`"output"`}}
   286  		A raw string constant.
   287  	{{printf "%q" "output"}}
   288  		A function call.
   289  	{{"output" | printf "%q"}}
   290  		A function call whose final argument comes from the previous
   291  		command.
   292  	{{printf "%q" (print "out" "put")}}
   293  		A parenthesized argument.
   294  	{{"put" | printf "%s%s" "out" | printf "%q"}}
   295  		A more elaborate call.
   296  	{{"output" | printf "%s" | printf "%q"}}
   297  		A longer chain.
   298  	{{with "output"}}{{printf "%q" .}}{{end}}
   299  		A with action using dot.
   300  	{{with $x := "output" | printf "%q"}}{{$x}}{{end}}
   301  		A with action that creates and uses a variable.
   302  	{{with $x := "output"}}{{printf "%q" $x}}{{end}}
   303  		A with action that uses the variable in another action.
   304  	{{with $x := "output"}}{{$x | printf "%q"}}{{end}}
   305  		The same, but pipelined.
   307  Functions
   309  During execution functions are found in two function maps: first in the
   310  template, then in the global function map. By default, no functions are defined
   311  in the template but the Funcs method can be used to add them.
   313  Predefined global functions are named as follows.
   315  	and
   316  		Returns the boolean AND of its arguments by returning the
   317  		first empty argument or the last argument. That is,
   318  		"and x y" behaves as "if x then y else x."
   319  		Evaluation proceeds through the arguments left to right
   320  		and returns when the result is determined.
   321  	call
   322  		Returns the result of calling the first argument, which
   323  		must be a function, with the remaining arguments as parameters.
   324  		Thus "call .X.Y 1 2" is, in Go notation, dot.X.Y(1, 2) where
   325  		Y is a func-valued field, map entry, or the like.
   326  		The first argument must be the result of an evaluation
   327  		that yields a value of function type (as distinct from
   328  		a predefined function such as print). The function must
   329  		return either one or two result values, the second of which
   330  		is of type error. If the arguments don't match the function
   331  		or the returned error value is non-nil, execution stops.
   332  	html
   333  		Returns the escaped HTML equivalent of the textual
   334  		representation of its arguments. This function is unavailable
   335  		in html/template, with a few exceptions.
   336  	index
   337  		Returns the result of indexing its first argument by the
   338  		following arguments. Thus "index x 1 2 3" is, in Go syntax,
   339  		x[1][2][3]. Each indexed item must be a map, slice, or array.
   340  	slice
   341  		slice returns the result of slicing its first argument by the
   342  		remaining arguments. Thus "slice x 1 2" is, in Go syntax, x[1:2],
   343  		while "slice x" is x[:], "slice x 1" is x[1:], and "slice x 1 2 3"
   344  		is x[1:2:3]. The first argument must be a string, slice, or array.
   345  	js
   346  		Returns the escaped JavaScript equivalent of the textual
   347  		representation of its arguments.
   348  	len
   349  		Returns the integer length of its argument.
   350  	not
   351  		Returns the boolean negation of its single argument.
   352  	or
   353  		Returns the boolean OR of its arguments by returning the
   354  		first non-empty argument or the last argument, that is,
   355  		"or x y" behaves as "if x then x else y".
   356  		Evaluation proceeds through the arguments left to right
   357  		and returns when the result is determined.
   358  	print
   359  		An alias for fmt.Sprint
   360  	printf
   361  		An alias for fmt.Sprintf
   362  	println
   363  		An alias for fmt.Sprintln
   364  	urlquery
   365  		Returns the escaped value of the textual representation of
   366  		its arguments in a form suitable for embedding in a URL query.
   367  		This function is unavailable in html/template, with a few
   368  		exceptions.
   370  The boolean functions take any zero value to be false and a non-zero
   371  value to be true.
   373  There is also a set of binary comparison operators defined as
   374  functions:
   376  	eq
   377  		Returns the boolean truth of arg1 == arg2
   378  	ne
   379  		Returns the boolean truth of arg1 != arg2
   380  	lt
   381  		Returns the boolean truth of arg1 < arg2
   382  	le
   383  		Returns the boolean truth of arg1 <= arg2
   384  	gt
   385  		Returns the boolean truth of arg1 > arg2
   386  	ge
   387  		Returns the boolean truth of arg1 >= arg2
   389  For simpler multi-way equality tests, eq (only) accepts two or more
   390  arguments and compares the second and subsequent to the first,
   391  returning in effect
   393  	arg1==arg2 || arg1==arg3 || arg1==arg4 ...
   395  (Unlike with || in Go, however, eq is a function call and all the
   396  arguments will be evaluated.)
   398  The comparison functions work on any values whose type Go defines as
   399  comparable. For basic types such as integers, the rules are relaxed:
   400  size and exact type are ignored, so any integer value, signed or unsigned,
   401  may be compared with any other integer value. (The arithmetic value is compared,
   402  not the bit pattern, so all negative integers are less than all unsigned integers.)
   403  However, as usual, one may not compare an int with a float32 and so on.
   405  Associated templates
   407  Each template is named by a string specified when it is created. Also, each
   408  template is associated with zero or more other templates that it may invoke by
   409  name; such associations are transitive and form a name space of templates.
   411  A template may use a template invocation to instantiate another associated
   412  template; see the explanation of the "template" action above. The name must be
   413  that of a template associated with the template that contains the invocation.
   415  Nested template definitions
   417  When parsing a template, another template may be defined and associated with the
   418  template being parsed. Template definitions must appear at the top level of the
   419  template, much like global variables in a Go program.
   421  The syntax of such definitions is to surround each template declaration with a
   422  "define" and "end" action.
   424  The define action names the template being created by providing a string
   425  constant. Here is a simple example:
   427  	{{define "T1"}}ONE{{end}}
   428  	{{define "T2"}}TWO{{end}}
   429  	{{define "T3"}}{{template "T1"}} {{template "T2"}}{{end}}
   430  	{{template "T3"}}
   432  This defines two templates, T1 and T2, and a third T3 that invokes the other two
   433  when it is executed. Finally it invokes T3. If executed this template will
   434  produce the text
   436  	ONE TWO
   438  By construction, a template may reside in only one association. If it's
   439  necessary to have a template addressable from multiple associations, the
   440  template definition must be parsed multiple times to create distinct *Template
   441  values, or must be copied with the Clone or AddParseTree method.
   443  Parse may be called multiple times to assemble the various associated templates;
   444  see the ParseFiles and ParseGlob functions and methods for simple ways to parse
   445  related templates stored in files.
   447  A template may be executed directly or through ExecuteTemplate, which executes
   448  an associated template identified by name. To invoke our example above, we
   449  might write,
   451  	err := tmpl.Execute(os.Stdout, "no data needed")
   452  	if err != nil {
   453  		log.Fatalf("execution failed: %s", err)
   454  	}
   456  or to invoke a particular template explicitly by name,
   458  	err := tmpl.ExecuteTemplate(os.Stdout, "T2", "no data needed")
   459  	if err != nil {
   460  		log.Fatalf("execution failed: %s", err)
   461  	}
   463  */
   464  package template

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