Black Lives Matter. Support the Equal Justice Initiative.

Source file src/text/template/doc.go

Documentation: text/template

     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.
    21  Except for raw strings, actions may not span newlines, although comments can.
    23  Once parsed, a template may be executed safely in parallel, although if parallel
    24  executions share a Writer the output may be interleaved.
    26  Here is a trivial example that prints "17 items are made of wool".
    28  	type Inventory struct {
    29  		Material string
    30  		Count    uint
    31  	}
    32  	sweaters := Inventory{"wool", 17}
    33  	tmpl, err := template.New("test").Parse("{{.Count}} items are made of {{.Material}}")
    34  	if err != nil { panic(err) }
    35  	err = tmpl.Execute(os.Stdout, sweaters)
    36  	if err != nil { panic(err) }
    38  More intricate examples appear below.
    40  Text and spaces
    42  By default, all text between actions is copied verbatim when the template is
    43  executed. For example, the string " items are made of " in the example above appears
    44  on standard output when the program is run.
    46  However, to aid in formatting template source code, if an action's left delimiter
    47  (by default "{{") is followed immediately by a minus sign and ASCII space character
    48  ("{{- "), all trailing white space is trimmed from the immediately preceding text.
    49  Similarly, if the right delimiter ("}}") is preceded by a space and minus sign
    50  (" -}}"), all leading white space is trimmed from the immediately following text.
    51  In these trim markers, the ASCII space must be present; "{{-3}}" parses as an
    52  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  	{{template "name"}}
   115  		The template with the specified name is executed with nil data.
   117  	{{template "name" pipeline}}
   118  		The template with the specified name is executed with dot set
   119  		to the value of the pipeline.
   121  	{{block "name" pipeline}} T1 {{end}}
   122  		A block is shorthand for defining a template
   123  			{{define "name"}} T1 {{end}}
   124  		and then executing it in place
   125  			{{template "name" pipeline}}
   126  		The typical use is to define a set of root templates that are
   127  		then customized by redefining the block templates within.
   129  	{{with pipeline}} T1 {{end}}
   130  		If the value of the pipeline is empty, no output is generated;
   131  		otherwise, dot is set to the value of the pipeline and T1 is
   132  		executed.
   134  	{{with pipeline}} T1 {{else}} T0 {{end}}
   135  		If the value of the pipeline is empty, dot is unaffected and T0
   136  		is executed; otherwise, dot is set to the value of the pipeline
   137  		and T1 is executed.
   139  Arguments
   141  An argument is a simple value, denoted by one of the following.
   143  	- A boolean, string, character, integer, floating-point, imaginary
   144  	  or complex constant in Go syntax. These behave like Go's untyped
   145  	  constants. Note that, as in Go, whether a large integer constant
   146  	  overflows when assigned or passed to a function can depend on whether
   147  	  the host machine's ints are 32 or 64 bits.
   148  	- The keyword nil, representing an untyped Go nil.
   149  	- The character '.' (period):
   150  		.
   151  	  The result is the value of dot.
   152  	- A variable name, which is a (possibly empty) alphanumeric string
   153  	  preceded by a dollar sign, such as
   154  		$piOver2
   155  	  or
   156  		$
   157  	  The result is the value of the variable.
   158  	  Variables are described below.
   159  	- The name of a field of the data, which must be a struct, preceded
   160  	  by a period, such as
   161  		.Field
   162  	  The result is the value of the field. Field invocations may be
   163  	  chained:
   164  	    .Field1.Field2
   165  	  Fields can also be evaluated on variables, including chaining:
   166  	    $x.Field1.Field2
   167  	- The name of a key of the data, which must be a map, preceded
   168  	  by a period, such as
   169  		.Key
   170  	  The result is the map element value indexed by the key.
   171  	  Key invocations may be chained and combined with fields to any
   172  	  depth:
   173  	    .Field1.Key1.Field2.Key2
   174  	  Although the key must be an alphanumeric identifier, unlike with
   175  	  field names they do not need to start with an upper case letter.
   176  	  Keys can also be evaluated on variables, including chaining:
   177  	    $x.key1.key2
   178  	- The name of a niladic method of the data, preceded by a period,
   179  	  such as
   180  		.Method
   181  	  The result is the value of invoking the method with dot as the
   182  	  receiver, dot.Method(). Such a method must have one return value (of
   183  	  any type) or two return values, the second of which is an error.
   184  	  If it has two and the returned error is non-nil, execution terminates
   185  	  and an error is returned to the caller as the value of Execute.
   186  	  Method invocations may be chained and combined with fields and keys
   187  	  to any depth:
   188  	    .Field1.Key1.Method1.Field2.Key2.Method2
   189  	  Methods can also be evaluated on variables, including chaining:
   190  	    $x.Method1.Field
   191  	- The name of a niladic function, such as
   192  		fun
   193  	  The result is the value of invoking the function, fun(). The return
   194  	  types and values behave as in methods. Functions and function
   195  	  names are described below.
   196  	- A parenthesized instance of one the above, for grouping. The result
   197  	  may be accessed by a field or map key invocation.
   198  		print (.F1 arg1) (.F2 arg2)
   199  		(.StructValuedMethod "arg").Field
   201  Arguments may evaluate to any type; if they are pointers the implementation
   202  automatically indirects to the base type when required.
   203  If an evaluation yields a function value, such as a function-valued
   204  field of a struct, the function is not invoked automatically, but it
   205  can be used as a truth value for an if action and the like. To invoke
   206  it, use the call function, defined below.
   208  Pipelines
   210  A pipeline is a possibly chained sequence of "commands". A command is a simple
   211  value (argument) or a function or method call, possibly with multiple arguments:
   213  	Argument
   214  		The result is the value of evaluating the argument.
   215  	.Method [Argument...]
   216  		The method can be alone or the last element of a chain but,
   217  		unlike methods in the middle of a chain, it can take arguments.
   218  		The result is the value of calling the method with the
   219  		arguments:
   220  			dot.Method(Argument1, etc.)
   221  	functionName [Argument...]
   222  		The result is the value of calling the function associated
   223  		with the name:
   224  			function(Argument1, etc.)
   225  		Functions and function names are described below.
   227  A pipeline may be "chained" by separating a sequence of commands with pipeline
   228  characters '|'. In a chained pipeline, the result of each command is
   229  passed as the last argument of the following command. The output of the final
   230  command in the pipeline is the value of the pipeline.
   232  The output of a command will be either one value or two values, the second of
   233  which has type error. If that second value is present and evaluates to
   234  non-nil, execution terminates and the error is returned to the caller of
   235  Execute.
   237  Variables
   239  A pipeline inside an action may initialize a variable to capture the result.
   240  The initialization has syntax
   242  	$variable := pipeline
   244  where $variable is the name of the variable. An action that declares a
   245  variable produces no output.
   247  Variables previously declared can also be assigned, using the syntax
   249  	$variable = pipeline
   251  If a "range" action initializes a variable, the variable is set to the
   252  successive elements of the iteration. Also, a "range" may declare two
   253  variables, separated by a comma:
   255  	range $index, $element := pipeline
   257  in which case $index and $element are set to the successive values of the
   258  array/slice index or map key and element, respectively. Note that if there is
   259  only one variable, it is assigned the element; this is opposite to the
   260  convention in Go range clauses.
   262  A variable's scope extends to the "end" action of the control structure ("if",
   263  "with", or "range") in which it is declared, or to the end of the template if
   264  there is no such control structure. A template invocation does not inherit
   265  variables from the point of its invocation.
   267  When execution begins, $ is set to the data argument passed to Execute, that is,
   268  to the starting value of dot.
   270  Examples
   272  Here are some example one-line templates demonstrating pipelines and variables.
   273  All produce the quoted word "output":
   275  	{{"\"output\""}}
   276  		A string constant.
   277  	{{`"output"`}}
   278  		A raw string constant.
   279  	{{printf "%q" "output"}}
   280  		A function call.
   281  	{{"output" | printf "%q"}}
   282  		A function call whose final argument comes from the previous
   283  		command.
   284  	{{printf "%q" (print "out" "put")}}
   285  		A parenthesized argument.
   286  	{{"put" | printf "%s%s" "out" | printf "%q"}}
   287  		A more elaborate call.
   288  	{{"output" | printf "%s" | printf "%q"}}
   289  		A longer chain.
   290  	{{with "output"}}{{printf "%q" .}}{{end}}
   291  		A with action using dot.
   292  	{{with $x := "output" | printf "%q"}}{{$x}}{{end}}
   293  		A with action that creates and uses a variable.
   294  	{{with $x := "output"}}{{printf "%q" $x}}{{end}}
   295  		A with action that uses the variable in another action.
   296  	{{with $x := "output"}}{{$x | printf "%q"}}{{end}}
   297  		The same, but pipelined.
   299  Functions
   301  During execution functions are found in two function maps: first in the
   302  template, then in the global function map. By default, no functions are defined
   303  in the template but the Funcs method can be used to add them.
   305  Predefined global functions are named as follows.
   307  	and
   308  		Returns the boolean AND of its arguments by returning the
   309  		first empty argument or the last argument, that is,
   310  		"and x y" behaves as "if x then y else x". All the
   311  		arguments are evaluated.
   312  	call
   313  		Returns the result of calling the first argument, which
   314  		must be a function, with the remaining arguments as parameters.
   315  		Thus "call .X.Y 1 2" is, in Go notation, dot.X.Y(1, 2) where
   316  		Y is a func-valued field, map entry, or the like.
   317  		The first argument must be the result of an evaluation
   318  		that yields a value of function type (as distinct from
   319  		a predefined function such as print). The function must
   320  		return either one or two result values, the second of which
   321  		is of type error. If the arguments don't match the function
   322  		or the returned error value is non-nil, execution stops.
   323  	html
   324  		Returns the escaped HTML equivalent of the textual
   325  		representation of its arguments. This function is unavailable
   326  		in html/template, with a few exceptions.
   327  	index
   328  		Returns the result of indexing its first argument by the
   329  		following arguments. Thus "index x 1 2 3" is, in Go syntax,
   330  		x[1][2][3]. Each indexed item must be a map, slice, or array.
   331  	slice
   332  		slice returns the result of slicing its first argument by the
   333  		remaining arguments. Thus "slice x 1 2" is, in Go syntax, x[1:2],
   334  		while "slice x" is x[:], "slice x 1" is x[1:], and "slice x 1 2 3"
   335  		is x[1:2:3]. The first argument must be a string, slice, or array.
   336  	js
   337  		Returns the escaped JavaScript equivalent of the textual
   338  		representation of its arguments.
   339  	len
   340  		Returns the integer length of its argument.
   341  	not
   342  		Returns the boolean negation of its single argument.
   343  	or
   344  		Returns the boolean OR of its arguments by returning the
   345  		first non-empty argument or the last argument, that is,
   346  		"or x y" behaves as "if x then x else y". All the
   347  		arguments are evaluated.
   348  	print
   349  		An alias for fmt.Sprint
   350  	printf
   351  		An alias for fmt.Sprintf
   352  	println
   353  		An alias for fmt.Sprintln
   354  	urlquery
   355  		Returns the escaped value of the textual representation of
   356  		its arguments in a form suitable for embedding in a URL query.
   357  		This function is unavailable in html/template, with a few
   358  		exceptions.
   360  The boolean functions take any zero value to be false and a non-zero
   361  value to be true.
   363  There is also a set of binary comparison operators defined as
   364  functions:
   366  	eq
   367  		Returns the boolean truth of arg1 == arg2
   368  	ne
   369  		Returns the boolean truth of arg1 != arg2
   370  	lt
   371  		Returns the boolean truth of arg1 < arg2
   372  	le
   373  		Returns the boolean truth of arg1 <= arg2
   374  	gt
   375  		Returns the boolean truth of arg1 > arg2
   376  	ge
   377  		Returns the boolean truth of arg1 >= arg2
   379  For simpler multi-way equality tests, eq (only) accepts two or more
   380  arguments and compares the second and subsequent to the first,
   381  returning in effect
   383  	arg1==arg2 || arg1==arg3 || arg1==arg4 ...
   385  (Unlike with || in Go, however, eq is a function call and all the
   386  arguments will be evaluated.)
   388  The comparison functions work on any values whose type Go defines as
   389  comparable. For basic types such as integers, the rules are relaxed:
   390  size and exact type are ignored, so any integer value, signed or unsigned,
   391  may be compared with any other integer value. (The arithmetic value is compared,
   392  not the bit pattern, so all negative integers are less than all unsigned integers.)
   393  However, as usual, one may not compare an int with a float32 and so on.
   395  Associated templates
   397  Each template is named by a string specified when it is created. Also, each
   398  template is associated with zero or more other templates that it may invoke by
   399  name; such associations are transitive and form a name space of templates.
   401  A template may use a template invocation to instantiate another associated
   402  template; see the explanation of the "template" action above. The name must be
   403  that of a template associated with the template that contains the invocation.
   405  Nested template definitions
   407  When parsing a template, another template may be defined and associated with the
   408  template being parsed. Template definitions must appear at the top level of the
   409  template, much like global variables in a Go program.
   411  The syntax of such definitions is to surround each template declaration with a
   412  "define" and "end" action.
   414  The define action names the template being created by providing a string
   415  constant. Here is a simple example:
   417  	`{{define "T1"}}ONE{{end}}
   418  	{{define "T2"}}TWO{{end}}
   419  	{{define "T3"}}{{template "T1"}} {{template "T2"}}{{end}}
   420  	{{template "T3"}}`
   422  This defines two templates, T1 and T2, and a third T3 that invokes the other two
   423  when it is executed. Finally it invokes T3. If executed this template will
   424  produce the text
   426  	ONE TWO
   428  By construction, a template may reside in only one association. If it's
   429  necessary to have a template addressable from multiple associations, the
   430  template definition must be parsed multiple times to create distinct *Template
   431  values, or must be copied with the Clone or AddParseTree method.
   433  Parse may be called multiple times to assemble the various associated templates;
   434  see the ParseFiles and ParseGlob functions and methods for simple ways to parse
   435  related templates stored in files.
   437  A template may be executed directly or through ExecuteTemplate, which executes
   438  an associated template identified by name. To invoke our example above, we
   439  might write,
   441  	err := tmpl.Execute(os.Stdout, "no data needed")
   442  	if err != nil {
   443  		log.Fatalf("execution failed: %s", err)
   444  	}
   446  or to invoke a particular template explicitly by name,
   448  	err := tmpl.ExecuteTemplate(os.Stdout, "T2", "no data needed")
   449  	if err != nil {
   450  		log.Fatalf("execution failed: %s", err)
   451  	}
   453  */
   454  package template

View as plain text