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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
    44  appears on standard output when the program is run.
    46  However, to aid in formatting template source code, if an action's left
    47  delimiter (by default "{{") is followed immediately by a minus sign and white
    48  space, all trailing white space is trimmed from the immediately preceding text.
    49  Similarly, if the right delimiter ("}}") is preceded by white space and a minus
    50  sign, all leading white space is trimmed from the immediately following text.
    51  In these trim markers, the white space must be present:
    52  "{{- 3}}" is like "{{3}}" but trims the immediately preceding text, while
    53  "{{-3}}" parses as an action containing the number -3.
    55  For instance, when executing the template whose source is
    57  	"{{23 -}} < {{- 45}}"
    59  the generated output would be
    61  	"23<45"
    63  For this trimming, the definition of white space characters is the same as in Go:
    64  space, horizontal tab, carriage return, and newline.
    66  Actions
    68  Here is the list of actions. "Arguments" and "pipelines" are evaluations of
    69  data, defined in detail in the corresponding sections that follow.
    71  */
    72  //	{{/* a comment */}}
    73  //	{{- /* a comment with white space trimmed from preceding and following text */ -}}
    74  //		A comment; discarded. May contain newlines.
    75  //		Comments do not nest and must start and end at the
    76  //		delimiters, as shown here.
    77  /*
    79  	{{pipeline}}
    80  		The default textual representation (the same as would be
    81  		printed by fmt.Print) of the value of the pipeline is copied
    82  		to the output.
    84  	{{if pipeline}} T1 {{end}}
    85  		If the value of the pipeline is empty, no output is generated;
    86  		otherwise, T1 is executed. The empty values are false, 0, any
    87  		nil pointer or interface value, and any array, slice, map, or
    88  		string of length zero.
    89  		Dot is unaffected.
    91  	{{if pipeline}} T1 {{else}} T0 {{end}}
    92  		If the value of the pipeline is empty, T0 is executed;
    93  		otherwise, T1 is executed. Dot is unaffected.
    95  	{{if pipeline}} T1 {{else if pipeline}} T0 {{end}}
    96  		To simplify the appearance of if-else chains, the else action
    97  		of an if may include another if directly; the effect is exactly
    98  		the same as writing
    99  			{{if pipeline}} T1 {{else}}{{if pipeline}} T0 {{end}}{{end}}
   101  	{{range pipeline}} T1 {{end}}
   102  		The value of the pipeline must be an array, slice, map, or channel.
   103  		If the value of the pipeline has length zero, nothing is output;
   104  		otherwise, dot is set to the successive elements of the array,
   105  		slice, or map and T1 is executed. If the value is a map and the
   106  		keys are of basic type with a defined order, the elements will be
   107  		visited in sorted key order.
   109  	{{range pipeline}} T1 {{else}} T0 {{end}}
   110  		The value of the pipeline must be an array, slice, map, or channel.
   111  		If the value of the pipeline has length zero, dot is unaffected and
   112  		T0 is executed; otherwise, dot is set to the successive elements
   113  		of the array, slice, or map and T1 is executed.
   115  	{{template "name"}}
   116  		The template with the specified name is executed with nil data.
   118  	{{template "name" pipeline}}
   119  		The template with the specified name is executed with dot set
   120  		to the value of the pipeline.
   122  	{{block "name" pipeline}} T1 {{end}}
   123  		A block is shorthand for defining a template
   124  			{{define "name"}} T1 {{end}}
   125  		and then executing it in place
   126  			{{template "name" pipeline}}
   127  		The typical use is to define a set of root templates that are
   128  		then customized by redefining the block templates within.
   130  	{{with pipeline}} T1 {{end}}
   131  		If the value of the pipeline is empty, no output is generated;
   132  		otherwise, dot is set to the value of the pipeline and T1 is
   133  		executed.
   135  	{{with pipeline}} T1 {{else}} T0 {{end}}
   136  		If the value of the pipeline is empty, dot is unaffected and T0
   137  		is executed; otherwise, dot is set to the value of the pipeline
   138  		and T1 is executed.
   140  Arguments
   142  An argument is a simple value, denoted by one of the following.
   144  	- A boolean, string, character, integer, floating-point, imaginary
   145  	  or complex constant in Go syntax. These behave like Go's untyped
   146  	  constants. Note that, as in Go, whether a large integer constant
   147  	  overflows when assigned or passed to a function can depend on whether
   148  	  the host machine's ints are 32 or 64 bits.
   149  	- The keyword nil, representing an untyped Go nil.
   150  	- The character '.' (period):
   151  		.
   152  	  The result is the value of dot.
   153  	- A variable name, which is a (possibly empty) alphanumeric string
   154  	  preceded by a dollar sign, such as
   155  		$piOver2
   156  	  or
   157  		$
   158  	  The result is the value of the variable.
   159  	  Variables are described below.
   160  	- The name of a field of the data, which must be a struct, preceded
   161  	  by a period, such as
   162  		.Field
   163  	  The result is the value of the field. Field invocations may be
   164  	  chained:
   165  	    .Field1.Field2
   166  	  Fields can also be evaluated on variables, including chaining:
   167  	    $x.Field1.Field2
   168  	- The name of a key of the data, which must be a map, preceded
   169  	  by a period, such as
   170  		.Key
   171  	  The result is the map element value indexed by the key.
   172  	  Key invocations may be chained and combined with fields to any
   173  	  depth:
   174  	    .Field1.Key1.Field2.Key2
   175  	  Although the key must be an alphanumeric identifier, unlike with
   176  	  field names they do not need to start with an upper case letter.
   177  	  Keys can also be evaluated on variables, including chaining:
   178  	    $x.key1.key2
   179  	- The name of a niladic method of the data, preceded by a period,
   180  	  such as
   181  		.Method
   182  	  The result is the value of invoking the method with dot as the
   183  	  receiver, dot.Method(). Such a method must have one return value (of
   184  	  any type) or two return values, the second of which is an error.
   185  	  If it has two and the returned error is non-nil, execution terminates
   186  	  and an error is returned to the caller as the value of Execute.
   187  	  Method invocations may be chained and combined with fields and keys
   188  	  to any depth:
   189  	    .Field1.Key1.Method1.Field2.Key2.Method2
   190  	  Methods can also be evaluated on variables, including chaining:
   191  	    $x.Method1.Field
   192  	- The name of a niladic function, such as
   193  		fun
   194  	  The result is the value of invoking the function, fun(). The return
   195  	  types and values behave as in methods. Functions and function
   196  	  names are described below.
   197  	- A parenthesized instance of one the above, for grouping. The result
   198  	  may be accessed by a field or map key invocation.
   199  		print (.F1 arg1) (.F2 arg2)
   200  		(.StructValuedMethod "arg").Field
   202  Arguments may evaluate to any type; if they are pointers the implementation
   203  automatically indirects to the base type when required.
   204  If an evaluation yields a function value, such as a function-valued
   205  field of a struct, the function is not invoked automatically, but it
   206  can be used as a truth value for an if action and the like. To invoke
   207  it, use the call function, defined below.
   209  Pipelines
   211  A pipeline is a possibly chained sequence of "commands". A command is a simple
   212  value (argument) or a function or method call, possibly with multiple arguments:
   214  	Argument
   215  		The result is the value of evaluating the argument.
   216  	.Method [Argument...]
   217  		The method can be alone or the last element of a chain but,
   218  		unlike methods in the middle of a chain, it can take arguments.
   219  		The result is the value of calling the method with the
   220  		arguments:
   221  			dot.Method(Argument1, etc.)
   222  	functionName [Argument...]
   223  		The result is the value of calling the function associated
   224  		with the name:
   225  			function(Argument1, etc.)
   226  		Functions and function names are described below.
   228  A pipeline may be "chained" by separating a sequence of commands with pipeline
   229  characters '|'. In a chained pipeline, the result of each command is
   230  passed as the last argument of the following command. The output of the final
   231  command in the pipeline is the value of the pipeline.
   233  The output of a command will be either one value or two values, the second of
   234  which has type error. If that second value is present and evaluates to
   235  non-nil, execution terminates and the error is returned to the caller of
   236  Execute.
   238  Variables
   240  A pipeline inside an action may initialize a variable to capture the result.
   241  The initialization has syntax
   243  	$variable := pipeline
   245  where $variable is the name of the variable. An action that declares a
   246  variable produces no output.
   248  Variables previously declared can also be assigned, using the syntax
   250  	$variable = pipeline
   252  If a "range" action initializes a variable, the variable is set to the
   253  successive elements of the iteration. Also, a "range" may declare two
   254  variables, separated by a comma:
   256  	range $index, $element := pipeline
   258  in which case $index and $element are set to the successive values of the
   259  array/slice index or map key and element, respectively. Note that if there is
   260  only one variable, it is assigned the element; this is opposite to the
   261  convention in Go range clauses.
   263  A variable's scope extends to the "end" action of the control structure ("if",
   264  "with", or "range") in which it is declared, or to the end of the template if
   265  there is no such control structure. A template invocation does not inherit
   266  variables from the point of its invocation.
   268  When execution begins, $ is set to the data argument passed to Execute, that is,
   269  to the starting value of dot.
   271  Examples
   273  Here are some example one-line templates demonstrating pipelines and variables.
   274  All produce the quoted word "output":
   276  	{{"\"output\""}}
   277  		A string constant.
   278  	{{`"output"`}}
   279  		A raw string constant.
   280  	{{printf "%q" "output"}}
   281  		A function call.
   282  	{{"output" | printf "%q"}}
   283  		A function call whose final argument comes from the previous
   284  		command.
   285  	{{printf "%q" (print "out" "put")}}
   286  		A parenthesized argument.
   287  	{{"put" | printf "%s%s" "out" | printf "%q"}}
   288  		A more elaborate call.
   289  	{{"output" | printf "%s" | printf "%q"}}
   290  		A longer chain.
   291  	{{with "output"}}{{printf "%q" .}}{{end}}
   292  		A with action using dot.
   293  	{{with $x := "output" | printf "%q"}}{{$x}}{{end}}
   294  		A with action that creates and uses a variable.
   295  	{{with $x := "output"}}{{printf "%q" $x}}{{end}}
   296  		A with action that uses the variable in another action.
   297  	{{with $x := "output"}}{{$x | printf "%q"}}{{end}}
   298  		The same, but pipelined.
   300  Functions
   302  During execution functions are found in two function maps: first in the
   303  template, then in the global function map. By default, no functions are defined
   304  in the template but the Funcs method can be used to add them.
   306  Predefined global functions are named as follows.
   308  	and
   309  		Returns the boolean AND of its arguments by returning the
   310  		first empty argument or the last argument, that is,
   311  		"and x y" behaves as "if x then y else x". All the
   312  		arguments are evaluated.
   313  	call
   314  		Returns the result of calling the first argument, which
   315  		must be a function, with the remaining arguments as parameters.
   316  		Thus "call .X.Y 1 2" is, in Go notation, dot.X.Y(1, 2) where
   317  		Y is a func-valued field, map entry, or the like.
   318  		The first argument must be the result of an evaluation
   319  		that yields a value of function type (as distinct from
   320  		a predefined function such as print). The function must
   321  		return either one or two result values, the second of which
   322  		is of type error. If the arguments don't match the function
   323  		or the returned error value is non-nil, execution stops.
   324  	html
   325  		Returns the escaped HTML equivalent of the textual
   326  		representation of its arguments. This function is unavailable
   327  		in html/template, with a few exceptions.
   328  	index
   329  		Returns the result of indexing its first argument by the
   330  		following arguments. Thus "index x 1 2 3" is, in Go syntax,
   331  		x[1][2][3]. Each indexed item must be a map, slice, or array.
   332  	slice
   333  		slice returns the result of slicing its first argument by the
   334  		remaining arguments. Thus "slice x 1 2" is, in Go syntax, x[1:2],
   335  		while "slice x" is x[:], "slice x 1" is x[1:], and "slice x 1 2 3"
   336  		is x[1:2:3]. The first argument must be a string, slice, or array.
   337  	js
   338  		Returns the escaped JavaScript equivalent of the textual
   339  		representation of its arguments.
   340  	len
   341  		Returns the integer length of its argument.
   342  	not
   343  		Returns the boolean negation of its single argument.
   344  	or
   345  		Returns the boolean OR of its arguments by returning the
   346  		first non-empty argument or the last argument, that is,
   347  		"or x y" behaves as "if x then x else y". All the
   348  		arguments are evaluated.
   349  	print
   350  		An alias for fmt.Sprint
   351  	printf
   352  		An alias for fmt.Sprintf
   353  	println
   354  		An alias for fmt.Sprintln
   355  	urlquery
   356  		Returns the escaped value of the textual representation of
   357  		its arguments in a form suitable for embedding in a URL query.
   358  		This function is unavailable in html/template, with a few
   359  		exceptions.
   361  The boolean functions take any zero value to be false and a non-zero
   362  value to be true.
   364  There is also a set of binary comparison operators defined as
   365  functions:
   367  	eq
   368  		Returns the boolean truth of arg1 == arg2
   369  	ne
   370  		Returns the boolean truth of arg1 != arg2
   371  	lt
   372  		Returns the boolean truth of arg1 < arg2
   373  	le
   374  		Returns the boolean truth of arg1 <= arg2
   375  	gt
   376  		Returns the boolean truth of arg1 > arg2
   377  	ge
   378  		Returns the boolean truth of arg1 >= arg2
   380  For simpler multi-way equality tests, eq (only) accepts two or more
   381  arguments and compares the second and subsequent to the first,
   382  returning in effect
   384  	arg1==arg2 || arg1==arg3 || arg1==arg4 ...
   386  (Unlike with || in Go, however, eq is a function call and all the
   387  arguments will be evaluated.)
   389  The comparison functions work on any values whose type Go defines as
   390  comparable. For basic types such as integers, the rules are relaxed:
   391  size and exact type are ignored, so any integer value, signed or unsigned,
   392  may be compared with any other integer value. (The arithmetic value is compared,
   393  not the bit pattern, so all negative integers are less than all unsigned integers.)
   394  However, as usual, one may not compare an int with a float32 and so on.
   396  Associated templates
   398  Each template is named by a string specified when it is created. Also, each
   399  template is associated with zero or more other templates that it may invoke by
   400  name; such associations are transitive and form a name space of templates.
   402  A template may use a template invocation to instantiate another associated
   403  template; see the explanation of the "template" action above. The name must be
   404  that of a template associated with the template that contains the invocation.
   406  Nested template definitions
   408  When parsing a template, another template may be defined and associated with the
   409  template being parsed. Template definitions must appear at the top level of the
   410  template, much like global variables in a Go program.
   412  The syntax of such definitions is to surround each template declaration with a
   413  "define" and "end" action.
   415  The define action names the template being created by providing a string
   416  constant. Here is a simple example:
   418  	`{{define "T1"}}ONE{{end}}
   419  	{{define "T2"}}TWO{{end}}
   420  	{{define "T3"}}{{template "T1"}} {{template "T2"}}{{end}}
   421  	{{template "T3"}}`
   423  This defines two templates, T1 and T2, and a third T3 that invokes the other two
   424  when it is executed. Finally it invokes T3. If executed this template will
   425  produce the text
   427  	ONE TWO
   429  By construction, a template may reside in only one association. If it's
   430  necessary to have a template addressable from multiple associations, the
   431  template definition must be parsed multiple times to create distinct *Template
   432  values, or must be copied with the Clone or AddParseTree method.
   434  Parse may be called multiple times to assemble the various associated templates;
   435  see the ParseFiles and ParseGlob functions and methods for simple ways to parse
   436  related templates stored in files.
   438  A template may be executed directly or through ExecuteTemplate, which executes
   439  an associated template identified by name. To invoke our example above, we
   440  might write,
   442  	err := tmpl.Execute(os.Stdout, "no data needed")
   443  	if err != nil {
   444  		log.Fatalf("execution failed: %s", err)
   445  	}
   447  or to invoke a particular template explicitly by name,
   449  	err := tmpl.ExecuteTemplate(os.Stdout, "T2", "no data needed")
   450  	if err != nil {
   451  		log.Fatalf("execution failed: %s", err)
   452  	}
   454  */
   455  package template

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