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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.
     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, although if parallel
    24  executions share a Writer the output may be interleaved.
    25  
    26  Here is a trivial example that prints "17 items are made of wool".
    27  
    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) }
    37  
    38  More intricate examples appear below.
    39  
    40  Text and spaces
    41  
    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.
    45  
    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.
    53  
    54  For instance, when executing the template whose source is
    55  
    56  	"{{23 -}} < {{- 45}}"
    57  
    58  the generated output would be
    59  
    60  	"23<45"
    61  
    62  For this trimming, the definition of white space characters is the same as in Go:
    63  space, horizontal tab, carriage return, and newline.
    64  
    65  Actions
    66  
    67  Here is the list of actions. "Arguments" and "pipelines" are evaluations of
    68  data, defined in detail in the corresponding sections that follow.
    69  
    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  /*
    77  
    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.
    82  
    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.
    89  
    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.
    93  
    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}}
    99  
   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 ("comparable"), the
   106  		elements will be visited in sorted key order.
   107  
   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.
   113  
   114  	{{template "name"}}
   115  		The template with the specified name is executed with nil data.
   116  
   117  	{{template "name" pipeline}}
   118  		The template with the specified name is executed with dot set
   119  		to the value of the pipeline.
   120  
   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.
   128  
   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.
   133  
   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.
   138  
   139  Arguments
   140  
   141  An argument is a simple value, denoted by one of the following.
   142  
   143  	- A boolean, string, character, integer, floating-point, imaginary
   144  	  or complex constant in Go syntax. These behave like Go's untyped
   145  	  constants.
   146  	- The keyword nil, representing an untyped Go nil.
   147  	- The character '.' (period):
   148  		.
   149  	  The result is the value of dot.
   150  	- A variable name, which is a (possibly empty) alphanumeric string
   151  	  preceded by a dollar sign, such as
   152  		$piOver2
   153  	  or
   154  		$
   155  	  The result is the value of the variable.
   156  	  Variables are described below.
   157  	- The name of a field of the data, which must be a struct, preceded
   158  	  by a period, such as
   159  		.Field
   160  	  The result is the value of the field. Field invocations may be
   161  	  chained:
   162  	    .Field1.Field2
   163  	  Fields can also be evaluated on variables, including chaining:
   164  	    $x.Field1.Field2
   165  	- The name of a key of the data, which must be a map, preceded
   166  	  by a period, such as
   167  		.Key
   168  	  The result is the map element value indexed by the key.
   169  	  Key invocations may be chained and combined with fields to any
   170  	  depth:
   171  	    .Field1.Key1.Field2.Key2
   172  	  Although the key must be an alphanumeric identifier, unlike with
   173  	  field names they do not need to start with an upper case letter.
   174  	  Keys can also be evaluated on variables, including chaining:
   175  	    $x.key1.key2
   176  	- The name of a niladic method of the data, preceded by a period,
   177  	  such as
   178  		.Method
   179  	  The result is the value of invoking the method with dot as the
   180  	  receiver, dot.Method(). Such a method must have one return value (of
   181  	  any type) or two return values, the second of which is an error.
   182  	  If it has two and the returned error is non-nil, execution terminates
   183  	  and an error is returned to the caller as the value of Execute.
   184  	  Method invocations may be chained and combined with fields and keys
   185  	  to any depth:
   186  	    .Field1.Key1.Method1.Field2.Key2.Method2
   187  	  Methods can also be evaluated on variables, including chaining:
   188  	    $x.Method1.Field
   189  	- The name of a niladic function, such as
   190  		fun
   191  	  The result is the value of invoking the function, fun(). The return
   192  	  types and values behave as in methods. Functions and function
   193  	  names are described below.
   194  	- A parenthesized instance of one the above, for grouping. The result
   195  	  may be accessed by a field or map key invocation.
   196  		print (.F1 arg1) (.F2 arg2)
   197  		(.StructValuedMethod "arg").Field
   198  
   199  Arguments may evaluate to any type; if they are pointers the implementation
   200  automatically indirects to the base type when required.
   201  If an evaluation yields a function value, such as a function-valued
   202  field of a struct, the function is not invoked automatically, but it
   203  can be used as a truth value for an if action and the like. To invoke
   204  it, use the call function, defined below.
   205  
   206  Pipelines
   207  
   208  A pipeline is a possibly chained sequence of "commands". A command is a simple
   209  value (argument) or a function or method call, possibly with multiple arguments:
   210  
   211  	Argument
   212  		The result is the value of evaluating the argument.
   213  	.Method [Argument...]
   214  		The method can be alone or the last element of a chain but,
   215  		unlike methods in the middle of a chain, it can take arguments.
   216  		The result is the value of calling the method with the
   217  		arguments:
   218  			dot.Method(Argument1, etc.)
   219  	functionName [Argument...]
   220  		The result is the value of calling the function associated
   221  		with the name:
   222  			function(Argument1, etc.)
   223  		Functions and function names are described below.
   224  
   225  A pipeline may be "chained" by separating a sequence of commands with pipeline
   226  characters '|'. In a chained pipeline, the result of each command is
   227  passed as the last argument of the following command. The output of the final
   228  command in the pipeline is the value of the pipeline.
   229  
   230  The output of a command will be either one value or two values, the second of
   231  which has type error. If that second value is present and evaluates to
   232  non-nil, execution terminates and the error is returned to the caller of
   233  Execute.
   234  
   235  Variables
   236  
   237  A pipeline inside an action may initialize a variable to capture the result.
   238  The initialization has syntax
   239  
   240  	$variable := pipeline
   241  
   242  where $variable is the name of the variable. An action that declares a
   243  variable produces no output.
   244  
   245  Variables previously declared can also be assigned, using the syntax
   246  
   247  	$variable = pipeline
   248  
   249  If a "range" action initializes a variable, the variable is set to the
   250  successive elements of the iteration. Also, a "range" may declare two
   251  variables, separated by a comma:
   252  
   253  	range $index, $element := pipeline
   254  
   255  in which case $index and $element are set to the successive values of the
   256  array/slice index or map key and element, respectively. Note that if there is
   257  only one variable, it is assigned the element; this is opposite to the
   258  convention in Go range clauses.
   259  
   260  A variable's scope extends to the "end" action of the control structure ("if",
   261  "with", or "range") in which it is declared, or to the end of the template if
   262  there is no such control structure. A template invocation does not inherit
   263  variables from the point of its invocation.
   264  
   265  When execution begins, $ is set to the data argument passed to Execute, that is,
   266  to the starting value of dot.
   267  
   268  Examples
   269  
   270  Here are some example one-line templates demonstrating pipelines and variables.
   271  All produce the quoted word "output":
   272  
   273  	{{"\"output\""}}
   274  		A string constant.
   275  	{{`"output"`}}
   276  		A raw string constant.
   277  	{{printf "%q" "output"}}
   278  		A function call.
   279  	{{"output" | printf "%q"}}
   280  		A function call whose final argument comes from the previous
   281  		command.
   282  	{{printf "%q" (print "out" "put")}}
   283  		A parenthesized argument.
   284  	{{"put" | printf "%s%s" "out" | printf "%q"}}
   285  		A more elaborate call.
   286  	{{"output" | printf "%s" | printf "%q"}}
   287  		A longer chain.
   288  	{{with "output"}}{{printf "%q" .}}{{end}}
   289  		A with action using dot.
   290  	{{with $x := "output" | printf "%q"}}{{$x}}{{end}}
   291  		A with action that creates and uses a variable.
   292  	{{with $x := "output"}}{{printf "%q" $x}}{{end}}
   293  		A with action that uses the variable in another action.
   294  	{{with $x := "output"}}{{$x | printf "%q"}}{{end}}
   295  		The same, but pipelined.
   296  
   297  Functions
   298  
   299  During execution functions are found in two function maps: first in the
   300  template, then in the global function map. By default, no functions are defined
   301  in the template but the Funcs method can be used to add them.
   302  
   303  Predefined global functions are named as follows.
   304  
   305  	and
   306  		Returns the boolean AND of its arguments by returning the
   307  		first empty argument or the last argument, that is,
   308  		"and x y" behaves as "if x then y else x". All the
   309  		arguments are evaluated.
   310  	call
   311  		Returns the result of calling the first argument, which
   312  		must be a function, with the remaining arguments as parameters.
   313  		Thus "call .X.Y 1 2" is, in Go notation, dot.X.Y(1, 2) where
   314  		Y is a func-valued field, map entry, or the like.
   315  		The first argument must be the result of an evaluation
   316  		that yields a value of function type (as distinct from
   317  		a predefined function such as print). The function must
   318  		return either one or two result values, the second of which
   319  		is of type error. If the arguments don't match the function
   320  		or the returned error value is non-nil, execution stops.
   321  	html
   322  		Returns the escaped HTML equivalent of the textual
   323  		representation of its arguments. This function is unavailable
   324  		in html/template, with a few exceptions.
   325  	index
   326  		Returns the result of indexing its first argument by the
   327  		following arguments. Thus "index x 1 2 3" is, in Go syntax,
   328  		x[1][2][3]. Each indexed item must be a map, slice, or array.
   329  	js
   330  		Returns the escaped JavaScript equivalent of the textual
   331  		representation of its arguments.
   332  	len
   333  		Returns the integer length of its argument.
   334  	not
   335  		Returns the boolean negation of its single argument.
   336  	or
   337  		Returns the boolean OR of its arguments by returning the
   338  		first non-empty argument or the last argument, that is,
   339  		"or x y" behaves as "if x then x else y". All the
   340  		arguments are evaluated.
   341  	print
   342  		An alias for fmt.Sprint
   343  	printf
   344  		An alias for fmt.Sprintf
   345  	println
   346  		An alias for fmt.Sprintln
   347  	urlquery
   348  		Returns the escaped value of the textual representation of
   349  		its arguments in a form suitable for embedding in a URL query.
   350  		This function is unavailable in html/template, with a few
   351  		exceptions.
   352  
   353  The boolean functions take any zero value to be false and a non-zero
   354  value to be true.
   355  
   356  There is also a set of binary comparison operators defined as
   357  functions:
   358  
   359  	eq
   360  		Returns the boolean truth of arg1 == arg2
   361  	ne
   362  		Returns the boolean truth of arg1 != arg2
   363  	lt
   364  		Returns the boolean truth of arg1 < arg2
   365  	le
   366  		Returns the boolean truth of arg1 <= arg2
   367  	gt
   368  		Returns the boolean truth of arg1 > arg2
   369  	ge
   370  		Returns the boolean truth of arg1 >= arg2
   371  
   372  For simpler multi-way equality tests, eq (only) accepts two or more
   373  arguments and compares the second and subsequent to the first,
   374  returning in effect
   375  
   376  	arg1==arg2 || arg1==arg3 || arg1==arg4 ...
   377  
   378  (Unlike with || in Go, however, eq is a function call and all the
   379  arguments will be evaluated.)
   380  
   381  The comparison functions work on basic types only (or named basic
   382  types, such as "type Celsius float32"). They implement the Go rules
   383  for comparison of values, except that size and exact type are
   384  ignored, so any integer value, signed or unsigned, may be compared
   385  with any other integer value. (The arithmetic value is compared,
   386  not the bit pattern, so all negative integers are less than all
   387  unsigned integers.) However, as usual, one may not compare an int
   388  with a float32 and so on.
   389  
   390  Associated templates
   391  
   392  Each template is named by a string specified when it is created. Also, each
   393  template is associated with zero or more other templates that it may invoke by
   394  name; such associations are transitive and form a name space of templates.
   395  
   396  A template may use a template invocation to instantiate another associated
   397  template; see the explanation of the "template" action above. The name must be
   398  that of a template associated with the template that contains the invocation.
   399  
   400  Nested template definitions
   401  
   402  When parsing a template, another template may be defined and associated with the
   403  template being parsed. Template definitions must appear at the top level of the
   404  template, much like global variables in a Go program.
   405  
   406  The syntax of such definitions is to surround each template declaration with a
   407  "define" and "end" action.
   408  
   409  The define action names the template being created by providing a string
   410  constant. Here is a simple example:
   411  
   412  	`{{define "T1"}}ONE{{end}}
   413  	{{define "T2"}}TWO{{end}}
   414  	{{define "T3"}}{{template "T1"}} {{template "T2"}}{{end}}
   415  	{{template "T3"}}`
   416  
   417  This defines two templates, T1 and T2, and a third T3 that invokes the other two
   418  when it is executed. Finally it invokes T3. If executed this template will
   419  produce the text
   420  
   421  	ONE TWO
   422  
   423  By construction, a template may reside in only one association. If it's
   424  necessary to have a template addressable from multiple associations, the
   425  template definition must be parsed multiple times to create distinct *Template
   426  values, or must be copied with the Clone or AddParseTree method.
   427  
   428  Parse may be called multiple times to assemble the various associated templates;
   429  see the ParseFiles and ParseGlob functions and methods for simple ways to parse
   430  related templates stored in files.
   431  
   432  A template may be executed directly or through ExecuteTemplate, which executes
   433  an associated template identified by name. To invoke our example above, we
   434  might write,
   435  
   436  	err := tmpl.Execute(os.Stdout, "no data needed")
   437  	if err != nil {
   438  		log.Fatalf("execution failed: %s", err)
   439  	}
   440  
   441  or to invoke a particular template explicitly by name,
   442  
   443  	err := tmpl.ExecuteTemplate(os.Stdout, "T2", "no data needed")
   444  	if err != nil {
   445  		log.Fatalf("execution failed: %s", err)
   446  	}
   447  
   448  */
   449  package template
   450  

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