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.
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 Actions may not span newlines, although comments can.
23 Once constructed, a template may be executed safely in parallel.
25 Here is a trivial example that prints "17 items are made of wool".
27 type Inventory struct {
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.
41 Here is the list of actions. "Arguments" and "pipelines" are evaluations of
42 data, defined in detail below.
45 // {{/* a comment */}}
46 // A comment; discarded. May contain newlines.
47 // Comments do not nest.
51 The default textual representation of the value of the pipeline
52 is copied to the output.
54 {{if pipeline}} T1 {{end}}
55 If the value of the pipeline is empty, no output is generated;
56 otherwise, T1 is executed. The empty values are false, 0, any
57 nil pointer or interface value, and any array, slice, map, or
58 string of length zero.
61 {{if pipeline}} T1 {{else}} T0 {{end}}
62 If the value of the pipeline is empty, T0 is executed;
63 otherwise, T1 is executed. Dot is unaffected.
65 {{range pipeline}} T1 {{end}}
66 The value of the pipeline must be an array, slice, map, or channel.
67 If the value of the pipeline has length zero, nothing is output;
68 otherwise, dot is set to the successive elements of the array,
69 slice, or map and T1 is executed. If the value is a map and the
70 keys are of basic type with a defined order ("comparable"), the
71 elements will be visited in sorted key order.
73 {{range pipeline}} T1 {{else}} T0 {{end}}
74 The value of the pipeline must be an array, slice, map, or channel.
75 If the value of the pipeline has length zero, dot is unaffected and
76 T0 is executed; otherwise, dot is set to the successive elements
77 of the array, slice, or map and T1 is executed.
80 The template with the specified name is executed with nil data.
82 {{template "name" pipeline}}
83 The template with the specified name is executed with dot set
84 to the value of the pipeline.
86 {{with pipeline}} T1 {{end}}
87 If the value of the pipeline is empty, no output is generated;
88 otherwise, dot is set to the value of the pipeline and T1 is
91 {{with pipeline}} T1 {{else}} T0 {{end}}
92 If the value of the pipeline is empty, dot is unaffected and T0
93 is executed; otherwise, dot is set to the value of the pipeline
98 An argument is a simple value, denoted by one of the following.
100 - A boolean, string, character, integer, floating-point, imaginary
101 or complex constant in Go syntax. These behave like Go's untyped
102 constants, although raw strings may not span newlines.
103 - The keyword nil, representing an untyped Go nil.
104 - The character '.' (period):
106 The result is the value of dot.
107 - A variable name, which is a (possibly empty) alphanumeric string
108 preceded by a dollar sign, such as
112 The result is the value of the variable.
113 Variables are described below.
114 - The name of a field of the data, which must be a struct, preceded
117 The result is the value of the field. Field invocations may be
120 Fields can also be evaluated on variables, including chaining:
122 - The name of a key of the data, which must be a map, preceded
125 The result is the map element value indexed by the key.
126 Key invocations may be chained and combined with fields to any
128 .Field1.Key1.Field2.Key2
129 Although the key must be an alphanumeric identifier, unlike with
130 field names they do not need to start with an upper case letter.
131 Keys can also be evaluated on variables, including chaining:
133 - The name of a niladic method of the data, preceded by a period,
136 The result is the value of invoking the method with dot as the
137 receiver, dot.Method(). Such a method must have one return value (of
138 any type) or two return values, the second of which is an error.
139 If it has two and the returned error is non-nil, execution terminates
140 and an error is returned to the caller as the value of Execute.
141 Method invocations may be chained and combined with fields and keys
143 .Field1.Key1.Method1.Field2.Key2.Method2
144 Methods can also be evaluated on variables, including chaining:
146 - The name of a niladic function, such as
148 The result is the value of invoking the function, fun(). The return
149 types and values behave as in methods. Functions and function
150 names are described below.
151 - A parenthesized instance of one the above, for grouping. The result
152 may be accessed by a field or map key invocation.
153 print (.F1 arg1) (.F2 arg2)
154 (.StructValuedMethod "arg").Field
156 Arguments may evaluate to any type; if they are pointers the implementation
157 automatically indirects to the base type when required.
158 If an evaluation yields a function value, such as a function-valued
159 field of a struct, the function is not invoked automatically, but it
160 can be used as a truth value for an if action and the like. To invoke
161 it, use the call function, defined below.
163 A pipeline is a possibly chained sequence of "commands". A command is a simple
164 value (argument) or a function or method call, possibly with multiple arguments:
167 The result is the value of evaluating the argument.
168 .Method [Argument...]
169 The method can be alone or the last element of a chain but,
170 unlike methods in the middle of a chain, it can take arguments.
171 The result is the value of calling the method with the
173 dot.Method(Argument1, etc.)
174 functionName [Argument...]
175 The result is the value of calling the function associated
177 function(Argument1, etc.)
178 Functions and function names are described below.
182 A pipeline may be "chained" by separating a sequence of commands with pipeline
183 characters '|'. In a chained pipeline, the result of the each command is
184 passed as the last argument of the following command. The output of the final
185 command in the pipeline is the value of the pipeline.
187 The output of a command will be either one value or two values, the second of
188 which has type error. If that second value is present and evaluates to
189 non-nil, execution terminates and the error is returned to the caller of
194 A pipeline inside an action may initialize a variable to capture the result.
195 The initialization has syntax
197 $variable := pipeline
199 where $variable is the name of the variable. An action that declares a
200 variable produces no output.
202 If a "range" action initializes a variable, the variable is set to the
203 successive elements of the iteration. Also, a "range" may declare two
204 variables, separated by a comma:
206 range $index, $element := pipeline
208 in which case $index and $element are set to the successive values of the
209 array/slice index or map key and element, respectively. Note that if there is
210 only one variable, it is assigned the element; this is opposite to the
211 convention in Go range clauses.
213 A variable's scope extends to the "end" action of the control structure ("if",
214 "with", or "range") in which it is declared, or to the end of the template if
215 there is no such control structure. A template invocation does not inherit
216 variables from the point of its invocation.
218 When execution begins, $ is set to the data argument passed to Execute, that is,
219 to the starting value of dot.
223 Here are some example one-line templates demonstrating pipelines and variables.
224 All produce the quoted word "output":
229 A raw string constant.
230 {{printf "%q" "output"}}
232 {{"output" | printf "%q"}}
233 A function call whose final argument comes from the previous
235 {{printf "%q" (print "out" "put")}}
236 A parenthesized argument.
237 {{"put" | printf "%s%s" "out" | printf "%q"}}
238 A more elaborate call.
239 {{"output" | printf "%s" | printf "%q"}}
241 {{with "output"}}{{printf "%q" .}}{{end}}
242 A with action using dot.
243 {{with $x := "output" | printf "%q"}}{{$x}}{{end}}
244 A with action that creates and uses a variable.
245 {{with $x := "output"}}{{printf "%q" $x}}{{end}}
246 A with action that uses the variable in another action.
247 {{with $x := "output"}}{{$x | printf "%q"}}{{end}}
248 The same, but pipelined.
252 During execution functions are found in two function maps: first in the
253 template, then in the global function map. By default, no functions are defined
254 in the template but the Funcs method can be used to add them.
256 Predefined global functions are named as follows.
259 Returns the boolean AND of its arguments by returning the
260 first empty argument or the last argument, that is,
261 "and x y" behaves as "if x then y else x". All the
262 arguments are evaluated.
264 Returns the result of calling the first argument, which
265 must be a function, with the remaining arguments as parameters.
266 Thus "call .X.Y 1 2" is, in Go notation, dot.X.Y(1, 2) where
267 Y is a func-valued field, map entry, or the like.
268 The first argument must be the result of an evaluation
269 that yields a value of function type (as distinct from
270 a predefined function such as print). The function must
271 return either one or two result values, the second of which
272 is of type error. If the arguments don't match the function
273 or the returned error value is non-nil, execution stops.
275 Returns the escaped HTML equivalent of the textual
276 representation of its arguments.
278 Returns the result of indexing its first argument by the
279 following arguments. Thus "index x 1 2 3" is, in Go syntax,
280 x[1][2][3]. Each indexed item must be a map, slice, or array.
282 Returns the escaped JavaScript equivalent of the textual
283 representation of its arguments.
285 Returns the integer length of its argument.
287 Returns the boolean negation of its single argument.
289 Returns the boolean OR of its arguments by returning the
290 first non-empty argument or the last argument, that is,
291 "or x y" behaves as "if x then x else y". All the
292 arguments are evaluated.
294 An alias for fmt.Sprint
296 An alias for fmt.Sprintf
298 An alias for fmt.Sprintln
300 Returns the escaped value of the textual representation of
301 its arguments in a form suitable for embedding in a URL query.
303 The boolean functions take any zero value to be false and a non-zero value to
308 Each template is named by a string specified when it is created. Also, each
309 template is associated with zero or more other templates that it may invoke by
310 name; such associations are transitive and form a name space of templates.
312 A template may use a template invocation to instantiate another associated
313 template; see the explanation of the "template" action above. The name must be
314 that of a template associated with the template that contains the invocation.
316 Nested template definitions
318 When parsing a template, another template may be defined and associated with the
319 template being parsed. Template definitions must appear at the top level of the
320 template, much like global variables in a Go program.
322 The syntax of such definitions is to surround each template declaration with a
323 "define" and "end" action.
325 The define action names the template being created by providing a string
326 constant. Here is a simple example:
328 `{{define "T1"}}ONE{{end}}
329 {{define "T2"}}TWO{{end}}
330 {{define "T3"}}{{template "T1"}} {{template "T2"}}{{end}}
333 This defines two templates, T1 and T2, and a third T3 that invokes the other two
334 when it is executed. Finally it invokes T3. If executed this template will
339 By construction, a template may reside in only one association. If it's
340 necessary to have a template addressable from multiple associations, the
341 template definition must be parsed multiple times to create distinct *Template
342 values, or must be copied with the Clone or AddParseTree method.
344 Parse may be called multiple times to assemble the various associated templates;
345 see the ParseFiles and ParseGlob functions and methods for simple ways to parse
346 related templates stored in files.
348 A template may be executed directly or through ExecuteTemplate, which executes
349 an associated template identified by name. To invoke our example above, we
352 err := tmpl.Execute(os.Stdout, "no data needed")
354 log.Fatalf("execution failed: %s", err)
357 or to invoke a particular template explicitly by name,
359 err := tmpl.ExecuteTemplate(os.Stdout, "T2", "no data needed")
361 log.Fatalf("execution failed: %s", err)