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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 [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
22 Once parsed, a template may be executed safely in parallel, although if parallel
23 executions share a Writer the output may be interleaved.
24
25 Here is a trivial example that prints "17 items are made of wool".
26
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) }
36
37 More intricate examples appear below.
38
39 Text and spaces
40
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.
44
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.
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, iter.Seq,
102 iter.Seq2, integer 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.
108
109 {{range pipeline}} T1 {{else}} T0 {{end}}
110 The value of the pipeline must be an array, slice, map, iter.Seq,
111 iter.Seq2, integer or channel.
112 If the value of the pipeline has length zero, dot is unaffected and
113 T0 is executed; otherwise, dot is set to the successive elements
114 of the array, slice, or map and T1 is executed.
115
116 {{break}}
117 The innermost {{range pipeline}} loop is ended early, stopping the
118 current iteration and bypassing all remaining iterations.
119
120 {{continue}}
121 The current iteration of the innermost {{range pipeline}} loop is
122 stopped, and the loop starts the next iteration.
123
124 {{template "name"}}
125 The template with the specified name is executed with nil data.
126
127 {{template "name" pipeline}}
128 The template with the specified name is executed with dot set
129 to the value of the pipeline.
130
131 {{block "name" pipeline}} T1 {{end}}
132 A block is shorthand for defining a template
133 {{define "name"}} T1 {{end}}
134 and then executing it in place
135 {{template "name" pipeline}}
136 The typical use is to define a set of root templates that are
137 then customized by redefining the block templates within.
138
139 {{with pipeline}} T1 {{end}}
140 If the value of the pipeline is empty, no output is generated;
141 otherwise, dot is set to the value of the pipeline and T1 is
142 executed.
143
144 {{with pipeline}} T1 {{else}} T0 {{end}}
145 If the value of the pipeline is empty, dot is unaffected and T0
146 is executed; otherwise, dot is set to the value of the pipeline
147 and T1 is executed.
148
149 {{with pipeline}} T1 {{else with pipeline}} T0 {{end}}
150 To simplify the appearance of with-else chains, the else action
151 of a with may include another with directly; the effect is exactly
152 the same as writing
153 {{with pipeline}} T1 {{else}}{{with pipeline}} T0 {{end}}{{end}}
154
155
156 Arguments
157
158 An argument is a simple value, denoted by one of the following.
159
160 - A boolean, string, character, integer, floating-point, imaginary
161 or complex constant in Go syntax. These behave like Go's untyped
162 constants. Note that, as in Go, whether a large integer constant
163 overflows when assigned or passed to a function can depend on whether
164 the host machine's ints are 32 or 64 bits.
165 - The keyword nil, representing an untyped Go nil.
166 - The character '.' (period):
167
168 .
169
170 The result is the value of dot.
171 - A variable name, which is a (possibly empty) alphanumeric string
172 preceded by a dollar sign, such as
173
174 $piOver2
175
176 or
177
178 $
179
180 The result is the value of the variable.
181 Variables are described below.
182 - The name of a field of the data, which must be a struct, preceded
183 by a period, such as
184
185 .Field
186
187 The result is the value of the field. Field invocations may be
188 chained:
189
190 .Field1.Field2
191
192 Fields can also be evaluated on variables, including chaining:
193
194 $x.Field1.Field2
195 - The name of a key of the data, which must be a map, preceded
196 by a period, such as
197
198 .Key
199
200 The result is the map element value indexed by the key.
201 Key invocations may be chained and combined with fields to any
202 depth:
203
204 .Field1.Key1.Field2.Key2
205
206 Although the key must be an alphanumeric identifier, unlike with
207 field names they do not need to start with an upper case letter.
208 Keys can also be evaluated on variables, including chaining:
209
210 $x.key1.key2
211 - The name of a niladic method of the data, preceded by a period,
212 such as
213
214 .Method
215
216 The result is the value of invoking the method with dot as the
217 receiver, dot.Method(). Such a method must have one return value (of
218 any type) or two return values, the second of which is an error.
219 If it has two and the returned error is non-nil, execution terminates
220 and an error is returned to the caller as the value of Execute.
221 Method invocations may be chained and combined with fields and keys
222 to any depth:
223
224 .Field1.Key1.Method1.Field2.Key2.Method2
225
226 Methods can also be evaluated on variables, including chaining:
227
228 $x.Method1.Field
229 - The name of a niladic function, such as
230
231 fun
232
233 The result is the value of invoking the function, fun(). The return
234 types and values behave as in methods. Functions and function
235 names are described below.
236 - A parenthesized instance of one the above, for grouping. The result
237 may be accessed by a field or map key invocation.
238
239 print (.F1 arg1) (.F2 arg2)
240 (.StructValuedMethod "arg").Field
241
242 Arguments may evaluate to any type; if they are pointers the implementation
243 automatically indirects to the base type when required.
244 If an evaluation yields a function value, such as a function-valued
245 field of a struct, the function is not invoked automatically, but it
246 can be used as a truth value for an if action and the like. To invoke
247 it, use the call function, defined below.
248
249 Pipelines
250
251 A pipeline is a possibly chained sequence of "commands". A command is a simple
252 value (argument) or a function or method call, possibly with multiple arguments:
253
254 Argument
255 The result is the value of evaluating the argument.
256 .Method [Argument...]
257 The method can be alone or the last element of a chain but,
258 unlike methods in the middle of a chain, it can take arguments.
259 The result is the value of calling the method with the
260 arguments:
261 dot.Method(Argument1, etc.)
262 functionName [Argument...]
263 The result is the value of calling the function associated
264 with the name:
265 function(Argument1, etc.)
266 Functions and function names are described below.
267
268 A pipeline may be "chained" by separating a sequence of commands with pipeline
269 characters '|'. In a chained pipeline, the result of each command is
270 passed as the last argument of the following command. The output of the final
271 command in the pipeline is the value of the pipeline.
272
273 The output of a command will be either one value or two values, the second of
274 which has type error. If that second value is present and evaluates to
275 non-nil, execution terminates and the error is returned to the caller of
276 Execute.
277
278 Variables
279
280 A pipeline inside an action may initialize a variable to capture the result.
281 The initialization has syntax
282
283 $variable := pipeline
284
285 where $variable is the name of the variable. An action that declares a
286 variable produces no output.
287
288 Variables previously declared can also be assigned, using the syntax
289
290 $variable = pipeline
291
292 If a "range" action initializes a variable, the variable is set to the
293 successive elements of the iteration. Also, a "range" may declare two
294 variables, separated by a comma:
295
296 range $index, $element := pipeline
297
298 in which case $index and $element are set to the successive values of the
299 array/slice index or map key and element, respectively. Note that if there is
300 only one variable, it is assigned the element; this is opposite to the
301 convention in Go range clauses.
302
303 A variable's scope extends to the "end" action of the control structure ("if",
304 "with", or "range") in which it is declared, or to the end of the template if
305 there is no such control structure. A template invocation does not inherit
306 variables from the point of its invocation.
307
308 When execution begins, $ is set to the data argument passed to Execute, that is,
309 to the starting value of dot.
310
311 Examples
312
313 Here are some example one-line templates demonstrating pipelines and variables.
314 All produce the quoted word "output":
315
316 {{"\"output\""}}
317 A string constant.
318 {{`"output"`}}
319 A raw string constant.
320 {{printf "%q" "output"}}
321 A function call.
322 {{"output" | printf "%q"}}
323 A function call whose final argument comes from the previous
324 command.
325 {{printf "%q" (print "out" "put")}}
326 A parenthesized argument.
327 {{"put" | printf "%s%s" "out" | printf "%q"}}
328 A more elaborate call.
329 {{"output" | printf "%s" | printf "%q"}}
330 A longer chain.
331 {{with "output"}}{{printf "%q" .}}{{end}}
332 A with action using dot.
333 {{with $x := "output" | printf "%q"}}{{$x}}{{end}}
334 A with action that creates and uses a variable.
335 {{with $x := "output"}}{{printf "%q" $x}}{{end}}
336 A with action that uses the variable in another action.
337 {{with $x := "output"}}{{$x | printf "%q"}}{{end}}
338 The same, but pipelined.
339
340 Functions
341
342 During execution functions are found in two function maps: first in the
343 template, then in the global function map. By default, no functions are defined
344 in the template but the Funcs method can be used to add them.
345
346 Predefined global functions are named as follows.
347
348 and
349 Returns the boolean AND of its arguments by returning the
350 first empty argument or the last argument. That is,
351 "and x y" behaves as "if x then y else x."
352 Evaluation proceeds through the arguments left to right
353 and returns when the result is determined.
354 call
355 Returns the result of calling the first argument, which
356 must be a function, with the remaining arguments as parameters.
357 Thus "call .X.Y 1 2" is, in Go notation, dot.X.Y(1, 2) where
358 Y is a func-valued field, map entry, or the like.
359 The first argument must be the result of an evaluation
360 that yields a value of function type (as distinct from
361 a predefined function such as print). The function must
362 return either one or two result values, the second of which
363 is of type error. If the arguments don't match the function
364 or the returned error value is non-nil, execution stops.
365 html
366 Returns the escaped HTML equivalent of the textual
367 representation of its arguments. This function is unavailable
368 in html/template, with a few exceptions.
369 index
370 Returns the result of indexing its first argument by the
371 following arguments. Thus "index x 1 2 3" is, in Go syntax,
372 x[1][2][3]. Each indexed item must be a map, slice, or array.
373 slice
374 slice returns the result of slicing its first argument by the
375 remaining arguments. Thus "slice x 1 2" is, in Go syntax, x[1:2],
376 while "slice x" is x[:], "slice x 1" is x[1:], and "slice x 1 2 3"
377 is x[1:2:3]. The first argument must be a string, slice, or array.
378 js
379 Returns the escaped JavaScript equivalent of the textual
380 representation of its arguments.
381 len
382 Returns the integer length of its argument.
383 not
384 Returns the boolean negation of its single argument.
385 or
386 Returns the boolean OR of its arguments by returning the
387 first non-empty argument or the last argument, that is,
388 "or x y" behaves as "if x then x else y".
389 Evaluation proceeds through the arguments left to right
390 and returns when the result is determined.
391 print
392 An alias for fmt.Sprint
393 printf
394 An alias for fmt.Sprintf
395 println
396 An alias for fmt.Sprintln
397 urlquery
398 Returns the escaped value of the textual representation of
399 its arguments in a form suitable for embedding in a URL query.
400 This function is unavailable in html/template, with a few
401 exceptions.
402
403 The boolean functions take any zero value to be false and a non-zero
404 value to be true.
405
406 There is also a set of binary comparison operators defined as
407 functions:
408
409 eq
410 Returns the boolean truth of arg1 == arg2
411 ne
412 Returns the boolean truth of arg1 != arg2
413 lt
414 Returns the boolean truth of arg1 < arg2
415 le
416 Returns the boolean truth of arg1 <= arg2
417 gt
418 Returns the boolean truth of arg1 > arg2
419 ge
420 Returns the boolean truth of arg1 >= arg2
421
422 For simpler multi-way equality tests, eq (only) accepts two or more
423 arguments and compares the second and subsequent to the first,
424 returning in effect
425
426 arg1==arg2 || arg1==arg3 || arg1==arg4 ...
427
428 (Unlike with || in Go, however, eq is a function call and all the
429 arguments will be evaluated.)
430
431 The comparison functions work on any values whose type Go defines as
432 comparable. For basic types such as integers, the rules are relaxed:
433 size and exact type are ignored, so any integer value, signed or unsigned,
434 may be compared with any other integer value. (The arithmetic value is compared,
435 not the bit pattern, so all negative integers are less than all unsigned integers.)
436 However, as usual, one may not compare an int with a float32 and so on.
437
438 Associated templates
439
440 Each template is named by a string specified when it is created. Also, each
441 template is associated with zero or more other templates that it may invoke by
442 name; such associations are transitive and form a name space of templates.
443
444 A template may use a template invocation to instantiate another associated
445 template; see the explanation of the "template" action above. The name must be
446 that of a template associated with the template that contains the invocation.
447
448 Nested template definitions
449
450 When parsing a template, another template may be defined and associated with the
451 template being parsed. Template definitions must appear at the top level of the
452 template, much like global variables in a Go program.
453
454 The syntax of such definitions is to surround each template declaration with a
455 "define" and "end" action.
456
457 The define action names the template being created by providing a string
458 constant. Here is a simple example:
459
460 {{define "T1"}}ONE{{end}}
461 {{define "T2"}}TWO{{end}}
462 {{define "T3"}}{{template "T1"}} {{template "T2"}}{{end}}
463 {{template "T3"}}
464
465 This defines two templates, T1 and T2, and a third T3 that invokes the other two
466 when it is executed. Finally it invokes T3. If executed this template will
467 produce the text
468
469 ONE TWO
470
471 By construction, a template may reside in only one association. If it's
472 necessary to have a template addressable from multiple associations, the
473 template definition must be parsed multiple times to create distinct *Template
474 values, or must be copied with [Template.Clone] or [Template.AddParseTree].
475
476 Parse may be called multiple times to assemble the various associated templates;
477 see [ParseFiles], [ParseGlob], [Template.ParseFiles] and [Template.ParseGlob]
478 for simple ways to parse related templates stored in files.
479
480 A template may be executed directly or through [Template.ExecuteTemplate], which executes
481 an associated template identified by name. To invoke our example above, we
482 might write,
483
484 err := tmpl.Execute(os.Stdout, "no data needed")
485 if err != nil {
486 log.Fatalf("execution failed: %s", err)
487 }
488
489 or to invoke a particular template explicitly by name,
490
491 err := tmpl.ExecuteTemplate(os.Stdout, "T2", "no data needed")
492 if err != nil {
493 log.Fatalf("execution failed: %s", err)
494 }
495
496 */
497 package template