doc.go (18053B)
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
44 appears on standard output when the program is run.
45
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.
54
55 For instance, when executing the template whose source is
56
57 "{{23 -}} < {{- 45}}"
58
59 the generated output would be
60
61 "23<45"
62
63 For this trimming, the definition of white space characters is the same as in Go:
64 space, horizontal tab, carriage return, and newline.
65
66 Actions
67
68 Here is the list of actions. "Arguments" and "pipelines" are evaluations of
69 data, defined in detail in the corresponding sections that follow.
70
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 /*
78
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.
83
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.
90
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.
94
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}}
100
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.
108
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.
114
115 {{break}}
116 The innermost {{range pipeline}} loop is ended early, stopping the
117 current iteration and bypassing all remaining iterations.
118
119 {{continue}}
120 The current iteration of the innermost {{range pipeline}} loop is
121 stopped, and the loop starts the next iteration.
122
123 {{template "name"}}
124 The template with the specified name is executed with nil data.
125
126 {{template "name" pipeline}}
127 The template with the specified name is executed with dot set
128 to the value of the pipeline.
129
130 {{block "name" pipeline}} T1 {{end}}
131 A block is shorthand for defining a template
132 {{define "name"}} T1 {{end}}
133 and then executing it in place
134 {{template "name" pipeline}}
135 The typical use is to define a set of root templates that are
136 then customized by redefining the block templates within.
137
138 {{with pipeline}} T1 {{end}}
139 If the value of the pipeline is empty, no output is generated;
140 otherwise, dot is set to the value of the pipeline and T1 is
141 executed.
142
143 {{with pipeline}} T1 {{else}} T0 {{end}}
144 If the value of the pipeline is empty, dot is unaffected and T0
145 is executed; otherwise, dot is set to the value of the pipeline
146 and T1 is executed.
147
148 Arguments
149
150 An argument is a simple value, denoted by one of the following.
151
152 - A boolean, string, character, integer, floating-point, imaginary
153 or complex constant in Go syntax. These behave like Go's untyped
154 constants. Note that, as in Go, whether a large integer constant
155 overflows when assigned or passed to a function can depend on whether
156 the host machine's ints are 32 or 64 bits.
157 - The keyword nil, representing an untyped Go nil.
158 - The character '.' (period):
159 .
160 The result is the value of dot.
161 - A variable name, which is a (possibly empty) alphanumeric string
162 preceded by a dollar sign, such as
163 $piOver2
164 or
165 $
166 The result is the value of the variable.
167 Variables are described below.
168 - The name of a field of the data, which must be a struct, preceded
169 by a period, such as
170 .Field
171 The result is the value of the field. Field invocations may be
172 chained:
173 .Field1.Field2
174 Fields can also be evaluated on variables, including chaining:
175 $x.Field1.Field2
176 - The name of a key of the data, which must be a map, preceded
177 by a period, such as
178 .Key
179 The result is the map element value indexed by the key.
180 Key invocations may be chained and combined with fields to any
181 depth:
182 .Field1.Key1.Field2.Key2
183 Although the key must be an alphanumeric identifier, unlike with
184 field names they do not need to start with an upper case letter.
185 Keys can also be evaluated on variables, including chaining:
186 $x.key1.key2
187 - The name of a niladic method of the data, preceded by a period,
188 such as
189 .Method
190 The result is the value of invoking the method with dot as the
191 receiver, dot.Method(). Such a method must have one return value (of
192 any type) or two return values, the second of which is an error.
193 If it has two and the returned error is non-nil, execution terminates
194 and an error is returned to the caller as the value of Execute.
195 Method invocations may be chained and combined with fields and keys
196 to any depth:
197 .Field1.Key1.Method1.Field2.Key2.Method2
198 Methods can also be evaluated on variables, including chaining:
199 $x.Method1.Field
200 - The name of a niladic function, such as
201 fun
202 The result is the value of invoking the function, fun(). The return
203 types and values behave as in methods. Functions and function
204 names are described below.
205 - A parenthesized instance of one the above, for grouping. The result
206 may be accessed by a field or map key invocation.
207 print (.F1 arg1) (.F2 arg2)
208 (.StructValuedMethod "arg").Field
209
210 Arguments may evaluate to any type; if they are pointers the implementation
211 automatically indirects to the base type when required.
212 If an evaluation yields a function value, such as a function-valued
213 field of a struct, the function is not invoked automatically, but it
214 can be used as a truth value for an if action and the like. To invoke
215 it, use the call function, defined below.
216
217 Pipelines
218
219 A pipeline is a possibly chained sequence of "commands". A command is a simple
220 value (argument) or a function or method call, possibly with multiple arguments:
221
222 Argument
223 The result is the value of evaluating the argument.
224 .Method [Argument...]
225 The method can be alone or the last element of a chain but,
226 unlike methods in the middle of a chain, it can take arguments.
227 The result is the value of calling the method with the
228 arguments:
229 dot.Method(Argument1, etc.)
230 functionName [Argument...]
231 The result is the value of calling the function associated
232 with the name:
233 function(Argument1, etc.)
234 Functions and function names are described below.
235
236 A pipeline may be "chained" by separating a sequence of commands with pipeline
237 characters '|'. In a chained pipeline, the result of each command is
238 passed as the last argument of the following command. The output of the final
239 command in the pipeline is the value of the pipeline.
240
241 The output of a command will be either one value or two values, the second of
242 which has type error. If that second value is present and evaluates to
243 non-nil, execution terminates and the error is returned to the caller of
244 Execute.
245
246 Variables
247
248 A pipeline inside an action may initialize a variable to capture the result.
249 The initialization has syntax
250
251 $variable := pipeline
252
253 where $variable is the name of the variable. An action that declares a
254 variable produces no output.
255
256 Variables previously declared can also be assigned, using the syntax
257
258 $variable = pipeline
259
260 If a "range" action initializes a variable, the variable is set to the
261 successive elements of the iteration. Also, a "range" may declare two
262 variables, separated by a comma:
263
264 range $index, $element := pipeline
265
266 in which case $index and $element are set to the successive values of the
267 array/slice index or map key and element, respectively. Note that if there is
268 only one variable, it is assigned the element; this is opposite to the
269 convention in Go range clauses.
270
271 A variable's scope extends to the "end" action of the control structure ("if",
272 "with", or "range") in which it is declared, or to the end of the template if
273 there is no such control structure. A template invocation does not inherit
274 variables from the point of its invocation.
275
276 When execution begins, $ is set to the data argument passed to Execute, that is,
277 to the starting value of dot.
278
279 Examples
280
281 Here are some example one-line templates demonstrating pipelines and variables.
282 All produce the quoted word "output":
283
284 {{"\"output\""}}
285 A string constant.
286 {{`"output"`}}
287 A raw string constant.
288 {{printf "%q" "output"}}
289 A function call.
290 {{"output" | printf "%q"}}
291 A function call whose final argument comes from the previous
292 command.
293 {{printf "%q" (print "out" "put")}}
294 A parenthesized argument.
295 {{"put" | printf "%s%s" "out" | printf "%q"}}
296 A more elaborate call.
297 {{"output" | printf "%s" | printf "%q"}}
298 A longer chain.
299 {{with "output"}}{{printf "%q" .}}{{end}}
300 A with action using dot.
301 {{with $x := "output" | printf "%q"}}{{$x}}{{end}}
302 A with action that creates and uses a variable.
303 {{with $x := "output"}}{{printf "%q" $x}}{{end}}
304 A with action that uses the variable in another action.
305 {{with $x := "output"}}{{$x | printf "%q"}}{{end}}
306 The same, but pipelined.
307
308 Functions
309
310 During execution functions are found in two function maps: first in the
311 template, then in the global function map. By default, no functions are defined
312 in the template but the Funcs method can be used to add them.
313
314 Predefined global functions are named as follows.
315
316 and
317 Returns the boolean AND of its arguments by returning the
318 first empty argument or the last argument. That is,
319 "and x y" behaves as "if x then y else x."
320 Evaluation proceeds through the arguments left to right
321 and returns when the result is determined.
322 call
323 Returns the result of calling the first argument, which
324 must be a function, with the remaining arguments as parameters.
325 Thus "call .X.Y 1 2" is, in Go notation, dot.X.Y(1, 2) where
326 Y is a func-valued field, map entry, or the like.
327 The first argument must be the result of an evaluation
328 that yields a value of function type (as distinct from
329 a predefined function such as print). The function must
330 return either one or two result values, the second of which
331 is of type error. If the arguments don't match the function
332 or the returned error value is non-nil, execution stops.
333 html
334 Returns the escaped HTML equivalent of the textual
335 representation of its arguments. This function is unavailable
336 in html/template, with a few exceptions.
337 index
338 Returns the result of indexing its first argument by the
339 following arguments. Thus "index x 1 2 3" is, in Go syntax,
340 x[1][2][3]. Each indexed item must be a map, slice, or array.
341 slice
342 slice returns the result of slicing its first argument by the
343 remaining arguments. Thus "slice x 1 2" is, in Go syntax, x[1:2],
344 while "slice x" is x[:], "slice x 1" is x[1:], and "slice x 1 2 3"
345 is x[1:2:3]. The first argument must be a string, slice, or array.
346 js
347 Returns the escaped JavaScript equivalent of the textual
348 representation of its arguments.
349 len
350 Returns the integer length of its argument.
351 not
352 Returns the boolean negation of its single argument.
353 or
354 Returns the boolean OR of its arguments by returning the
355 first non-empty argument or the last argument, that is,
356 "or x y" behaves as "if x then x else y".
357 Evaluation proceeds through the arguments left to right
358 and returns when the result is determined.
359 print
360 An alias for fmt.Sprint
361 printf
362 An alias for fmt.Sprintf
363 println
364 An alias for fmt.Sprintln
365 urlquery
366 Returns the escaped value of the textual representation of
367 its arguments in a form suitable for embedding in a URL query.
368 This function is unavailable in html/template, with a few
369 exceptions.
370
371 The boolean functions take any zero value to be false and a non-zero
372 value to be true.
373
374 There is also a set of binary comparison operators defined as
375 functions:
376
377 eq
378 Returns the boolean truth of arg1 == arg2
379 ne
380 Returns the boolean truth of arg1 != arg2
381 lt
382 Returns the boolean truth of arg1 < arg2
383 le
384 Returns the boolean truth of arg1 <= arg2
385 gt
386 Returns the boolean truth of arg1 > arg2
387 ge
388 Returns the boolean truth of arg1 >= arg2
389
390 For simpler multi-way equality tests, eq (only) accepts two or more
391 arguments and compares the second and subsequent to the first,
392 returning in effect
393
394 arg1==arg2 || arg1==arg3 || arg1==arg4 ...
395
396 (Unlike with || in Go, however, eq is a function call and all the
397 arguments will be evaluated.)
398
399 The comparison functions work on any values whose type Go defines as
400 comparable. For basic types such as integers, the rules are relaxed:
401 size and exact type are ignored, so any integer value, signed or unsigned,
402 may be compared with any other integer value. (The arithmetic value is compared,
403 not the bit pattern, so all negative integers are less than all unsigned integers.)
404 However, as usual, one may not compare an int with a float32 and so on.
405
406 Associated templates
407
408 Each template is named by a string specified when it is created. Also, each
409 template is associated with zero or more other templates that it may invoke by
410 name; such associations are transitive and form a name space of templates.
411
412 A template may use a template invocation to instantiate another associated
413 template; see the explanation of the "template" action above. The name must be
414 that of a template associated with the template that contains the invocation.
415
416 Nested template definitions
417
418 When parsing a template, another template may be defined and associated with the
419 template being parsed. Template definitions must appear at the top level of the
420 template, much like global variables in a Go program.
421
422 The syntax of such definitions is to surround each template declaration with a
423 "define" and "end" action.
424
425 The define action names the template being created by providing a string
426 constant. Here is a simple example:
427
428 `{{define "T1"}}ONE{{end}}
429 {{define "T2"}}TWO{{end}}
430 {{define "T3"}}{{template "T1"}} {{template "T2"}}{{end}}
431 {{template "T3"}}`
432
433 This defines two templates, T1 and T2, and a third T3 that invokes the other two
434 when it is executed. Finally it invokes T3. If executed this template will
435 produce the text
436
437 ONE TWO
438
439 By construction, a template may reside in only one association. If it's
440 necessary to have a template addressable from multiple associations, the
441 template definition must be parsed multiple times to create distinct *Template
442 values, or must be copied with the Clone or AddParseTree method.
443
444 Parse may be called multiple times to assemble the various associated templates;
445 see the ParseFiles and ParseGlob functions and methods for simple ways to parse
446 related templates stored in files.
447
448 A template may be executed directly or through ExecuteTemplate, which executes
449 an associated template identified by name. To invoke our example above, we
450 might write,
451
452 err := tmpl.Execute(os.Stdout, "no data needed")
453 if err != nil {
454 log.Fatalf("execution failed: %s", err)
455 }
456
457 or to invoke a particular template explicitly by name,
458
459 err := tmpl.ExecuteTemplate(os.Stdout, "T2", "no data needed")
460 if err != nil {
461 log.Fatalf("execution failed: %s", err)
462 }
463
464 */
465 package template