Source file src/text/template/doc.go
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 security model used by this package assumes that template authors are 19 trusted. The package does not auto-escape output, so injecting code into 20 a template can lead to arbitrary code execution if the template is executed 21 by an untrusted source. 22 23 The input text for a template is UTF-8-encoded text in any format. 24 "Actions"--data evaluations or control structures--are delimited by 25 "{{" and "}}"; all text outside actions is copied to the output unchanged. 26 27 Once parsed, a template may be executed safely in parallel, although if parallel 28 executions share a Writer the output may be interleaved. 29 30 Here is a trivial example that prints "17 items are made of wool". 31 32 type Inventory struct { 33 Material string 34 Count uint 35 } 36 sweaters := Inventory{"wool", 17} 37 tmpl, err := template.New("test").Parse("{{.Count}} items are made of {{.Material}}") 38 if err != nil { panic(err) } 39 err = tmpl.Execute(os.Stdout, sweaters) 40 if err != nil { panic(err) } 41 42 More intricate examples appear below. 43 44 Text and spaces 45 46 By default, all text between actions is copied verbatim when the template is 47 executed. For example, the string " items are made of " in the example above 48 appears on standard output when the program is run. 49 50 However, to aid in formatting template source code, if an action's left 51 delimiter (by default "{{") is followed immediately by a minus sign and white 52 space, all trailing white space is trimmed from the immediately preceding text. 53 Similarly, if the right delimiter ("}}") is preceded by white space and a minus 54 sign, all leading white space is trimmed from the immediately following text. 55 In these trim markers, the white space must be present: 56 "{{- 3}}" is like "{{3}}" but trims the immediately preceding text, while 57 "{{-3}}" parses as an action containing the number -3. 58 59 For instance, when executing the template whose source is 60 61 "{{23 -}} < {{- 45}}" 62 63 the generated output would be 64 65 "23<45" 66 67 For this trimming, the definition of white space characters is the same as in Go: 68 space, horizontal tab, carriage return, and newline. 69 70 Actions 71 72 Here is the list of actions. "Arguments" and "pipelines" are evaluations of 73 data, defined in detail in the corresponding sections that follow. 74 75 */ 76 // {{/* a comment */}} 77 // {{- /* a comment with white space trimmed from preceding and following text */ -}} 78 // A comment; discarded. May contain newlines. 79 // Comments do not nest and must start and end at the 80 // delimiters, as shown here. 81 /* 82 83 {{pipeline}} 84 The default textual representation (the same as would be 85 printed by fmt.Print) of the value of the pipeline is copied 86 to the output. 87 88 {{if pipeline}} T1 {{end}} 89 If the value of the pipeline is empty, no output is generated; 90 otherwise, T1 is executed. The empty values are false, 0, any 91 nil pointer or interface value, and any array, slice, map, or 92 string of length zero. 93 Dot is unaffected. 94 95 {{if pipeline}} T1 {{else}} T0 {{end}} 96 If the value of the pipeline is empty, T0 is executed; 97 otherwise, T1 is executed. Dot is unaffected. 98 99 {{if pipeline}} T1 {{else if pipeline}} T0 {{end}} 100 To simplify the appearance of if-else chains, the else action 101 of an if may include another if directly; the effect is exactly 102 the same as writing 103 {{if pipeline}} T1 {{else}}{{if pipeline}} T0 {{end}}{{end}} 104 105 {{range pipeline}} T1 {{end}} 106 The value of the pipeline must be an array, slice, map, iter.Seq, 107 iter.Seq2, integer or channel. 108 If the value of the pipeline has length zero, nothing is output; 109 otherwise, dot is set to the successive elements of the array, 110 slice, or map and T1 is executed. If the value is a map and the 111 keys are of basic type with a defined order, the elements will be 112 visited in sorted key order. 113 114 {{range pipeline}} T1 {{else}} T0 {{end}} 115 The value of the pipeline must be an array, slice, map, iter.Seq, 116 iter.Seq2, integer or channel. 117 If the value of the pipeline has length zero, dot is unaffected and 118 T0 is executed; otherwise, dot is set to the successive elements 119 of the array, slice, or map and T1 is executed. 120 121 {{break}} 122 The innermost {{range pipeline}} loop is ended early, stopping the 123 current iteration and bypassing all remaining iterations. 124 125 {{continue}} 126 The current iteration of the innermost {{range pipeline}} loop is 127 stopped, and the loop starts the next iteration. 128 129 {{template "name"}} 130 The template with the specified name is executed with nil data. 131 132 {{template "name" pipeline}} 133 The template with the specified name is executed with dot set 134 to the value of the pipeline. 135 136 {{block "name" pipeline}} T1 {{end}} 137 A block is shorthand for defining a template 138 {{define "name"}} T1 {{end}} 139 and then executing it in place 140 {{template "name" pipeline}} 141 The typical use is to define a set of root templates that are 142 then customized by redefining the block templates within. 143 144 {{with pipeline}} T1 {{end}} 145 If the value of the pipeline is empty, no output is generated; 146 otherwise, dot is set to the value of the pipeline and T1 is 147 executed. 148 149 {{with pipeline}} T1 {{else}} T0 {{end}} 150 If the value of the pipeline is empty, dot is unaffected and T0 151 is executed; otherwise, dot is set to the value of the pipeline 152 and T1 is executed. 153 154 {{with pipeline}} T1 {{else with pipeline}} T0 {{end}} 155 To simplify the appearance of with-else chains, the else action 156 of a with may include another with directly; the effect is exactly 157 the same as writing 158 {{with pipeline}} T1 {{else}}{{with pipeline}} T0 {{end}}{{end}} 159 160 161 Arguments 162 163 An argument is a simple value, denoted by one of the following. 164 165 - A boolean, string, character, integer, floating-point, imaginary 166 or complex constant in Go syntax. These behave like Go's untyped 167 constants. Note that, as in Go, whether a large integer constant 168 overflows when assigned or passed to a function can depend on whether 169 the host machine's ints are 32 or 64 bits. 170 - The keyword nil, representing an untyped Go nil. 171 - The character '.' (period): 172 173 . 174 175 The result is the value of dot. 176 - A variable name, which is a (possibly empty) alphanumeric string 177 preceded by a dollar sign, such as 178 179 $piOver2 180 181 or 182 183 $ 184 185 The result is the value of the variable. 186 Variables are described below. 187 - The name of a field of the data, which must be a struct, preceded 188 by a period, such as 189 190 .Field 191 192 The result is the value of the field. Field invocations may be 193 chained: 194 195 .Field1.Field2 196 197 Fields can also be evaluated on variables, including chaining: 198 199 $x.Field1.Field2 200 - The name of a key of the data, which must be a map, preceded 201 by a period, such as 202 203 .Key 204 205 The result is the map element value indexed by the key. 206 Key invocations may be chained and combined with fields to any 207 depth: 208 209 .Field1.Key1.Field2.Key2 210 211 Although the key must be an alphanumeric identifier, unlike with 212 field names they do not need to start with an upper case letter. 213 Keys can also be evaluated on variables, including chaining: 214 215 $x.key1.key2 216 - The name of a niladic method of the data, preceded by a period, 217 such as 218 219 .Method 220 221 The result is the value of invoking the method with dot as the 222 receiver, dot.Method(). Such a method must have one return value (of 223 any type) or two return values, the second of which is an error. 224 If it has two and the returned error is non-nil, execution terminates 225 and an error is returned to the caller as the value of Execute. 226 Method invocations may be chained and combined with fields and keys 227 to any depth: 228 229 .Field1.Key1.Method1.Field2.Key2.Method2 230 231 Methods can also be evaluated on variables, including chaining: 232 233 $x.Method1.Field 234 - The name of a niladic function, such as 235 236 fun 237 238 The result is the value of invoking the function, fun(). The return 239 types and values behave as in methods. Functions and function 240 names are described below. 241 - A parenthesized instance of one the above, for grouping. The result 242 may be accessed by a field or map key invocation. 243 244 print (.F1 arg1) (.F2 arg2) 245 (.StructValuedMethod "arg").Field 246 247 Arguments may evaluate to any type; if they are pointers the implementation 248 automatically indirects to the base type when required. 249 If an evaluation yields a function value, such as a function-valued 250 field of a struct, the function is not invoked automatically, but it 251 can be used as a truth value for an if action and the like. To invoke 252 it, use the call function, defined below. 253 254 Pipelines 255 256 A pipeline is a possibly chained sequence of "commands". A command is a simple 257 value (argument) or a function or method call, possibly with multiple arguments: 258 259 Argument 260 The result is the value of evaluating the argument. 261 .Method [Argument...] 262 The method can be alone or the last element of a chain but, 263 unlike methods in the middle of a chain, it can take arguments. 264 The result is the value of calling the method with the 265 arguments: 266 dot.Method(Argument1, etc.) 267 functionName [Argument...] 268 The result is the value of calling the function associated 269 with the name: 270 function(Argument1, etc.) 271 Functions and function names are described below. 272 273 A pipeline may be "chained" by separating a sequence of commands with pipeline 274 characters '|'. In a chained pipeline, the result of each command is 275 passed as the last argument of the following command. The output of the final 276 command in the pipeline is the value of the pipeline. 277 278 The output of a command will be either one value or two values, the second of 279 which has type error. If that second value is present and evaluates to 280 non-nil, execution terminates and the error is returned to the caller of 281 Execute. 282 283 Variables 284 285 A pipeline inside an action may initialize a variable to capture the result. 286 The initialization has syntax 287 288 $variable := pipeline 289 290 where $variable is the name of the variable. An action that declares a 291 variable produces no output. 292 293 Variables previously declared can also be assigned, using the syntax 294 295 $variable = pipeline 296 297 If a "range" action initializes a variable, the variable is set to the 298 successive elements of the iteration. Also, a "range" may declare two 299 variables, separated by a comma: 300 301 range $index, $element := pipeline 302 303 in which case $index and $element are set to the successive values of the 304 array/slice index or map key and element, respectively. Note that if there is 305 only one variable, it is assigned the V of any map[K]V type, 306 or the first value of any iter.Seq2[E1, E2] type; this is opposite to the 307 convention in Go range clauses. 308 309 A variable's scope extends to the "end" action of the control structure ("if", 310 "with", or "range") in which it is declared, or to the end of the template if 311 there is no such control structure. A template invocation does not inherit 312 variables from the point of its invocation. 313 314 When execution begins, $ is set to the data argument passed to Execute, that is, 315 to the starting value of dot. 316 317 Examples 318 319 Here are some example one-line templates demonstrating pipelines and variables. 320 All produce the quoted word "output": 321 322 {{"\"output\""}} 323 A string constant. 324 {{`"output"`}} 325 A raw string constant. 326 {{printf "%q" "output"}} 327 A function call. 328 {{"output" | printf "%q"}} 329 A function call whose final argument comes from the previous 330 command. 331 {{printf "%q" (print "out" "put")}} 332 A parenthesized argument. 333 {{"put" | printf "%s%s" "out" | printf "%q"}} 334 A more elaborate call. 335 {{"output" | printf "%s" | printf "%q"}} 336 A longer chain. 337 {{with "output"}}{{printf "%q" .}}{{end}} 338 A with action using dot. 339 {{with $x := "output" | printf "%q"}}{{$x}}{{end}} 340 A with action that creates and uses a variable. 341 {{with $x := "output"}}{{printf "%q" $x}}{{end}} 342 A with action that uses the variable in another action. 343 {{with $x := "output"}}{{$x | printf "%q"}}{{end}} 344 The same, but pipelined. 345 346 Functions 347 348 During execution functions are found in two function maps: first in the 349 template, then in the global function map. By default, no functions are defined 350 in the template but the Funcs method can be used to add them. 351 352 Predefined global functions are named as follows. 353 354 and 355 Returns the boolean AND of its arguments by returning the 356 first empty argument or the last argument. That is, 357 "and x y" behaves as "if x then y else x." 358 Evaluation proceeds through the arguments left to right 359 and returns when the result is determined. 360 call 361 Returns the result of calling the first argument, which 362 must be a function, with the remaining arguments as parameters. 363 Thus "call .X.Y 1 2" is, in Go notation, dot.X.Y(1, 2) where 364 Y is a func-valued field, map entry, or the like. 365 The first argument must be the result of an evaluation 366 that yields a value of function type (as distinct from 367 a predefined function such as print). The function must 368 return either one or two result values, the second of which 369 is of type error. If the arguments don't match the function 370 or the returned error value is non-nil, execution stops. 371 html 372 Returns the escaped HTML equivalent of the textual 373 representation of its arguments. This function is unavailable 374 in html/template, with a few exceptions. 375 index 376 Returns the result of indexing its first argument by the 377 following arguments. Thus "index x 1 2 3" is, in Go syntax, 378 x[1][2][3]. Each indexed item must be a map, slice, or array. 379 slice 380 slice returns the result of slicing its first argument by the 381 remaining arguments. Thus "slice x 1 2" is, in Go syntax, x[1:2], 382 while "slice x" is x[:], "slice x 1" is x[1:], and "slice x 1 2 3" 383 is x[1:2:3]. The first argument must be a string, slice, or array. 384 js 385 Returns the escaped JavaScript equivalent of the textual 386 representation of its arguments. 387 len 388 Returns the integer length of its argument. 389 not 390 Returns the boolean negation of its single argument. 391 or 392 Returns the boolean OR of its arguments by returning the 393 first non-empty argument or the last argument, that is, 394 "or x y" behaves as "if x then x else y". 395 Evaluation proceeds through the arguments left to right 396 and returns when the result is determined. 397 print 398 An alias for fmt.Sprint 399 printf 400 An alias for fmt.Sprintf 401 println 402 An alias for fmt.Sprintln 403 urlquery 404 Returns the escaped value of the textual representation of 405 its arguments in a form suitable for embedding in a URL query. 406 This function is unavailable in html/template, with a few 407 exceptions. 408 409 The boolean functions take any zero value to be false and a non-zero 410 value to be true. 411 412 There is also a set of binary comparison operators defined as 413 functions: 414 415 eq 416 Returns the boolean truth of arg1 == arg2 417 ne 418 Returns the boolean truth of arg1 != arg2 419 lt 420 Returns the boolean truth of arg1 < arg2 421 le 422 Returns the boolean truth of arg1 <= arg2 423 gt 424 Returns the boolean truth of arg1 > arg2 425 ge 426 Returns the boolean truth of arg1 >= arg2 427 428 For simpler multi-way equality tests, eq (only) accepts two or more 429 arguments and compares the second and subsequent to the first, 430 returning in effect 431 432 arg1==arg2 || arg1==arg3 || arg1==arg4 ... 433 434 (Unlike with || in Go, however, eq is a function call and all the 435 arguments will be evaluated.) 436 437 The comparison functions work on any values whose type Go defines as 438 comparable. For basic types such as integers, the rules are relaxed: 439 size and exact type are ignored, so any integer value, signed or unsigned, 440 may be compared with any other integer value. (The arithmetic value is compared, 441 not the bit pattern, so all negative integers are less than all unsigned integers.) 442 However, as usual, one may not compare an int with a float32 and so on. 443 444 Associated templates 445 446 Each template is named by a string specified when it is created. Also, each 447 template is associated with zero or more other templates that it may invoke by 448 name; such associations are transitive and form a name space of templates. 449 450 A template may use a template invocation to instantiate another associated 451 template; see the explanation of the "template" action above. The name must be 452 that of a template associated with the template that contains the invocation. 453 454 Nested template definitions 455 456 When parsing a template, another template may be defined and associated with the 457 template being parsed. Template definitions must appear at the top level of the 458 template, much like global variables in a Go program. 459 460 The syntax of such definitions is to surround each template declaration with a 461 "define" and "end" action. 462 463 The define action names the template being created by providing a string 464 constant. Here is a simple example: 465 466 {{define "T1"}}ONE{{end}} 467 {{define "T2"}}TWO{{end}} 468 {{define "T3"}}{{template "T1"}} {{template "T2"}}{{end}} 469 {{template "T3"}} 470 471 This defines two templates, T1 and T2, and a third T3 that invokes the other two 472 when it is executed. Finally it invokes T3. If executed this template will 473 produce the text 474 475 ONE TWO 476 477 By construction, a template may reside in only one association. If it's 478 necessary to have a template addressable from multiple associations, the 479 template definition must be parsed multiple times to create distinct *Template 480 values, or must be copied with [Template.Clone] or [Template.AddParseTree]. 481 482 Parse may be called multiple times to assemble the various associated templates; 483 see [ParseFiles], [ParseGlob], [Template.ParseFiles] and [Template.ParseGlob] 484 for simple ways to parse related templates stored in files. 485 486 A template may be executed directly or through [Template.ExecuteTemplate], which executes 487 an associated template identified by name. To invoke our example above, we 488 might write, 489 490 err := tmpl.Execute(os.Stdout, "no data needed") 491 if err != nil { 492 log.Fatalf("execution failed: %s", err) 493 } 494 495 or to invoke a particular template explicitly by name, 496 497 err := tmpl.ExecuteTemplate(os.Stdout, "T2", "no data needed") 498 if err != nil { 499 log.Fatalf("execution failed: %s", err) 500 } 501 502 */ 503 package template 504