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 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, 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, the elements will be 106 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 {{break}} 115 The innermost {{range pipeline}} loop is ended early, stopping the 116 current iteration and bypassing all remaining iterations. 117 118 {{continue}} 119 The current iteration of the innermost {{range pipeline}} loop is 120 stopped, and the loop starts the next iteration. 121 122 {{template "name"}} 123 The template with the specified name is executed with nil data. 124 125 {{template "name" pipeline}} 126 The template with the specified name is executed with dot set 127 to the value of the pipeline. 128 129 {{block "name" pipeline}} T1 {{end}} 130 A block is shorthand for defining a template 131 {{define "name"}} T1 {{end}} 132 and then executing it in place 133 {{template "name" pipeline}} 134 The typical use is to define a set of root templates that are 135 then customized by redefining the block templates within. 136 137 {{with pipeline}} T1 {{end}} 138 If the value of the pipeline is empty, no output is generated; 139 otherwise, dot is set to the value of the pipeline and T1 is 140 executed. 141 142 {{with pipeline}} T1 {{else}} T0 {{end}} 143 If the value of the pipeline is empty, dot is unaffected and T0 144 is executed; otherwise, dot is set to the value of the pipeline 145 and T1 is executed. 146 147 {{with pipeline}} T1 {{else with pipeline}} T0 {{end}} 148 To simplify the appearance of with-else chains, the else action 149 of a with may include another with directly; the effect is exactly 150 the same as writing 151 {{with pipeline}} T1 {{else}}{{with pipeline}} T0 {{end}}{{end}} 152 153 154 Arguments 155 156 An argument is a simple value, denoted by one of the following. 157 158 - A boolean, string, character, integer, floating-point, imaginary 159 or complex constant in Go syntax. These behave like Go's untyped 160 constants. Note that, as in Go, whether a large integer constant 161 overflows when assigned or passed to a function can depend on whether 162 the host machine's ints are 32 or 64 bits. 163 - The keyword nil, representing an untyped Go nil. 164 - The character '.' (period): 165 . 166 The result is the value of dot. 167 - A variable name, which is a (possibly empty) alphanumeric string 168 preceded by a dollar sign, such as 169 $piOver2 170 or 171 $ 172 The result is the value of the variable. 173 Variables are described below. 174 - The name of a field of the data, which must be a struct, preceded 175 by a period, such as 176 .Field 177 The result is the value of the field. Field invocations may be 178 chained: 179 .Field1.Field2 180 Fields can also be evaluated on variables, including chaining: 181 $x.Field1.Field2 182 - The name of a key of the data, which must be a map, preceded 183 by a period, such as 184 .Key 185 The result is the map element value indexed by the key. 186 Key invocations may be chained and combined with fields to any 187 depth: 188 .Field1.Key1.Field2.Key2 189 Although the key must be an alphanumeric identifier, unlike with 190 field names they do not need to start with an upper case letter. 191 Keys can also be evaluated on variables, including chaining: 192 $x.key1.key2 193 - The name of a niladic method of the data, preceded by a period, 194 such as 195 .Method 196 The result is the value of invoking the method with dot as the 197 receiver, dot.Method(). Such a method must have one return value (of 198 any type) or two return values, the second of which is an error. 199 If it has two and the returned error is non-nil, execution terminates 200 and an error is returned to the caller as the value of Execute. 201 Method invocations may be chained and combined with fields and keys 202 to any depth: 203 .Field1.Key1.Method1.Field2.Key2.Method2 204 Methods can also be evaluated on variables, including chaining: 205 $x.Method1.Field 206 - The name of a niladic function, such as 207 fun 208 The result is the value of invoking the function, fun(). The return 209 types and values behave as in methods. Functions and function 210 names are described below. 211 - A parenthesized instance of one the above, for grouping. The result 212 may be accessed by a field or map key invocation. 213 print (.F1 arg1) (.F2 arg2) 214 (.StructValuedMethod "arg").Field 215 216 Arguments may evaluate to any type; if they are pointers the implementation 217 automatically indirects to the base type when required. 218 If an evaluation yields a function value, such as a function-valued 219 field of a struct, the function is not invoked automatically, but it 220 can be used as a truth value for an if action and the like. To invoke 221 it, use the call function, defined below. 222 223 Pipelines 224 225 A pipeline is a possibly chained sequence of "commands". A command is a simple 226 value (argument) or a function or method call, possibly with multiple arguments: 227 228 Argument 229 The result is the value of evaluating the argument. 230 .Method [Argument...] 231 The method can be alone or the last element of a chain but, 232 unlike methods in the middle of a chain, it can take arguments. 233 The result is the value of calling the method with the 234 arguments: 235 dot.Method(Argument1, etc.) 236 functionName [Argument...] 237 The result is the value of calling the function associated 238 with the name: 239 function(Argument1, etc.) 240 Functions and function names are described below. 241 242 A pipeline may be "chained" by separating a sequence of commands with pipeline 243 characters '|'. In a chained pipeline, the result of each command is 244 passed as the last argument of the following command. The output of the final 245 command in the pipeline is the value of the pipeline. 246 247 The output of a command will be either one value or two values, the second of 248 which has type error. If that second value is present and evaluates to 249 non-nil, execution terminates and the error is returned to the caller of 250 Execute. 251 252 Variables 253 254 A pipeline inside an action may initialize a variable to capture the result. 255 The initialization has syntax 256 257 $variable := pipeline 258 259 where $variable is the name of the variable. An action that declares a 260 variable produces no output. 261 262 Variables previously declared can also be assigned, using the syntax 263 264 $variable = pipeline 265 266 If a "range" action initializes a variable, the variable is set to the 267 successive elements of the iteration. Also, a "range" may declare two 268 variables, separated by a comma: 269 270 range $index, $element := pipeline 271 272 in which case $index and $element are set to the successive values of the 273 array/slice index or map key and element, respectively. Note that if there is 274 only one variable, it is assigned the element; this is opposite to the 275 convention in Go range clauses. 276 277 A variable's scope extends to the "end" action of the control structure ("if", 278 "with", or "range") in which it is declared, or to the end of the template if 279 there is no such control structure. A template invocation does not inherit 280 variables from the point of its invocation. 281 282 When execution begins, $ is set to the data argument passed to Execute, that is, 283 to the starting value of dot. 284 285 Examples 286 287 Here are some example one-line templates demonstrating pipelines and variables. 288 All produce the quoted word "output": 289 290 {{"\"output\""}} 291 A string constant. 292 {{`"output"`}} 293 A raw string constant. 294 {{printf "%q" "output"}} 295 A function call. 296 {{"output" | printf "%q"}} 297 A function call whose final argument comes from the previous 298 command. 299 {{printf "%q" (print "out" "put")}} 300 A parenthesized argument. 301 {{"put" | printf "%s%s" "out" | printf "%q"}} 302 A more elaborate call. 303 {{"output" | printf "%s" | printf "%q"}} 304 A longer chain. 305 {{with "output"}}{{printf "%q" .}}{{end}} 306 A with action using dot. 307 {{with $x := "output" | printf "%q"}}{{$x}}{{end}} 308 A with action that creates and uses a variable. 309 {{with $x := "output"}}{{printf "%q" $x}}{{end}} 310 A with action that uses the variable in another action. 311 {{with $x := "output"}}{{$x | printf "%q"}}{{end}} 312 The same, but pipelined. 313 314 Functions 315 316 During execution functions are found in two function maps: first in the 317 template, then in the global function map. By default, no functions are defined 318 in the template but the Funcs method can be used to add them. 319 320 Predefined global functions are named as follows. 321 322 and 323 Returns the boolean AND of its arguments by returning the 324 first empty argument or the last argument. That is, 325 "and x y" behaves as "if x then y else x." 326 Evaluation proceeds through the arguments left to right 327 and returns when the result is determined. 328 call 329 Returns the result of calling the first argument, which 330 must be a function, with the remaining arguments as parameters. 331 Thus "call .X.Y 1 2" is, in Go notation, dot.X.Y(1, 2) where 332 Y is a func-valued field, map entry, or the like. 333 The first argument must be the result of an evaluation 334 that yields a value of function type (as distinct from 335 a predefined function such as print). The function must 336 return either one or two result values, the second of which 337 is of type error. If the arguments don't match the function 338 or the returned error value is non-nil, execution stops. 339 html 340 Returns the escaped HTML equivalent of the textual 341 representation of its arguments. This function is unavailable 342 in html/template, with a few exceptions. 343 index 344 Returns the result of indexing its first argument by the 345 following arguments. Thus "index x 1 2 3" is, in Go syntax, 346 x[1][2][3]. Each indexed item must be a map, slice, or array. 347 slice 348 slice returns the result of slicing its first argument by the 349 remaining arguments. Thus "slice x 1 2" is, in Go syntax, x[1:2], 350 while "slice x" is x[:], "slice x 1" is x[1:], and "slice x 1 2 3" 351 is x[1:2:3]. The first argument must be a string, slice, or array. 352 js 353 Returns the escaped JavaScript equivalent of the textual 354 representation of its arguments. 355 len 356 Returns the integer length of its argument. 357 not 358 Returns the boolean negation of its single argument. 359 or 360 Returns the boolean OR of its arguments by returning the 361 first non-empty argument or the last argument, that is, 362 "or x y" behaves as "if x then x else y". 363 Evaluation proceeds through the arguments left to right 364 and returns when the result is determined. 365 print 366 An alias for fmt.Sprint 367 printf 368 An alias for fmt.Sprintf 369 println 370 An alias for fmt.Sprintln 371 urlquery 372 Returns the escaped value of the textual representation of 373 its arguments in a form suitable for embedding in a URL query. 374 This function is unavailable in html/template, with a few 375 exceptions. 376 377 The boolean functions take any zero value to be false and a non-zero 378 value to be true. 379 380 There is also a set of binary comparison operators defined as 381 functions: 382 383 eq 384 Returns the boolean truth of arg1 == arg2 385 ne 386 Returns the boolean truth of arg1 != arg2 387 lt 388 Returns the boolean truth of arg1 < arg2 389 le 390 Returns the boolean truth of arg1 <= arg2 391 gt 392 Returns the boolean truth of arg1 > arg2 393 ge 394 Returns the boolean truth of arg1 >= arg2 395 396 For simpler multi-way equality tests, eq (only) accepts two or more 397 arguments and compares the second and subsequent to the first, 398 returning in effect 399 400 arg1==arg2 || arg1==arg3 || arg1==arg4 ... 401 402 (Unlike with || in Go, however, eq is a function call and all the 403 arguments will be evaluated.) 404 405 The comparison functions work on any values whose type Go defines as 406 comparable. For basic types such as integers, the rules are relaxed: 407 size and exact type are ignored, so any integer value, signed or unsigned, 408 may be compared with any other integer value. (The arithmetic value is compared, 409 not the bit pattern, so all negative integers are less than all unsigned integers.) 410 However, as usual, one may not compare an int with a float32 and so on. 411 412 Associated templates 413 414 Each template is named by a string specified when it is created. Also, each 415 template is associated with zero or more other templates that it may invoke by 416 name; such associations are transitive and form a name space of templates. 417 418 A template may use a template invocation to instantiate another associated 419 template; see the explanation of the "template" action above. The name must be 420 that of a template associated with the template that contains the invocation. 421 422 Nested template definitions 423 424 When parsing a template, another template may be defined and associated with the 425 template being parsed. Template definitions must appear at the top level of the 426 template, much like global variables in a Go program. 427 428 The syntax of such definitions is to surround each template declaration with a 429 "define" and "end" action. 430 431 The define action names the template being created by providing a string 432 constant. Here is a simple example: 433 434 {{define "T1"}}ONE{{end}} 435 {{define "T2"}}TWO{{end}} 436 {{define "T3"}}{{template "T1"}} {{template "T2"}}{{end}} 437 {{template "T3"}} 438 439 This defines two templates, T1 and T2, and a third T3 that invokes the other two 440 when it is executed. Finally it invokes T3. If executed this template will 441 produce the text 442 443 ONE TWO 444 445 By construction, a template may reside in only one association. If it's 446 necessary to have a template addressable from multiple associations, the 447 template definition must be parsed multiple times to create distinct *Template 448 values, or must be copied with [Template.Clone] or [Template.AddParseTree]. 449 450 Parse may be called multiple times to assemble the various associated templates; 451 see [ParseFiles], [ParseGlob], [Template.ParseFiles] and [Template.ParseGlob] 452 for simple ways to parse related templates stored in files. 453 454 A template may be executed directly or through [Template.ExecuteTemplate], which executes 455 an associated template identified by name. To invoke our example above, we 456 might write, 457 458 err := tmpl.Execute(os.Stdout, "no data needed") 459 if err != nil { 460 log.Fatalf("execution failed: %s", err) 461 } 462 463 or to invoke a particular template explicitly by name, 464 465 err := tmpl.ExecuteTemplate(os.Stdout, "T2", "no data needed") 466 if err != nil { 467 log.Fatalf("execution failed: %s", err) 468 } 469 470 */ 471 package template 472