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 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 466