Source file src/text/template/doc.go
Documentation: text/template
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 appears 44 on standard output when the program is run. 45 46 However, to aid in formatting template source code, if an action's left delimiter 47 (by default "{{") is followed immediately by a minus sign and ASCII space character 48 ("{{- "), all trailing white space is trimmed from the immediately preceding text. 49 Similarly, if the right delimiter ("}}") is preceded by a space and minus sign 50 (" -}}"), all leading white space is trimmed from the immediately following text. 51 In these trim markers, the ASCII space must be present; "{{-3}}" parses as an 52 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 {{template "name"}} 115 The template with the specified name is executed with nil data. 116 117 {{template "name" pipeline}} 118 The template with the specified name is executed with dot set 119 to the value of the pipeline. 120 121 {{block "name" pipeline}} T1 {{end}} 122 A block is shorthand for defining a template 123 {{define "name"}} T1 {{end}} 124 and then executing it in place 125 {{template "name" pipeline}} 126 The typical use is to define a set of root templates that are 127 then customized by redefining the block templates within. 128 129 {{with pipeline}} T1 {{end}} 130 If the value of the pipeline is empty, no output is generated; 131 otherwise, dot is set to the value of the pipeline and T1 is 132 executed. 133 134 {{with pipeline}} T1 {{else}} T0 {{end}} 135 If the value of the pipeline is empty, dot is unaffected and T0 136 is executed; otherwise, dot is set to the value of the pipeline 137 and T1 is executed. 138 139 Arguments 140 141 An argument is a simple value, denoted by one of the following. 142 143 - A boolean, string, character, integer, floating-point, imaginary 144 or complex constant in Go syntax. These behave like Go's untyped 145 constants. Note that, as in Go, whether a large integer constant 146 overflows when assigned or passed to a function can depend on whether 147 the host machine's ints are 32 or 64 bits. 148 - The keyword nil, representing an untyped Go nil. 149 - The character '.' (period): 150 . 151 The result is the value of dot. 152 - A variable name, which is a (possibly empty) alphanumeric string 153 preceded by a dollar sign, such as 154 $piOver2 155 or 156 $ 157 The result is the value of the variable. 158 Variables are described below. 159 - The name of a field of the data, which must be a struct, preceded 160 by a period, such as 161 .Field 162 The result is the value of the field. Field invocations may be 163 chained: 164 .Field1.Field2 165 Fields can also be evaluated on variables, including chaining: 166 $x.Field1.Field2 167 - The name of a key of the data, which must be a map, preceded 168 by a period, such as 169 .Key 170 The result is the map element value indexed by the key. 171 Key invocations may be chained and combined with fields to any 172 depth: 173 .Field1.Key1.Field2.Key2 174 Although the key must be an alphanumeric identifier, unlike with 175 field names they do not need to start with an upper case letter. 176 Keys can also be evaluated on variables, including chaining: 177 $x.key1.key2 178 - The name of a niladic method of the data, preceded by a period, 179 such as 180 .Method 181 The result is the value of invoking the method with dot as the 182 receiver, dot.Method(). Such a method must have one return value (of 183 any type) or two return values, the second of which is an error. 184 If it has two and the returned error is non-nil, execution terminates 185 and an error is returned to the caller as the value of Execute. 186 Method invocations may be chained and combined with fields and keys 187 to any depth: 188 .Field1.Key1.Method1.Field2.Key2.Method2 189 Methods can also be evaluated on variables, including chaining: 190 $x.Method1.Field 191 - The name of a niladic function, such as 192 fun 193 The result is the value of invoking the function, fun(). The return 194 types and values behave as in methods. Functions and function 195 names are described below. 196 - A parenthesized instance of one the above, for grouping. The result 197 may be accessed by a field or map key invocation. 198 print (.F1 arg1) (.F2 arg2) 199 (.StructValuedMethod "arg").Field 200 201 Arguments may evaluate to any type; if they are pointers the implementation 202 automatically indirects to the base type when required. 203 If an evaluation yields a function value, such as a function-valued 204 field of a struct, the function is not invoked automatically, but it 205 can be used as a truth value for an if action and the like. To invoke 206 it, use the call function, defined below. 207 208 Pipelines 209 210 A pipeline is a possibly chained sequence of "commands". A command is a simple 211 value (argument) or a function or method call, possibly with multiple arguments: 212 213 Argument 214 The result is the value of evaluating the argument. 215 .Method [Argument...] 216 The method can be alone or the last element of a chain but, 217 unlike methods in the middle of a chain, it can take arguments. 218 The result is the value of calling the method with the 219 arguments: 220 dot.Method(Argument1, etc.) 221 functionName [Argument...] 222 The result is the value of calling the function associated 223 with the name: 224 function(Argument1, etc.) 225 Functions and function names are described below. 226 227 A pipeline may be "chained" by separating a sequence of commands with pipeline 228 characters '|'. In a chained pipeline, the result of each command is 229 passed as the last argument of the following command. The output of the final 230 command in the pipeline is the value of the pipeline. 231 232 The output of a command will be either one value or two values, the second of 233 which has type error. If that second value is present and evaluates to 234 non-nil, execution terminates and the error is returned to the caller of 235 Execute. 236 237 Variables 238 239 A pipeline inside an action may initialize a variable to capture the result. 240 The initialization has syntax 241 242 $variable := pipeline 243 244 where $variable is the name of the variable. An action that declares a 245 variable produces no output. 246 247 Variables previously declared can also be assigned, using the syntax 248 249 $variable = pipeline 250 251 If a "range" action initializes a variable, the variable is set to the 252 successive elements of the iteration. Also, a "range" may declare two 253 variables, separated by a comma: 254 255 range $index, $element := pipeline 256 257 in which case $index and $element are set to the successive values of the 258 array/slice index or map key and element, respectively. Note that if there is 259 only one variable, it is assigned the element; this is opposite to the 260 convention in Go range clauses. 261 262 A variable's scope extends to the "end" action of the control structure ("if", 263 "with", or "range") in which it is declared, or to the end of the template if 264 there is no such control structure. A template invocation does not inherit 265 variables from the point of its invocation. 266 267 When execution begins, $ is set to the data argument passed to Execute, that is, 268 to the starting value of dot. 269 270 Examples 271 272 Here are some example one-line templates demonstrating pipelines and variables. 273 All produce the quoted word "output": 274 275 {{"\"output\""}} 276 A string constant. 277 {{`"output"`}} 278 A raw string constant. 279 {{printf "%q" "output"}} 280 A function call. 281 {{"output" | printf "%q"}} 282 A function call whose final argument comes from the previous 283 command. 284 {{printf "%q" (print "out" "put")}} 285 A parenthesized argument. 286 {{"put" | printf "%s%s" "out" | printf "%q"}} 287 A more elaborate call. 288 {{"output" | printf "%s" | printf "%q"}} 289 A longer chain. 290 {{with "output"}}{{printf "%q" .}}{{end}} 291 A with action using dot. 292 {{with $x := "output" | printf "%q"}}{{$x}}{{end}} 293 A with action that creates and uses a variable. 294 {{with $x := "output"}}{{printf "%q" $x}}{{end}} 295 A with action that uses the variable in another action. 296 {{with $x := "output"}}{{$x | printf "%q"}}{{end}} 297 The same, but pipelined. 298 299 Functions 300 301 During execution functions are found in two function maps: first in the 302 template, then in the global function map. By default, no functions are defined 303 in the template but the Funcs method can be used to add them. 304 305 Predefined global functions are named as follows. 306 307 and 308 Returns the boolean AND of its arguments by returning the 309 first empty argument or the last argument, that is, 310 "and x y" behaves as "if x then y else x". All the 311 arguments are evaluated. 312 call 313 Returns the result of calling the first argument, which 314 must be a function, with the remaining arguments as parameters. 315 Thus "call .X.Y 1 2" is, in Go notation, dot.X.Y(1, 2) where 316 Y is a func-valued field, map entry, or the like. 317 The first argument must be the result of an evaluation 318 that yields a value of function type (as distinct from 319 a predefined function such as print). The function must 320 return either one or two result values, the second of which 321 is of type error. If the arguments don't match the function 322 or the returned error value is non-nil, execution stops. 323 html 324 Returns the escaped HTML equivalent of the textual 325 representation of its arguments. This function is unavailable 326 in html/template, with a few exceptions. 327 index 328 Returns the result of indexing its first argument by the 329 following arguments. Thus "index x 1 2 3" is, in Go syntax, 330 x[1][2][3]. Each indexed item must be a map, slice, or array. 331 slice 332 slice returns the result of slicing its first argument by the 333 remaining arguments. Thus "slice x 1 2" is, in Go syntax, x[1:2], 334 while "slice x" is x[:], "slice x 1" is x[1:], and "slice x 1 2 3" 335 is x[1:2:3]. The first argument must be a string, slice, or array. 336 js 337 Returns the escaped JavaScript equivalent of the textual 338 representation of its arguments. 339 len 340 Returns the integer length of its argument. 341 not 342 Returns the boolean negation of its single argument. 343 or 344 Returns the boolean OR of its arguments by returning the 345 first non-empty argument or the last argument, that is, 346 "or x y" behaves as "if x then x else y". All the 347 arguments are evaluated. 348 print 349 An alias for fmt.Sprint 350 printf 351 An alias for fmt.Sprintf 352 println 353 An alias for fmt.Sprintln 354 urlquery 355 Returns the escaped value of the textual representation of 356 its arguments in a form suitable for embedding in a URL query. 357 This function is unavailable in html/template, with a few 358 exceptions. 359 360 The boolean functions take any zero value to be false and a non-zero 361 value to be true. 362 363 There is also a set of binary comparison operators defined as 364 functions: 365 366 eq 367 Returns the boolean truth of arg1 == arg2 368 ne 369 Returns the boolean truth of arg1 != arg2 370 lt 371 Returns the boolean truth of arg1 < arg2 372 le 373 Returns the boolean truth of arg1 <= arg2 374 gt 375 Returns the boolean truth of arg1 > arg2 376 ge 377 Returns the boolean truth of arg1 >= arg2 378 379 For simpler multi-way equality tests, eq (only) accepts two or more 380 arguments and compares the second and subsequent to the first, 381 returning in effect 382 383 arg1==arg2 || arg1==arg3 || arg1==arg4 ... 384 385 (Unlike with || in Go, however, eq is a function call and all the 386 arguments will be evaluated.) 387 388 The comparison functions work on any values whose type Go defines as 389 comparable. For basic types such as integers, the rules are relaxed: 390 size and exact type are ignored, so any integer value, signed or unsigned, 391 may be compared with any other integer value. (The arithmetic value is compared, 392 not the bit pattern, so all negative integers are less than all unsigned integers.) 393 However, as usual, one may not compare an int with a float32 and so on. 394 395 Associated templates 396 397 Each template is named by a string specified when it is created. Also, each 398 template is associated with zero or more other templates that it may invoke by 399 name; such associations are transitive and form a name space of templates. 400 401 A template may use a template invocation to instantiate another associated 402 template; see the explanation of the "template" action above. The name must be 403 that of a template associated with the template that contains the invocation. 404 405 Nested template definitions 406 407 When parsing a template, another template may be defined and associated with the 408 template being parsed. Template definitions must appear at the top level of the 409 template, much like global variables in a Go program. 410 411 The syntax of such definitions is to surround each template declaration with a 412 "define" and "end" action. 413 414 The define action names the template being created by providing a string 415 constant. Here is a simple example: 416 417 `{{define "T1"}}ONE{{end}} 418 {{define "T2"}}TWO{{end}} 419 {{define "T3"}}{{template "T1"}} {{template "T2"}}{{end}} 420 {{template "T3"}}` 421 422 This defines two templates, T1 and T2, and a third T3 that invokes the other two 423 when it is executed. Finally it invokes T3. If executed this template will 424 produce the text 425 426 ONE TWO 427 428 By construction, a template may reside in only one association. If it's 429 necessary to have a template addressable from multiple associations, the 430 template definition must be parsed multiple times to create distinct *Template 431 values, or must be copied with the Clone or AddParseTree method. 432 433 Parse may be called multiple times to assemble the various associated templates; 434 see the ParseFiles and ParseGlob functions and methods for simple ways to parse 435 related templates stored in files. 436 437 A template may be executed directly or through ExecuteTemplate, which executes 438 an associated template identified by name. To invoke our example above, we 439 might write, 440 441 err := tmpl.Execute(os.Stdout, "no data needed") 442 if err != nil { 443 log.Fatalf("execution failed: %s", err) 444 } 445 446 or to invoke a particular template explicitly by name, 447 448 err := tmpl.ExecuteTemplate(os.Stdout, "T2", "no data needed") 449 if err != nil { 450 log.Fatalf("execution failed: %s", err) 451 } 452 453 */ 454 package template 455