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