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