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