Source file src/pkg/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 Actions may not span newlines, although comments can. 22 23 Once constructed, a template may be executed safely in parallel. 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 Actions 40 41 Here is the list of actions. "Arguments" and "pipelines" are evaluations of 42 data, defined in detail below. 43 44 */ 45 // {{/* a comment */}} 46 // A comment; discarded. May contain newlines. 47 // Comments do not nest. 48 /* 49 50 {{pipeline}} 51 The default textual representation of the value of the pipeline 52 is copied to the output. 53 54 {{if pipeline}} T1 {{end}} 55 If the value of the pipeline is empty, no output is generated; 56 otherwise, T1 is executed. The empty values are false, 0, any 57 nil pointer or interface value, and any array, slice, map, or 58 string of length zero. 59 Dot is unaffected. 60 61 {{if pipeline}} T1 {{else}} T0 {{end}} 62 If the value of the pipeline is empty, T0 is executed; 63 otherwise, T1 is executed. Dot is unaffected. 64 65 {{range pipeline}} T1 {{end}} 66 The value of the pipeline must be an array, slice, or map. If 67 the value of the pipeline has length zero, nothing is output; 68 otherwise, dot is set to the successive elements of the array, 69 slice, or map and T1 is executed. If the value is a map and the 70 keys are of basic type with a defined order ("comparable"), the 71 elements will be visited in sorted key order. 72 73 {{range pipeline}} T1 {{else}} T0 {{end}} 74 The value of the pipeline must be an array, slice, or map. If 75 the value of the pipeline has length zero, dot is unaffected and 76 T0 is executed; otherwise, dot is set to the successive elements 77 of the array, slice, or map and T1 is executed. 78 79 {{template "name"}} 80 The template with the specified name is executed with nil data. 81 82 {{template "name" pipeline}} 83 The template with the specified name is executed with dot set 84 to the value of the pipeline. 85 86 {{with pipeline}} T1 {{end}} 87 If the value of the pipeline is empty, no output is generated; 88 otherwise, dot is set to the value of the pipeline and T1 is 89 executed. 90 91 {{with pipeline}} T1 {{else}} T0 {{end}} 92 If the value of the pipeline is empty, dot is unaffected and T0 93 is executed; otherwise, dot is set to the value of the pipeline 94 and T1 is executed. 95 96 Arguments 97 98 An argument is a simple value, denoted by one of the following. 99 100 - A boolean, string, character, integer, floating-point, imaginary 101 or complex constant in Go syntax. These behave like Go's untyped 102 constants, although raw strings may not span newlines. 103 - The character '.' (period): 104 . 105 The result is the value of dot. 106 - A variable name, which is a (possibly empty) alphanumeric string 107 preceded by a dollar sign, such as 108 $piOver2 109 or 110 $ 111 The result is the value of the variable. 112 Variables are described below. 113 - The name of a field of the data, which must be a struct, preceded 114 by a period, such as 115 .Field 116 The result is the value of the field. Field invocations may be 117 chained: 118 .Field1.Field2 119 Fields can also be evaluated on variables, including chaining: 120 $x.Field1.Field2 121 - The name of a key of the data, which must be a map, preceded 122 by a period, such as 123 .Key 124 The result is the map element value indexed by the key. 125 Key invocations may be chained and combined with fields to any 126 depth: 127 .Field1.Key1.Field2.Key2 128 Although the key must be an alphanumeric identifier, unlike with 129 field names they do not need to start with an upper case letter. 130 Keys can also be evaluated on variables, including chaining: 131 $x.key1.key2 132 - The name of a niladic method of the data, preceded by a period, 133 such as 134 .Method 135 The result is the value of invoking the method with dot as the 136 receiver, dot.Method(). Such a method must have one return value (of 137 any type) or two return values, the second of which is an error. 138 If it has two and the returned error is non-nil, execution terminates 139 and an error is returned to the caller as the value of Execute. 140 Method invocations may be chained and combined with fields and keys 141 to any depth: 142 .Field1.Key1.Method1.Field2.Key2.Method2 143 Methods can also be evaluated on variables, including chaining: 144 $x.Method1.Field 145 - The name of a niladic function, such as 146 fun 147 The result is the value of invoking the function, fun(). The return 148 types and values behave as in methods. Functions and function 149 names are described below. 150 151 Arguments may evaluate to any type; if they are pointers the implementation 152 automatically indirects to the base type when required. 153 If an evaluation yields a function value, such as a function-valued 154 field of a struct, the function is not invoked automatically, but it 155 can be used as a truth value for an if action and the like. To invoke 156 it, use the call function, defined below. 157 158 A pipeline is a possibly chained sequence of "commands". A command is a simple 159 value (argument) or a function or method call, possibly with multiple arguments: 160 161 Argument 162 The result is the value of evaluating the argument. 163 .Method [Argument...] 164 The method can be alone or the last element of a chain but, 165 unlike methods in the middle of a chain, it can take arguments. 166 The result is the value of calling the method with the 167 arguments: 168 dot.Method(Argument1, etc.) 169 functionName [Argument...] 170 The result is the value of calling the function associated 171 with the name: 172 function(Argument1, etc.) 173 Functions and function names are described below. 174 175 Pipelines 176 177 A pipeline may be "chained" by separating a sequence of commands with pipeline 178 characters '|'. In a chained pipeline, the result of the each command is 179 passed as the last argument of the following command. The output of the final 180 command in the pipeline is the value of the pipeline. 181 182 The output of a command will be either one value or two values, the second of 183 which has type error. If that second value is present and evaluates to 184 non-nil, execution terminates and the error is returned to the caller of 185 Execute. 186 187 Variables 188 189 A pipeline inside an action may initialize a variable to capture the result. 190 The initialization has syntax 191 192 $variable := pipeline 193 194 where $variable is the name of the variable. An action that declares a 195 variable produces no output. 196 197 If a "range" action initializes a variable, the variable is set to the 198 successive elements of the iteration. Also, a "range" may declare two 199 variables, separated by a comma: 200 201 $index, $element := pipeline 202 203 in which case $index and $element are set to the successive values of the 204 array/slice index or map key and element, respectively. Note that if there is 205 only one variable, it is assigned the element; this is opposite to the 206 convention in Go range clauses. 207 208 A variable's scope extends to the "end" action of the control structure ("if", 209 "with", or "range") in which it is declared, or to the end of the template if 210 there is no such control structure. A template invocation does not inherit 211 variables from the point of its invocation. 212 213 When execution begins, $ is set to the data argument passed to Execute, that is, 214 to the starting value of dot. 215 216 Examples 217 218 Here are some example one-line templates demonstrating pipelines and variables. 219 All produce the quoted word "output": 220 221 {{"\"output\""}} 222 A string constant. 223 {{`"output"`}} 224 A raw string constant. 225 {{printf "%q" "output"}} 226 A function call. 227 {{"output" | printf "%q"}} 228 A function call whose final argument comes from the previous 229 command. 230 {{"put" | printf "%s%s" "out" | printf "%q"}} 231 A more elaborate call. 232 {{"output" | printf "%s" | printf "%q"}} 233 A longer chain. 234 {{with "output"}}{{printf "%q" .}}{{end}} 235 A with action using dot. 236 {{with $x := "output" | printf "%q"}}{{$x}}{{end}} 237 A with action that creates and uses a variable. 238 {{with $x := "output"}}{{printf "%q" $x}}{{end}} 239 A with action that uses the variable in another action. 240 {{with $x := "output"}}{{$x | printf "%q"}}{{end}} 241 The same, but pipelined. 242 243 Functions 244 245 During execution functions are found in two function maps: first in the 246 template, then in the global function map. By default, no functions are defined 247 in the template but the Funcs method can be used to add them. 248 249 Predefined global functions are named as follows. 250 251 and 252 Returns the boolean AND of its arguments by returning the 253 first empty argument or the last argument, that is, 254 "and x y" behaves as "if x then y else x". All the 255 arguments are evaluated. 256 call 257 Returns the result of calling the first argument, which 258 must be a function, with the remaining arguments as parameters. 259 Thus "call .X.Y 1 2" is, in Go notation, dot.X.Y(1, 2) where 260 Y is a func-valued field, map entry, or the like. 261 The first argument must be the result of an evaluation 262 that yields a value of function type (as distinct from 263 a predefined function such as print). The function must 264 return either one or two result values, the second of which 265 is of type error. If the arguments don't match the function 266 or the returned error value is non-nil, execution stops. 267 html 268 Returns the escaped HTML equivalent of the textual 269 representation of its arguments. 270 index 271 Returns the result of indexing its first argument by the 272 following arguments. Thus "index x 1 2 3" is, in Go syntax, 273 x[1][2][3]. Each indexed item must be a map, slice, or array. 274 js 275 Returns the escaped JavaScript equivalent of the textual 276 representation of its arguments. 277 len 278 Returns the integer length of its argument. 279 not 280 Returns the boolean negation of its single argument. 281 or 282 Returns the boolean OR of its arguments by returning the 283 first non-empty argument or the last argument, that is, 284 "or x y" behaves as "if x then x else y". All the 285 arguments are evaluated. 286 print 287 An alias for fmt.Sprint 288 printf 289 An alias for fmt.Sprintf 290 println 291 An alias for fmt.Sprintln 292 urlquery 293 Returns the escaped value of the textual representation of 294 its arguments in a form suitable for embedding in a URL query. 295 296 The boolean functions take any zero value to be false and a non-zero value to 297 be true. 298 299 Associated templates 300 301 Each template is named by a string specified when it is created. Also, each 302 template is associated with zero or more other templates that it may invoke by 303 name; such associations are transitive and form a name space of templates. 304 305 A template may use a template invocation to instantiate another associated 306 template; see the explanation of the "template" action above. The name must be 307 that of a template associated with the template that contains the invocation. 308 309 Nested template definitions 310 311 When parsing a template, another template may be defined and associated with the 312 template being parsed. Template definitions must appear at the top level of the 313 template, much like global variables in a Go program. 314 315 The syntax of such definitions is to surround each template declaration with a 316 "define" and "end" action. 317 318 The define action names the template being created by providing a string 319 constant. Here is a simple example: 320 321 `{{define "T1"}}ONE{{end}} 322 {{define "T2"}}TWO{{end}} 323 {{define "T3"}}{{template "T1"}} {{template "T2"}}{{end}} 324 {{template "T3"}}` 325 326 This defines two templates, T1 and T2, and a third T3 that invokes the other two 327 when it is executed. Finally it invokes T3. If executed this template will 328 produce the text 329 330 ONE TWO 331 332 By construction, a template may reside in only one association. If it's 333 necessary to have a template addressable from multiple associations, the 334 template definition must be parsed multiple times to create distinct *Template 335 values, or must be copied with the Clone or AddParseTree method. 336 337 Parse may be called multiple times to assemble the various associated templates; 338 see the ParseFiles and ParseGlob functions and methods for simple ways to parse 339 related templates stored in files. 340 341 A template may be executed directly or through ExecuteTemplate, which executes 342 an associated template identified by name. To invoke our example above, we 343 might write, 344 345 err := tmpl.Execute(os.Stdout, "no data needed") 346 if err != nil { 347 log.Fatalf("execution failed: %s", err) 348 } 349 350 or to invoke a particular template explicitly by name, 351 352 err := tmpl.ExecuteTemplate(os.Stdout, "T2", "no data needed") 353 if err != nil { 354 log.Fatalf("execution failed: %s", err) 355 } 356 357 */ 358 package template