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diff --git a/src/text/template/doc.go b/src/text/template/doc.go new file mode 100644 index 0000000..7b30294 --- /dev/null +++ b/src/text/template/doc.go @@ -0,0 +1,455 @@ +// Copyright 2011 The Go Authors. All rights reserved. +// Use of this source code is governed by a BSD-style +// license that can be found in the LICENSE file. + +/* +Package template implements data-driven templates for generating textual output. + +To generate HTML output, see package html/template, which has the same interface +as this package but automatically secures HTML output against certain attacks. + +Templates are executed by applying them to a data structure. Annotations in the +template refer to elements of the data structure (typically a field of a struct +or a key in a map) to control execution and derive values to be displayed. +Execution of the template walks the structure and sets the cursor, represented +by a period '.' and called "dot", to the value at the current location in the +structure as execution proceeds. + +The input text for a template is UTF-8-encoded text in any format. +"Actions"--data evaluations or control structures--are delimited by +"{{" and "}}"; all text outside actions is copied to the output unchanged. +Except for raw strings, actions may not span newlines, although comments can. + +Once parsed, a template may be executed safely in parallel, although if parallel +executions share a Writer the output may be interleaved. + +Here is a trivial example that prints "17 items are made of wool". + + type Inventory struct { + Material string + Count uint + } + sweaters := Inventory{"wool", 17} + tmpl, err := template.New("test").Parse("{{.Count}} items are made of {{.Material}}") + if err != nil { panic(err) } + err = tmpl.Execute(os.Stdout, sweaters) + if err != nil { panic(err) } + +More intricate examples appear below. + +Text and spaces + +By default, all text between actions is copied verbatim when the template is +executed. For example, the string " items are made of " in the example above +appears on standard output when the program is run. + +However, to aid in formatting template source code, if an action's left +delimiter (by default "{{") is followed immediately by a minus sign and white +space, all trailing white space is trimmed from the immediately preceding text. +Similarly, if the right delimiter ("}}") is preceded by white space and a minus +sign, all leading white space is trimmed from the immediately following text. +In these trim markers, the white space must be present: +"{{- 3}}" is like "{{3}}" but trims the immediately preceding text, while +"{{-3}}" parses as an action containing the number -3. + +For instance, when executing the template whose source is + + "{{23 -}} < {{- 45}}" + +the generated output would be + + "23<45" + +For this trimming, the definition of white space characters is the same as in Go: +space, horizontal tab, carriage return, and newline. + +Actions + +Here is the list of actions. "Arguments" and "pipelines" are evaluations of +data, defined in detail in the corresponding sections that follow. + +*/ +// {{/* a comment */}} +// {{- /* a comment with white space trimmed from preceding and following text */ -}} +// A comment; discarded. May contain newlines. +// Comments do not nest and must start and end at the +// delimiters, as shown here. +/* + + {{pipeline}} + The default textual representation (the same as would be + printed by fmt.Print) of the value of the pipeline is copied + to the output. + + {{if pipeline}} T1 {{end}} + If the value of the pipeline is empty, no output is generated; + otherwise, T1 is executed. The empty values are false, 0, any + nil pointer or interface value, and any array, slice, map, or + string of length zero. + Dot is unaffected. + + {{if pipeline}} T1 {{else}} T0 {{end}} + If the value of the pipeline is empty, T0 is executed; + otherwise, T1 is executed. Dot is unaffected. + + {{if pipeline}} T1 {{else if pipeline}} T0 {{end}} + To simplify the appearance of if-else chains, the else action + of an if may include another if directly; the effect is exactly + the same as writing + {{if pipeline}} T1 {{else}}{{if pipeline}} T0 {{end}}{{end}} + + {{range pipeline}} T1 {{end}} + The value of the pipeline must be an array, slice, map, or channel. + If the value of the pipeline has length zero, nothing is output; + otherwise, dot is set to the successive elements of the array, + slice, or map and T1 is executed. If the value is a map and the + keys are of basic type with a defined order, the elements will be + visited in sorted key order. + + {{range pipeline}} T1 {{else}} T0 {{end}} + The value of the pipeline must be an array, slice, map, or channel. + If the value of the pipeline has length zero, dot is unaffected and + T0 is executed; otherwise, dot is set to the successive elements + of the array, slice, or map and T1 is executed. + + {{template "name"}} + The template with the specified name is executed with nil data. + + {{template "name" pipeline}} + The template with the specified name is executed with dot set + to the value of the pipeline. + + {{block "name" pipeline}} T1 {{end}} + A block is shorthand for defining a template + {{define "name"}} T1 {{end}} + and then executing it in place + {{template "name" pipeline}} + The typical use is to define a set of root templates that are + then customized by redefining the block templates within. + + {{with pipeline}} T1 {{end}} + If the value of the pipeline is empty, no output is generated; + otherwise, dot is set to the value of the pipeline and T1 is + executed. + + {{with pipeline}} T1 {{else}} T0 {{end}} + If the value of the pipeline is empty, dot is unaffected and T0 + is executed; otherwise, dot is set to the value of the pipeline + and T1 is executed. + +Arguments + +An argument is a simple value, denoted by one of the following. + + - A boolean, string, character, integer, floating-point, imaginary + or complex constant in Go syntax. These behave like Go's untyped + constants. Note that, as in Go, whether a large integer constant + overflows when assigned or passed to a function can depend on whether + the host machine's ints are 32 or 64 bits. + - The keyword nil, representing an untyped Go nil. + - The character '.' (period): + . + The result is the value of dot. + - A variable name, which is a (possibly empty) alphanumeric string + preceded by a dollar sign, such as + $piOver2 + or + $ + The result is the value of the variable. + Variables are described below. + - The name of a field of the data, which must be a struct, preceded + by a period, such as + .Field + The result is the value of the field. Field invocations may be + chained: + .Field1.Field2 + Fields can also be evaluated on variables, including chaining: + $x.Field1.Field2 + - The name of a key of the data, which must be a map, preceded + by a period, such as + .Key + The result is the map element value indexed by the key. + Key invocations may be chained and combined with fields to any + depth: + .Field1.Key1.Field2.Key2 + Although the key must be an alphanumeric identifier, unlike with + field names they do not need to start with an upper case letter. + Keys can also be evaluated on variables, including chaining: + $x.key1.key2 + - The name of a niladic method of the data, preceded by a period, + such as + .Method + The result is the value of invoking the method with dot as the + receiver, dot.Method(). Such a method must have one return value (of + any type) or two return values, the second of which is an error. + If it has two and the returned error is non-nil, execution terminates + and an error is returned to the caller as the value of Execute. + Method invocations may be chained and combined with fields and keys + to any depth: + .Field1.Key1.Method1.Field2.Key2.Method2 + Methods can also be evaluated on variables, including chaining: + $x.Method1.Field + - The name of a niladic function, such as + fun + The result is the value of invoking the function, fun(). The return + types and values behave as in methods. Functions and function + names are described below. + - A parenthesized instance of one the above, for grouping. The result + may be accessed by a field or map key invocation. + print (.F1 arg1) (.F2 arg2) + (.StructValuedMethod "arg").Field + +Arguments may evaluate to any type; if they are pointers the implementation +automatically indirects to the base type when required. +If an evaluation yields a function value, such as a function-valued +field of a struct, the function is not invoked automatically, but it +can be used as a truth value for an if action and the like. To invoke +it, use the call function, defined below. + +Pipelines + +A pipeline is a possibly chained sequence of "commands". A command is a simple +value (argument) or a function or method call, possibly with multiple arguments: + + Argument + The result is the value of evaluating the argument. + .Method [Argument...] + The method can be alone or the last element of a chain but, + unlike methods in the middle of a chain, it can take arguments. + The result is the value of calling the method with the + arguments: + dot.Method(Argument1, etc.) + functionName [Argument...] + The result is the value of calling the function associated + with the name: + function(Argument1, etc.) + Functions and function names are described below. + +A pipeline may be "chained" by separating a sequence of commands with pipeline +characters '|'. In a chained pipeline, the result of each command is +passed as the last argument of the following command. The output of the final +command in the pipeline is the value of the pipeline. + +The output of a command will be either one value or two values, the second of +which has type error. If that second value is present and evaluates to +non-nil, execution terminates and the error is returned to the caller of +Execute. + +Variables + +A pipeline inside an action may initialize a variable to capture the result. +The initialization has syntax + + $variable := pipeline + +where $variable is the name of the variable. An action that declares a +variable produces no output. + +Variables previously declared can also be assigned, using the syntax + + $variable = pipeline + +If a "range" action initializes a variable, the variable is set to the +successive elements of the iteration. Also, a "range" may declare two +variables, separated by a comma: + + range $index, $element := pipeline + +in which case $index and $element are set to the successive values of the +array/slice index or map key and element, respectively. Note that if there is +only one variable, it is assigned the element; this is opposite to the +convention in Go range clauses. + +A variable's scope extends to the "end" action of the control structure ("if", +"with", or "range") in which it is declared, or to the end of the template if +there is no such control structure. A template invocation does not inherit +variables from the point of its invocation. + +When execution begins, $ is set to the data argument passed to Execute, that is, +to the starting value of dot. + +Examples + +Here are some example one-line templates demonstrating pipelines and variables. +All produce the quoted word "output": + + {{"\"output\""}} + A string constant. + {{`"output"`}} + A raw string constant. + {{printf "%q" "output"}} + A function call. + {{"output" | printf "%q"}} + A function call whose final argument comes from the previous + command. + {{printf "%q" (print "out" "put")}} + A parenthesized argument. + {{"put" | printf "%s%s" "out" | printf "%q"}} + A more elaborate call. + {{"output" | printf "%s" | printf "%q"}} + A longer chain. + {{with "output"}}{{printf "%q" .}}{{end}} + A with action using dot. + {{with $x := "output" | printf "%q"}}{{$x}}{{end}} + A with action that creates and uses a variable. + {{with $x := "output"}}{{printf "%q" $x}}{{end}} + A with action that uses the variable in another action. + {{with $x := "output"}}{{$x | printf "%q"}}{{end}} + The same, but pipelined. + +Functions + +During execution functions are found in two function maps: first in the +template, then in the global function map. By default, no functions are defined +in the template but the Funcs method can be used to add them. + +Predefined global functions are named as follows. + + and + Returns the boolean AND of its arguments by returning the + first empty argument or the last argument, that is, + "and x y" behaves as "if x then y else x". All the + arguments are evaluated. + call + Returns the result of calling the first argument, which + must be a function, with the remaining arguments as parameters. + Thus "call .X.Y 1 2" is, in Go notation, dot.X.Y(1, 2) where + Y is a func-valued field, map entry, or the like. + The first argument must be the result of an evaluation + that yields a value of function type (as distinct from + a predefined function such as print). The function must + return either one or two result values, the second of which + is of type error. If the arguments don't match the function + or the returned error value is non-nil, execution stops. + html + Returns the escaped HTML equivalent of the textual + representation of its arguments. This function is unavailable + in html/template, with a few exceptions. + index + Returns the result of indexing its first argument by the + following arguments. Thus "index x 1 2 3" is, in Go syntax, + x[1][2][3]. Each indexed item must be a map, slice, or array. + slice + slice returns the result of slicing its first argument by the + remaining arguments. Thus "slice x 1 2" is, in Go syntax, x[1:2], + while "slice x" is x[:], "slice x 1" is x[1:], and "slice x 1 2 3" + is x[1:2:3]. The first argument must be a string, slice, or array. + js + Returns the escaped JavaScript equivalent of the textual + representation of its arguments. + len + Returns the integer length of its argument. + not + Returns the boolean negation of its single argument. + or + Returns the boolean OR of its arguments by returning the + first non-empty argument or the last argument, that is, + "or x y" behaves as "if x then x else y". All the + arguments are evaluated. + print + An alias for fmt.Sprint + printf + An alias for fmt.Sprintf + println + An alias for fmt.Sprintln + urlquery + Returns the escaped value of the textual representation of + its arguments in a form suitable for embedding in a URL query. + This function is unavailable in html/template, with a few + exceptions. + +The boolean functions take any zero value to be false and a non-zero +value to be true. + +There is also a set of binary comparison operators defined as +functions: + + eq + Returns the boolean truth of arg1 == arg2 + ne + Returns the boolean truth of arg1 != arg2 + lt + Returns the boolean truth of arg1 < arg2 + le + Returns the boolean truth of arg1 <= arg2 + gt + Returns the boolean truth of arg1 > arg2 + ge + Returns the boolean truth of arg1 >= arg2 + +For simpler multi-way equality tests, eq (only) accepts two or more +arguments and compares the second and subsequent to the first, +returning in effect + + arg1==arg2 || arg1==arg3 || arg1==arg4 ... + +(Unlike with || in Go, however, eq is a function call and all the +arguments will be evaluated.) + +The comparison functions work on any values whose type Go defines as +comparable. For basic types such as integers, the rules are relaxed: +size and exact type are ignored, so any integer value, signed or unsigned, +may be compared with any other integer value. (The arithmetic value is compared, +not the bit pattern, so all negative integers are less than all unsigned integers.) +However, as usual, one may not compare an int with a float32 and so on. + +Associated templates + +Each template is named by a string specified when it is created. Also, each +template is associated with zero or more other templates that it may invoke by +name; such associations are transitive and form a name space of templates. + +A template may use a template invocation to instantiate another associated +template; see the explanation of the "template" action above. The name must be +that of a template associated with the template that contains the invocation. + +Nested template definitions + +When parsing a template, another template may be defined and associated with the +template being parsed. Template definitions must appear at the top level of the +template, much like global variables in a Go program. + +The syntax of such definitions is to surround each template declaration with a +"define" and "end" action. + +The define action names the template being created by providing a string +constant. Here is a simple example: + + `{{define "T1"}}ONE{{end}} + {{define "T2"}}TWO{{end}} + {{define "T3"}}{{template "T1"}} {{template "T2"}}{{end}} + {{template "T3"}}` + +This defines two templates, T1 and T2, and a third T3 that invokes the other two +when it is executed. Finally it invokes T3. If executed this template will +produce the text + + ONE TWO + +By construction, a template may reside in only one association. If it's +necessary to have a template addressable from multiple associations, the +template definition must be parsed multiple times to create distinct *Template +values, or must be copied with the Clone or AddParseTree method. + +Parse may be called multiple times to assemble the various associated templates; +see the ParseFiles and ParseGlob functions and methods for simple ways to parse +related templates stored in files. + +A template may be executed directly or through ExecuteTemplate, which executes +an associated template identified by name. To invoke our example above, we +might write, + + err := tmpl.Execute(os.Stdout, "no data needed") + if err != nil { + log.Fatalf("execution failed: %s", err) + } + +or to invoke a particular template explicitly by name, + + err := tmpl.ExecuteTemplate(os.Stdout, "T2", "no data needed") + if err != nil { + log.Fatalf("execution failed: %s", err) + } + +*/ +package template |