diff options
Diffstat (limited to 'src/pkg/template/doc.go')
| -rw-r--r-- | src/pkg/template/doc.go | 392 |
1 files changed, 307 insertions, 85 deletions
diff --git a/src/pkg/template/doc.go b/src/pkg/template/doc.go index e778d801d..a52f32d91 100644 --- a/src/pkg/template/doc.go +++ b/src/pkg/template/doc.go @@ -1,91 +1,313 @@ -// Copyright 2009 The Go Authors. All rights reserved. +// 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 such as HTML. - - 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. - The template walks the structure as it executes and the - "cursor" @ represents the value at the current location - in the structure. - - Data items may be values or pointers; the interface hides the - indirection. - - In the following, 'Field' is one of several things, according to the data. - - - The name of a field of a struct (result = data.Field), - - The value stored in a map under that key (result = data["Field"]), or - - The result of invoking a niladic single-valued method with that name - (result = data.Field()) - - If Field is a struct field or method name, it must be an exported - (capitalized) name. - - Major constructs ({} are the default delimiters for template actions; - [] are the notation in this comment for optional elements): - - {# comment } - - A one-line comment. - - {.section field} XXX [ {.or} YYY ] {.end} - - Set @ to the value of the field. It may be an explicit @ - to stay at the same point in the data. If the field is nil - or empty, execute YYY; otherwise execute XXX. - - {.repeated section field} XXX [ {.alternates with} ZZZ ] [ {.or} YYY ] {.end} - - Like .section, but field must be an array or slice. XXX - is executed for each element. If the array is nil or empty, - YYY is executed instead. If the {.alternates with} marker - is present, ZZZ is executed between iterations of XXX. - - {field} - {field1 field2 ...} - {field|formatter} - {field1 field2...|formatter} - {field|formatter1|formatter2} - - Insert the value of the fields into the output. Each field is - first looked for in the cursor, as in .section and .repeated. - If it is not found, the search continues in outer sections - until the top level is reached. - - If the field value is a pointer, leading asterisks indicate - that the value to be inserted should be evaluated through the - pointer. For example, if x.p is of type *int, {x.p} will - insert the value of the pointer but {*x.p} will insert the - value of the underlying integer. If the value is nil or not a - pointer, asterisks have no effect. - - If a formatter is specified, it must be named in the formatter - map passed to the template set up routines or in the default - set ("html","str","") and is used to process the data for - output. The formatter function has signature - func(wr io.Writer, formatter string, data ...interface{}) - where wr is the destination for output, data holds the field - values at the instantiation, and formatter is its name at - the invocation site. The default formatter just concatenates - the string representations of the fields. - - Multiple formatters separated by the pipeline character | are - executed sequentially, with each formatter receiving the bytes - emitted by the one to its left. - - As well as field names, one may use literals with Go syntax. - Integer, floating-point, and string literals are supported. - Raw strings may not span newlines. - - The delimiter strings get their default value, "{" and "}", from - JSON-template. They may be set to any non-empty, space-free - string using the SetDelims method. Their value can be printed - in the output using {.meta-left} and {.meta-right}. +Package template implements data-driven templates for generating textual output +such as HTML. + +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. +Actions may not span newlines, although comments can. + +Once constructed, templates and template sets can be executed safely in +parallel. + +Actions + +Here is the list of actions. "Arguments" and "pipelines" are evaluations of +data, defined in detail below. + +*/ +// {{/* a comment */}} +// A comment; discarded. May contain newlines. +// Comments do not nest. +/* + + {{pipeline}} + The default textual representation 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. + + {{range pipeline}} T1 {{end}} + The value of the pipeline must be an array, slice, or map. 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. + + {{range pipeline}} T1 {{else}} T0 {{end}} + The value of the pipeline must be an array, slice, or map. 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. + + {{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, although raw strings may not span newlines. + - 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 os.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. + +Arguments may evaluate to any type; if they are pointers the implementation +automatically indirects to the base type when required. + +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. + +Pipelines + +A pipeline may be "chained" by separating a sequence of commands with pipeline +characters '|'. In a chained pipeline, the result of the 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 os.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. + +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: + + $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. + {{"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 three function maps: first in the +template, then in the "template set" (described below), and finally in the +global function map. By default, no functions are defined in the template or +the set but the Funcs methods 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. + html + Returns the escaped HTML equivalent of the textual + representation of its arguments. + 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. + 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. + +The boolean functions take any zero value to be false and a non-zero value to +be true. + +Template sets + +Each template is named by a string specified when it is created. A template may +use a template invocation to instantiate another template directly or by its +name; see the explanation of the template action above. The name is looked up +in the template set associated with the template. + +If no template invocation actions occur in the template, the issue of template +sets can be ignored. If it does contain invocations, though, the template +containing the invocations must be part of a template set in which to look up +the names. + +There are two ways to construct template sets. + +The first is to use a Set's Parse method to create a set of named templates from +a single input defining multiple templates. The syntax of the 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 of input to Set.Parse: + + `{{define "T1"}} definition of template T1 {{end}} + {{define "T2"}} definition of template T2 {{end}} + {{define "T3"}} {{template "T1"}} {{template "T2"}} {{end}}` + +This defines two templates, T1 and T2, and a third T3 that invokes the other two +when it is executed. + +The second way to build a template set is to use Set's Add method to add a +parsed template to a set. A template may be bound to at most one set. If it's +necessary to have a template in multiple sets, the template definition must be +parsed multiple times to create distinct *Template values. + +Set.Parse may be called multiple times on different inputs to construct the set. +Two sets may therefore be constructed with a common base set of templates plus, +through a second Parse call each, specializations for some elements. + +A template may be executed directly or through Set.Execute, which executes a +named template from the set. To invoke our example above, we might write, + + err := set.Execute(os.Stdout, "T3", "no data needed") + if err != nil { + log.Fatalf("execution failed: %s", err) + } */ package template |