314 lines
12 KiB
Go
314 lines
12 KiB
Go
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// Copyright 2011 The Go Authors. All rights reserved.
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// Use of this source code is governed by a BSD-style
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// license that can be found in the LICENSE file.
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/*
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Package template implements data-driven templates for generating textual output
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such as HTML.
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Templates are executed by applying them to a data structure. Annotations in the
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template refer to elements of the data structure (typically a field of a struct
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or a key in a map) to control execution and derive values to be displayed.
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Execution of the template walks the structure and sets the cursor, represented
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by a period '.' and called "dot", to the value at the current location in the
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structure as execution proceeds.
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The input text for a template is UTF-8-encoded text in any format.
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"Actions"--data evaluations or control structures--are delimited by
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"{{" and "}}"; all text outside actions is copied to the output unchanged.
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Actions may not span newlines, although comments can.
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Once constructed, templates and template sets can be executed safely in
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parallel.
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Actions
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Here is the list of actions. "Arguments" and "pipelines" are evaluations of
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data, defined in detail below.
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*/
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// {{/* a comment */}}
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// A comment; discarded. May contain newlines.
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// Comments do not nest.
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/*
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{{pipeline}}
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The default textual representation of the value of the pipeline
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is copied to the output.
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{{if pipeline}} T1 {{end}}
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If the value of the pipeline is empty, no output is generated;
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otherwise, T1 is executed. The empty values are false, 0, any
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nil pointer or interface value, and any array, slice, map, or
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string of length zero.
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Dot is unaffected.
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{{if pipeline}} T1 {{else}} T0 {{end}}
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If the value of the pipeline is empty, T0 is executed;
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otherwise, T1 is executed. Dot is unaffected.
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{{range pipeline}} T1 {{end}}
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The value of the pipeline must be an array, slice, or map. If
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the value of the pipeline has length zero, nothing is output;
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otherwise, dot is set to the successive elements of the array,
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slice, or map and T1 is executed.
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{{range pipeline}} T1 {{else}} T0 {{end}}
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The value of the pipeline must be an array, slice, or map. If
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the value of the pipeline has length zero, dot is unaffected and
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T0 is executed; otherwise, dot is set to the successive elements
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of the array, slice, or map and T1 is executed.
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{{template "name"}}
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The template with the specified name is executed with nil data.
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{{template "name" pipeline}}
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The template with the specified name is executed with dot set
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to the value of the pipeline.
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{{with pipeline}} T1 {{end}}
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If the value of the pipeline is empty, no output is generated;
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otherwise, dot is set to the value of the pipeline and T1 is
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executed.
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{{with pipeline}} T1 {{else}} T0 {{end}}
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If the value of the pipeline is empty, dot is unaffected and T0
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is executed; otherwise, dot is set to the value of the pipeline
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and T1 is executed.
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Arguments
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An argument is a simple value, denoted by one of the following.
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- A boolean, string, character, integer, floating-point, imaginary
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or complex constant in Go syntax. These behave like Go's untyped
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constants, although raw strings may not span newlines.
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- The character '.' (period):
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.
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The result is the value of dot.
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- A variable name, which is a (possibly empty) alphanumeric string
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preceded by a dollar sign, such as
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$piOver2
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or
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$
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The result is the value of the variable.
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Variables are described below.
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- The name of a field of the data, which must be a struct, preceded
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by a period, such as
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.Field
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The result is the value of the field. Field invocations may be
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chained:
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.Field1.Field2
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Fields can also be evaluated on variables, including chaining:
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$x.Field1.Field2
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- The name of a key of the data, which must be a map, preceded
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by a period, such as
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.Key
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The result is the map element value indexed by the key.
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Key invocations may be chained and combined with fields to any
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depth:
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.Field1.Key1.Field2.Key2
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Although the key must be an alphanumeric identifier, unlike with
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field names they do not need to start with an upper case letter.
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Keys can also be evaluated on variables, including chaining:
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$x.key1.key2
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- The name of a niladic method of the data, preceded by a period,
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such as
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.Method
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The result is the value of invoking the method with dot as the
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receiver, dot.Method(). Such a method must have one return value (of
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any type) or two return values, the second of which is an os.Error.
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If it has two and the returned error is non-nil, execution terminates
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and an error is returned to the caller as the value of Execute.
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Method invocations may be chained and combined with fields and keys
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to any depth:
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.Field1.Key1.Method1.Field2.Key2.Method2
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Methods can also be evaluated on variables, including chaining:
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$x.Method1.Field
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- The name of a niladic function, such as
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fun
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The result is the value of invoking the function, fun(). The return
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types and values behave as in methods. Functions and function
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names are described below.
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Arguments may evaluate to any type; if they are pointers the implementation
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automatically indirects to the base type when required.
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A pipeline is a possibly chained sequence of "commands". A command is a simple
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value (argument) or a function or method call, possibly with multiple arguments:
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Argument
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The result is the value of evaluating the argument.
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.Method [Argument...]
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The method can be alone or the last element of a chain but,
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unlike methods in the middle of a chain, it can take arguments.
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The result is the value of calling the method with the
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arguments:
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dot.Method(Argument1, etc.)
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functionName [Argument...]
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The result is the value of calling the function associated
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with the name:
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function(Argument1, etc.)
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Functions and function names are described below.
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Pipelines
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A pipeline may be "chained" by separating a sequence of commands with pipeline
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characters '|'. In a chained pipeline, the result of the each command is
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passed as the last argument of the following command. The output of the final
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command in the pipeline is the value of the pipeline.
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The output of a command will be either one value or two values, the second of
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which has type os.Error. If that second value is present and evaluates to
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non-nil, execution terminates and the error is returned to the caller of
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Execute.
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Variables
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A pipeline inside an action may initialize a variable to capture the result.
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The initialization has syntax
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$variable := pipeline
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where $variable is the name of the variable. An action that declares a
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variable produces no output.
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If a "range" action initializes a variable, the variable is set to the
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successive elements of the iteration. Also, a "range" may declare two
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variables, separated by a comma:
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$index, $element := pipeline
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in which case $index and $element are set to the successive values of the
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array/slice index or map key and element, respectively. Note that if there is
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only one variable, it is assigned the element; this is opposite to the
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convention in Go range clauses.
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A variable's scope extends to the "end" action of the control structure ("if",
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"with", or "range") in which it is declared, or to the end of the template if
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there is no such control structure. A template invocation does not inherit
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variables from the point of its invocation.
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When execution begins, $ is set to the data argument passed to Execute, that is,
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to the starting value of dot.
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Examples
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Here are some example one-line templates demonstrating pipelines and variables.
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All produce the quoted word "output":
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{{"\"output\""}}
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A string constant.
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{{`"output"`}}
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A raw string constant.
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{{printf "%q" "output"}}
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A function call.
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{{"output" | printf "%q"}}
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A function call whose final argument comes from the previous
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command.
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{{"put" | printf "%s%s" "out" | printf "%q"}}
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A more elaborate call.
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{{"output" | printf "%s" | printf "%q"}}
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A longer chain.
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{{with "output"}}{{printf "%q" .}}{{end}}
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A with action using dot.
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{{with $x := "output" | printf "%q"}}{{$x}}{{end}}
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A with action that creates and uses a variable.
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{{with $x := "output"}}{{printf "%q" $x}}{{end}}
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A with action that uses the variable in another action.
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{{with $x := "output"}}{{$x | printf "%q"}}{{end}}
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The same, but pipelined.
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Functions
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During execution functions are found in three function maps: first in the
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template, then in the "template set" (described below), and finally in the
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global function map. By default, no functions are defined in the template or
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the set but the Funcs methods can be used to add them.
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Predefined global functions are named as follows.
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and
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Returns the boolean AND of its arguments by returning the
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first empty argument or the last argument, that is,
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"and x y" behaves as "if x then y else x". All the
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arguments are evaluated.
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html
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Returns the escaped HTML equivalent of the textual
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representation of its arguments.
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index
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Returns the result of indexing its first argument by the
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following arguments. Thus "index x 1 2 3" is, in Go syntax,
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x[1][2][3]. Each indexed item must be a map, slice, or array.
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js
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Returns the escaped JavaScript equivalent of the textual
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representation of its arguments.
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len
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Returns the integer length of its argument.
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not
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Returns the boolean negation of its single argument.
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or
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Returns the boolean OR of its arguments by returning the
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first non-empty argument or the last argument, that is,
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"or x y" behaves as "if x then x else y". All the
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arguments are evaluated.
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print
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An alias for fmt.Sprint
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printf
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An alias for fmt.Sprintf
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println
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An alias for fmt.Sprintln
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urlquery
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Returns the escaped value of the textual representation of
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its arguments in a form suitable for embedding in a URL query.
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The boolean functions take any zero value to be false and a non-zero value to
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be true.
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Template sets
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Each template is named by a string specified when it is created. A template may
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use a template invocation to instantiate another template directly or by its
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name; see the explanation of the template action above. The name is looked up
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in the template set associated with the template.
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If no template invocation actions occur in the template, the issue of template
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sets can be ignored. If it does contain invocations, though, the template
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containing the invocations must be part of a template set in which to look up
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the names.
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There are two ways to construct template sets.
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The first is to use a Set's Parse method to create a set of named templates from
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a single input defining multiple templates. The syntax of the definitions is to
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surround each template declaration with a define and end action.
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The define action names the template being created by providing a string
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constant. Here is a simple example of input to Set.Parse:
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`{{define "T1"}} definition of template T1 {{end}}
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{{define "T2"}} definition of template T2 {{end}}
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{{define "T3"}} {{template "T1"}} {{template "T2"}} {{end}}`
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This defines two templates, T1 and T2, and a third T3 that invokes the other two
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when it is executed.
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The second way to build a template set is to use Set's Add method to add a
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parsed template to a set. A template may be bound to at most one set. If it's
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necessary to have a template in multiple sets, the template definition must be
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parsed multiple times to create distinct *Template values.
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Set.Parse may be called multiple times on different inputs to construct the set.
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Two sets may therefore be constructed with a common base set of templates plus,
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through a second Parse call each, specializations for some elements.
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A template may be executed directly or through Set.Execute, which executes a
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named template from the set. To invoke our example above, we might write,
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err := set.Execute(os.Stdout, "T3", "no data needed")
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if err != nil {
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log.Fatalf("execution failed: %s", err)
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}
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*/
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package template
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