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// 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.
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, a template may 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. If the value is a map and the
                keys are of basic type with a defined order ("comparable"), 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, 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 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 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 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 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.

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.

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