Built-In Tags

Inversion’s tags support the Pluggability API, allowing for the easy addition of custom tags, but it comes with a number of built-in ones too.

Tag Syntax

Tags can be in either of two formats:

  • XML Pre-processing instruction style: <?tagname tagdata ?>

  • or the same thing, but with square brackets instead: [?tagname tagdata ?]

The second form is especially useful if you’re generating HTML and want to put an Inversion tag inside the attribute of an HTML tag, but still want the template to be well-formed:

<a href="[?call article.permalink ?]">Permalink</a>

You can mix tag forms in a single document.

Placeholder Tags

Placeholder tags represent the main functionality of Inversion; they create a placeholder in the output text which can be filled in via a method on the template object with the same name.


The attr tag is the primary placeholder tag for injecting dynamic values into your templates. The most basic form is analogous to attr_accessor; it defines a method on the template object that, when set, replaces all occurrences of the tag in the template with the value:

Title: <?attr title ?>

Calling the template object’s #title= method will inject the stringified value into that part of the output when it’s rendered, e.g.,

template.title = "How to Breed Kangaroos for Milk and Meat"
# => "Title: How to Breed Kangaroos for Milk and Meat"

The rendered values of an attr tag can also be the result of calling methods on the attr value:

ISBN: <?attr book.isbn ?>

Attributes can be sprintf formatted using Ruby’s String#% method:

Book price: <?attr "%0.2f" % book.price ?>

Attributes can also contain other template objects, which allows templates to be nested within each other easily.

layout  = Inversion::Template.load( 'layout.tmpl' )
content = Inversion::Template.load( 'content.tmpl' )
content.caption = "Your kids will love their new Kangaroo family!"
layout.body = content


call is just an alias for attr. Use whichever strikes your fancy.


escape works just like attr, but it escapes the content inserted into the template, using the configured escaping behavior. The supported escaping behaviors are defined in a mixin called Inversion::Escaping. The behavior to use can be set using the :escape_format option on the template or in a ‘config+ tag; it defaults to HTML escaping.

<p>Company name: <?escape company.name ?></p>

If the company was "AT&T", the output would look like:

<p>Company name: AT&amp;T</p>


The urlencode tag is another attr-like tag, but this one does URI encoding:

<nav>Edit <a href="/profile?name=[?uriencode person.name ?]">your profile</a></nav>

Special Placeholders


If you need to automatically generate a human-readable description of the interval between two times, you can use the timedelta tag:

<article class="blogentry">
        <p>Posted: <?timedelta entry.date_posted ?>.</p>

The tag supports any object which responds to the #to_time method, so standard Time, Date, and DateTime objects all work.

Dates are compared against the current time, and render to approximate descriptions of the interval, e.g.,

  • 4 days ago

  • about an hour from now

  • 6 weeks ago

  • less than a minute from now

Inter-template Tags

These tags operate on nested templates, allowing you to selectively use or send attributes or content from other templates.


Occasionally, you’ll want to compose output from several different templates by nesting them, but you don’t want to have to set common objects on all of them from code. The import tag lets you copy the values from a container template into one intended to be nested within it:

<!-- layout.tmpl -->
Logged in as: <?attr request.authenticated_user ?>
<?attr body ?>

<!-- body.tmpl -->
<?import request ?>
<p>You can check your balance using <a href="[?call request.path_info ?]/accounts">the
   accounts tool</a>.</p>

When the content template is nested in the container, you only need to set the request attribute on the container to set it in both places:

layout = Inversion::Template.load( 'layout.tmpl' )
body = Inversion::Template.load( 'body.tmpl' )

layout.body = body
layout.request = request

puts layout.render

Without the use of import, you’d need to similarly set the request attribute on the body template.

The imported attribute’s value is determined at render time, so you can also use it to import values from an iteration.

<!-- Container template (table.tmpl)" -->
    <?for user in assigned_users ?>
    <?attr row ?>
    <?end for ?>

<!-- Content template (row.tmpl)" -->
<?import user ?>
      <th>Username:</th><td><?escape user.username ?></td>
      <th>UID:</th><td><?escape user.uid ?></td>
      <th>GID:</th><td><?escape user.gid ?></td>

and the code:

usertable = Inversion::Template.load( 'table.tmpl' )
userrow = Inversion::Template.load( 'row.tmpl' )

usertable.row = userrow
usertable.assigned_users = User.assigned.all

puts usertable.render

When the row.tmpl is rendered each time, its imported user is set to whatever the user in the container is, in this case the next object in assigned_users.

You can import values into deeply-nested templates, provided each container imports it as well.


Often you’ll want to set up a generic layout template to establish a global look-and-feel, and then modify it based on the content of an inner template.

Look and feel template (layout.tmpl)

<html lang="en">
  <title><?subscribe title || Untitled ?></title>
  <link rel="stylesheet" href="/css/base.css" type="text/css" media="screen"
    title="Base Stylesheet" charset="utf-8" />
    <?subscribe stylesheets ?>

  <script defer="defer" src="/js/jquery-1.4.2.min.js"
      type="text/javascript" charset="utf-8"></script>
    <?subscribe scripts ?>

<body><?attr body ?></body>

A content template (content.tmpl)

<?publish title ?>I make stuff up<?end publish?>

<?publish stylesheets ?>
  <link rel="stylesheet" href="/css/content.css" type="text/css" media="screen"
    title="Content Style Overrides" charset="utf-8" />
<?end publish?>

<?publish scripts ?>
  <script defer="defer" src="/js/content.js" type="text/javascript" charset="utf-8"></script>
<?end publish?>

<div>Hi, there.</div>

Template setup

layout  = Inversion::Template.load( 'layout.tmpl' )
content = Inversion::Template.load( 'content.tmpl' )

layout.body = content

puts layout.render

subscribe renders to an empty string if there is no matching publish, or to the value of a default if supplied (as in the HTML title example above.) In this fashion, you can dynamically switch out different content pages, with each having the ability to optionally override various HTML elements.


The include tag allows inclusion of other template files from within a template. This supports separation of a template into several reusable components. The included template becomes a part of the including template, along with any defaults, attributes and configuration.

include setup

email = Inversion::Template.load( 'email.tmpl' )

email.greeting = "Kudos"
email.company  = Company[ :spime_thorpe ]
email.user     = User[ :jrandom ]

puts main.render

Including template (email.tmpl)

Subject: Great news, everybody!
From: <?attr company.email ?>
To: <?attr user.email ?>

<?attr greeting ?>, <?attr user.first_name ?>!

We are excited to inform you that you have been selected to participate
in a challenging and exciting career displacement opportunity!

Please attend the mandatory Man Overboard (tm) session we have scheduled
for you at 8:45AM on Thursday in the Sunshine Room. Light refreshments
and computer-aided aptitude testing will be provided.

<?include signature.tmpl ?>

Included template (signature.tmpl)

Your Friends at <?attr company.name ?>!

The rendered output

Subject: Great news, everybody!
From: "Spime-Thorpe, Inc." <salesteam2@spime-thorpe.com>
To: "James Random" <jrandom@compusa.com>

Kudos, James!

We are excited to inform you that you have been selected to participate
in a challenging and exciting career displacement opportunity!

Please attend the mandatory Man Overboard (tm) session we have scheduled
for you at 8:45AM on Thursday in the Sunshine Room. Light refreshments
and computer-aided aptitude testing will be provided.

Your Friends at Spime Thorpe!


A fragment tag also sets an attribute from within the template, but under the scope of the global template itself. A fragment can use other Inversion tags, and the attribute is both usable elsewhere in the template, and accessible from calling code after rendering.

template = Inversion::Template.new <<-TMPL
<?fragment subject ?>Your order status (Order #<?call order.number ?>)<?end ?>

Dear <?call order.customer.name ?>,

Your recent order was modified by our Order Fulfillment Team.

After careful deliberation, it was decided that no one should have need for that many hot dogs
with overnight shipping.  Frankly, we're more than a little concerned for your health.

Rowe's Meat Emporium
(Buy!  Sell!  Consignment!)

template.order = order

template.fragments[ :subject ] #=> "Your order status (Order #3492)"

Flow Control

The following tags are used to alter the flow of rendering from within templates.


The for tag iterates over the objects in a collection, rendering its template section once for each iteration. Its attribute can be set to anything that responds to @#each@. The iteration variable(s) are scoped to the block, and temporarily override any template attributes of the same name.

for tag setup

overhead_list = Inversion::Template.load( 'employee_list.tmpl' )
overhead_list.users = User.
  filter { start_date < 6.months.ago }.
  filter { department = 'Information Technology' }

puts overhead_list.render

The for tag’s iteration works just like Ruby’s for; if the enumerated value has more than one value, you can give a list of iteration variables to be assigned to.

Employee list using for

    <?for user, i in users.each_with_index ?>
        <tr class="[?if i.even? ?]even[?else?]odd[?end if?]-row">
            <td><?attr user.first_name ?></td>
            <td><?attr user.last_name ?></td>
            <td><?attr user.title ?></td>
            <td><?attr user.start_date ?></td>
            <td><?attr user.salary ?></td>
    <?end for ?>

The example above uses a Ruby enumerator for the #each_with_index method to set the class of the row to 'even-row' or 'odd-row'.

This works with the keys and values of Hashes, too:

Display hash of notes keyed by author using for

<?for user, content in user.notes ?>
<section class="note">
    Note by <?call user.username ?>
  <p><?escape content ?></p>

<?end for ?>

Note that you can also use Ruby’s “external iterator” syntax to iterate, too:

Iterate over each byte of a string with an index using for

<section class="hexdump">
<?for byte, index in frame.header.each_byte.with_index ?>
  <?if index.modulo(8).zero? ?>
    <?if index.nonzero? ?>
  </span><br />
    <?end if ?>
  <span class="row"><?attr "0x%08x" % index ?>:
  <?end if ?>
  &nbsp;`<?attr "0x%02x" % byte ?>`
<?end for ?>


The if tag can be used to conditionally render a section of the template based on the value of an attribute or the value of a method called on it.

Conditional block

<?if user.has_stock_options? ?>
You will have 21 days to exercise your stock options.
<?else ?>
You have a week to optionally take home a handful of supplies from the
office cabinet.
<?end if ?>


Unless is like the if tag, but with inverted logic. Note that an unless can have an else tag, but cannot have any elsif tags within it.


The yield tag is used to defer rendering of some part of the template to the code that is calling render on it. If a block is passed to #render, then the yield tag will call it with the Inversion::RenderState object that is currently in effect, and will render the return value in its place.

Using yield to defer an expensive database lookup (report.tmpl)

<?if extra_details_enabled ?>
<?yield ?>
<?end if ?>

report = Inversion::Template.load( 'report.tmpl' )
report.extra_details_enabled = true if $DEBUG
puts report.render do
  report_table = Inversion::Template.load( 'table.tmpl' )
  report_table.rows = an_expensive_database_query()

This will insert the report_table template in place of the yield, but only if $DEBUG is true.


These tags work as you’d expect from their ruby counterparts.

The begin section of the template to be rendered only if no exceptions are raised while it’s being rendered. If an exception is raised, it is checked against any rescue sections, and the first with a matching exception is rendered instead. If no rescue block is found, the exception is handled by the configured exception behavior for the template (see Template Options at Inversion::Template).

<?begin ?><?call employees.length ?><?end?>

<?begin ?>
  <?for employee in employees.all ?>
    <?attr employee.name ?> --> <?attr employee.title ?>
  <?end for?>
<?rescue DatabaseError => err ?>
  Oh no!! I can't talk to the database for some reason.  The
  error was as follows:
    <?attr err.message ?>

Control Tags

There are a few tags that can be used to set values in the templating system itself.


The config tag can be used to override template options on a per-template basis. It allows for convenient, inline settings from within a template rather than from the code in which the template is loaded.

For example, if you want to enable debugging comments on a single template:

<?config debugging_comments: true ?>

Multiple template options can be set simultaneously by using a YAML hash:

    on_render_error: propagate
    debugging_comments: true
    comment_start: /*
    comment_end: */

Note that this also allows you to set multiple options on a single line, if you wrap them in braces:

<?config { comment_start: "/*", comment_end: "*/" } ?>


The default tag sets an attribute from within the template, and this value is used if the attribute is set to nil or otherwise unset.

template = Inversion::Template.new <<-TMPL
  <?default adjective to "cruel" ?>
  <?default noun to "world" ?>
  Goodbye, <?attr adjective ?> <?attr noun ?>!


template.adjective = "delicious"

template.adjective = nil
template.noun = "banana"

Would produce the output:

Goodbye, cruel world!
Goodbye, delicious world!
Goodbye, cruel banana!



The pp tag uses the PP library to output an escaped representation of its argument.

Creating an object to inspect

content = Inversion::Template.load( 'content.tmpl' )
content.file = File.stat( '/tmp/example.txt' )

puts content.render

Inspecting an object from within a template (content.tmpl)

<div class="debugging">
    The file's stat attributes:
    <?pp file ?>

The output is escaped according to the current setting of the :escape_format option.

The rendered result

<div class="debugging">
    The file's stat attributes:
 mode=0100644 (file rw-r--r--),
 uid=501 (mahlon),
 gid=0 (wheel),
 rdev=0x0 (0, 0),
 atime=2011-08-12 08:43:15 -0700 (1313163795),
 mtime=2011-08-12 08:43:15 -0700 (1313163795),
 ctime=2011-08-12 08:43:15 -0700 (1313163795)&gt;</div>

Custom Tags

We have a lot of documentation work to do for this still, but the basics are:

Unfortunately, the tag superclasses aren’t currently documented very well, so the best way to accomplish what you want is to find an existing tag that does something similar and look at how it does it.