Haml

Haml
	File:Haml 1-5 logo.gif
Developer(s)	Nathan Weizenbaum, Hampton Catlin
Stable release	3.0.1 / May 10, 2010
Repository	github.com/haml/haml ;
Operating system	Cross-platform
Type	Template Engine
License	MIT License
Website	http://haml-lang.com/

Haml (HTML Abstraction Markup Language) is a markup language that is used to cleanly and simply describe the XHTML of any web document without the use of traditional inline coding. It’s designed to address many of the flaws in traditional templating engines, as well as making markup as elegant as it can be. Haml functions as a replacement for inline page templating systems such as PHP, RHTML, and ASP. However, Haml avoids the need for explicitly coding XHTML into the template, because it is itself a description of the XHTML, with some code to generate dynamic content.

Haml's equivalent for CSS is Sass, which comes with Haml.

Principles

Markup should be beautiful: Markup should not be used merely as a tool to get browsers to render a page the way its author wants it rendered. The rendering isn't the only thing people have to see; they have to see, modify, and understand the markup as well. Thus, the markup should be just as user-friendly and pleasant as the rendered result.
Markup should be DRY: XHTML involves major repetition. Most elements have to be named twice: once before its content and once after. ERB adds even more repetition and unnecessary characters. Haml avoids all of this by relying on indentation, not text, to determine where elements and blocks of code begin and end. Not only does this result in smaller templates, it makes the code much cleaner to look at.
Markup should be well-indented: One of the major problems with traditional templating languages is that not only do they not encourage well-indented code, they actively make it challenging, or even impossible, to write. The result is confusing, unreadable XHTML. Haml formats tags so they are well indented and reflect the underlying structure of the document.
XHTML structure should be clear: XML and XHTML are formats built upon the idea of a structured document. That structure is reflected in their markup, and it should likewise be reflected in meta-markup such as Haml. Because Haml's logic is based on indentation of child elements, this structure is naturally preserved, making the document much easier and more logical for mere humans to read.

Disadvantages and criticisms

Haml is "whitespace active", and relies on consistent spacing for each indent level. Tabs or spaces must be maintained consistently throughout the document.
There are no WYSIWYG Haml editors, whereas there are many XHTML and HTML editors. However, the growing community of Haml users has released syntax bundle addons to many popular programming IDEs, such as Eclipse (which includes the spinoff IDEs RadRails and Aptana), jEdit, NetBeans, and editors like TextMate, Emacs and vim. This allows developers to edit Haml in an environment that is aware of the indenting rules and syntax.
While HAML provides visual aids in structuring XML and XHTML, this structure is not always necessary when creating (non-XHTML) HTML templates.
HAML's syntax is quite cryptic, causing a barrier to entry for those familiar most with standard HTML and CSS.
The authors of Haml have skewed the definition of DRY to essentially mean "don't type as much" rather than simply "avoid repetition." Typing out a closing tag is not a redundant operation. The code typed in isn't duplicating functionality, rather, it is completing a statement.

Example

Note: This is a simple preview example and may not reflect the current version of the language.

!!!
%html{ :xmlns => "http://www.w3.org/1999/xhtml", :lang => "en", "xml:lang" => "en"}
  %head
    %title BoBlog
    %meta{"http-equiv" => "Content-Type", :content => "text/html; charset=utf-8"}
    = stylesheet_link_tag 'main'
  %body
    #header
      %h1 BoBlog
      %h2 Bob's Blog
    #content
      - @entries.each do |entry|
        .entry
          %h3.title= entry.title
          %p.date= entry.posted.strftime("%A, %B %d, %Y")
          %p.body= entry.body
    #footer
      %p
        All content copyright © Bob

The above Haml would produce this XHTML:

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html lang='en' xml:lang='en' xmlns='http://www.w3.org/1999/xhtml'>
  <head>
    <title>BoBlog</title>
    <meta content='text/html; charset=utf-8' http-equiv='Content-Type' />
    <link href="/stylesheets/main.css" media="screen" rel="Stylesheet" type="text/css" />
  </head>
  <body>
    <div id='header'>
      <h1>BoBlog</h1>
      <h2>Bob's Blog</h2>
    </div>
    <div id='content'>
      <div class='entry'>
        <h3 class='title'>Halloween</h3>
        <p class='date'>Tuesday, October 31, 2006</p>
        <p class='body'>
          Happy Halloween, glorious readers! I'm going to a party this evening... I'm very excited.
        </p>
      </div>
      <div class='entry'>
        <h3 class='title'>New Rails Templating Engine</h3>
        <p class='date'>Friday, August 11, 2006</p>
        <p class='body'>
          There's a very cool new Templating Engine out for Ruby on Rails. It's called Haml.
        </p>
      </div>
    </div>
    <div id='footer'>
      <p>
        All content copyright © Bob
      </p>
    </div>
  </body>
</html>

Implementations

The official implementation of Haml has been built for Ruby with plugins for Ruby on Rails and Merb, but the Ruby implementation also functions independently.

There are also implementations in other languages:

Development

Haml was invented in May 2006 by Hampton Catlin, who continues to work on the implementation and the ideas behind Haml. However, Nathan Weizenbaum is responsible for much of the recent growth and maturation of the Haml codebase.

External links