Haml and Sass

Haml and Sass are templating engines for the two most common types of documents on the web: HTML and CSS, respectively. They are designed to make it both easier and more pleasant to code HTML and CSS documents, by eliminating redundancy, reflecting the underlying structure that the document represents, and providing elegant, easily understandable, and powerful syntax.

Using

Haml and Sass can be used from the command line or as part of a Ruby web framework. The first step is to install the gem:

gem install haml

After you convert some HTML to Haml or some CSS to Sass, you can run

haml document.haml
sass style.sass

to compile them. For more information on these commands, check out

haml --help
sass --help

To install Haml and Sass as a Rails plugin, just run haml --rails path/to/rails/app and both Haml and Sass will be installed. Views with the .html.haml extension will automatically use Haml. Sass is a little more complicated; .sass files should be placed in public/stylesheets/sass, where they'll be automatically compiled to corresponding CSS files in public/stylesheets when needed (the Sass template directory is customizable... see the Sass reference for details).

For Merb, .html.haml views will work without any further modification. To enable Sass, you also need to add a dependency. To do so, just add

dependency "merb-haml"

to config/dependencies.rb (or config/init.rb in a flat/very flat Merb application). Then it'll work just like it does in Rails.

To use Haml and Sass programatically, check out the YARD documentation.

Formatting

Haml

The most basic element of Haml is a shorthand for creating HTML:

%tagname{:attr1 => 'value1', :attr2 => 'value2'} Contents

No end-tag is needed; Haml handles that automatically. If you prefer HTML-style attributes, you can also use:

%tagname(attr1='value1' attr2='value2') Contents

Adding class and id attributes is even easier. Haml uses the same syntax as the CSS that styles the document:

%tagname#id.class

In fact, when you're using the <div> tag, it becomes even easier. Because <div> is such a common element, a tag without a name defaults to a div. So

#foo Hello!

becomes

<div id='foo'>Hello!</div>

Haml uses indentation to bring the individual elements to represent the HTML structure. A tag's children are indented beneath than the parent tag. Again, a closing tag is automatically added. For example:

%ul
  %li Salt
  %li Pepper

becomes:

<ul>
  <li>Salt</li>
  <li>Pepper</li>
</ul>

You can also put plain text as a child of an element:

%p
  Hello,
  World!

It's also possible to embed Ruby code into Haml documents. An equals sign, =, will output the result of the code. A hyphen, -, will run the code but not output the result. You can even use control statements like if and while:

%p
  Date/Time:
  - now = DateTime.now
  %strong= now
  - if now > DateTime.parse("December 31, 2006")
    = "Happy new " + "year!"

Haml provides far more tools than those presented here. Check out the reference documentation in the Haml module.

Sass

At its most basic, Sass is just another way of writing CSS. Although it's very much like normal CSS, the basic syntax offers a few helpful features: indentation indicates the properties in a rule, rather than non-DRY brackets; and newlines indicate the end of a properties, rather than a semicolon. For example:

#main
  background-color: #f00
  width: 98%

becomes:

#main {
  background-color: #f00;
  width: 98% }

However, Sass provides much more than a way to make CSS look nice. In CSS, it's important to have accurate selectors, so your styles don't just apply to everything. However, in order to do this, you need to use nested element selectors. These get very ugly very quickly. I'm sure everyone's had to write something like "#main .sidebar .top p h1 a", followed by "#main .sidebar .top p h1 a:visited" and "#main .sidebar .top p h1 a:hover". Well, Sass gets rid of that. Like Haml, it uses indentation to indicate the structure of the document. So, what was:

#main {
  width: 90%;
}
#main p {
  border-style: solid;
  border-width: 1px;
  border-color: #00f;
}
#main p a {
  text-decoration: none;
  font-weight: bold;
}
#main p a:hover {
  text-decoration: underline;
}

becomes:

#main
  width: 90%
  p
    border-style: solid
    border-width: 1px
    border-color: #00f
    a
      text-decoration: none
      font-weight: bold
    a:hover
      text-decoration: underline

Pretty nice, no? Well, it gets better. One of the main complaints against CSS is that it doesn't allow variables. What if have a color or a width you re-use all the time? In CSS, you just have to re-type it each time, which is a nightmare when you decide to change it later. Not so for Sass! You can use the ! character to set variables. Then, if you put = after your property name, you can set it to a variable. For example:

!note_bg= #55aaff

#main
  width: 70%
  .note
    background-color = !note_bg
  p
    width: 5em
    background-color = !note_bg

becomes:

#main {
  width: 70%; }
  #main .note {
    background-color: #55aaff; }
  #main p {
    width: 5em;
    background-color: #55aaff; }

You can even do simple arithmetic operations with variables, adding numbers and even colors together:

!main_bg= #46ar12
!main_width= 40em

#main
  background-color = !main_bg
  width = !main_width
  .sidebar
    background-color = !main_bg + #333333
    width = !main_width - 25em

becomes:

#main {
  background-color: #46a312;
  width: 40em; }
  #main .sidebar {
    background-color: #79d645;
    width: 15em; }

Taking the idea of variables a bit further are mixins. These let you group whole bunches of CSS properties into a single directive and then include those anywhere you want:

=blue-border
  border:
    color: blue
    width: 2px
    style: dotted

.comment
  +blue-border
  padding: 2px
  margin: 10px 0

.reply
  +blue-border

becomes:

.comment {
  border-color: blue;
  border-width: 2px;
  border-style: dotted;
  padding: 2px;
  margin: 10px 0;
}

.reply {
  border-color: blue;
  border-width: 2px;
  border-style: dotted;
}

A comprehensive list of features is in the documentation for the Sass module.

Indentation

Indentation can be made up of one or more tabs or spaces. However, indentation must be consistent within a given document. Hard tabs and spaces can't be mixed, and the same number of tabs or spaces must be used throughout.

Executables

The Haml gem includes several executables that are useful for dealing with Haml and Sass from the command line.

haml

The haml executable transforms a source Haml file into HTML. See haml --help for further information and options.

sass

The sass executable transforms a source Sass file into CSS. See sass --help for further information and options.

html2haml

The html2haml executable attempts to transform HTML, optionally with ERB markup, into Haml code. Since HTML is so variable, this transformation is not always perfect; it's a good idea to have a human check the output of this tool. See html2haml --help for further information and options.

css2sass

The css2sass executable attempts to transform CSS into Sass code. This transformation attempts to use Sass nesting where possible. See css2sass --help for further information and options.

Authors

Haml and Sass were created by Hampton Catlin (hcatlin) and he is the author of the original implementation. However, Hampton doesn't even know his way around the code anymore and now occasionally consults on the language issues. Hampton lives in Jacksonville, Florida and is the lead mobile developer for Wikimedia.

Nathan Weizenbaum is the primary developer and architect of the "modern" Ruby implementation of Haml. His hard work has kept the project alive by endlessly answering forum posts, fixing bugs, refactoring, finding speed improvements, writing documentation, implementing new features, and getting Hampton coffee (a fitting task for a boy-genius). Nathan lives in Seattle, Washington and while not being a student at the University of Washington or working at an internship, he consults for Unspace Interactive.

Chris Eppstein is a core contributor to Sass and the creator of Compass, the first Sass-based framework. Chris focuses on making Sass more powerful, easy to use, and on ways to speed its adoption through the web development community. Chris lives in San Jose, California with his wife and daughter. He is the Software Architect for Caring.com, a website devoted to the 34 Million caregivers whose parents are sick or elderly, that uses Haml and Sass.

If you use this software, you must pay Hampton a compliment. And buy Nathan some jelly beans. Maybe pet a kitten. Yeah. Pet that kitty.

Some of the work on Haml was supported by Unspace Interactive.

Beyond that, the implementation is licensed under the MIT License. Okay, fine, I guess that means compliments aren't required.