Agile Web Development

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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.


There are two ways to use Haml and Sass. The easiest is as the Rails plugin. See for installation instructions. Views with the .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 module docs for details).



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

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

No end-tag is needed; Haml handles that automatically. Adding class and id attributes is even easier. Haml uses the same syntax as the CSS that styles the document:


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

  #foo Hello!



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

    %li Salt
    %li Pepper



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


It’s even 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:

    - 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.


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: tabulation (using *two spaces*) indicates the attributes in a rule, rather than non-DRY brackets; and newlines indicate the end of an attribute, rather than a semicolon. For example:

    :background-color #f00
    :width 98%


  #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;


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

Pretty nice, no? Well, it gets better. One of the main complaints against CSS is that it doesn’t allow constants. 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 constants. Then, if you put "=" after your attribute name, you can set it to a constant. For example:

  !note_bg= #55aaff

    :width 70%
      :background-color= !note_bg
      :width 5em
      :background-color= !note_bg


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

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

  !main_bg= #46ar12
  !main_width= 40em

    :background-color= !main_bg
    :width= !main_width
      :background-color= !main_bg + #333333
      :width= !main_width - 25em


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

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


Haml and Sass are designed by Hampton Catlin (hcatlin). Help with the Ruby On Rails implementation and much of the documentation by Jeff Hardy (packagethief).

Nathan Weizenbaum (Nex3) contributed the buffered-engine code to Haml, along with many other enhancements (including the silent-line syntax: "-"). He continues to actively work on both Haml and Sass.

If you use this software, you must pay Hampton a compliment. Say something nice about it. Beyond that, the implementation is licensed under the MIT License. Ok, fine, I guess that means compliments aren’t required.


License Rails' (MIT)
Rating (27 votes)
Owner Hampton Catlin
Created 1 September 2007


  • meekish
    2 September 2007

    The URL for the plugin install should be:

  • Avatar
    26 October 2007

    "Is that a HAML template or some old perl program?"

    That's how I feel after recently working with / inheriting some HAML templates on a project.

  • Avatar
    26 November 2007

    Is it only me that gets invalid markup like this:


    %p{:id => "product5" }, or %p{:id => 'product5' }

    Rendered XHTML:

    <p id='product_5'></p>

    Valid XHTML:

    <p id="product_5"></p>


    I hope that it's me that do something wrong, because there's nothing worse than non-standard solutions. (Rails programmers in general seem to have missed semantics - even the XHTML in 37signals producs makes me cry blood.)

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