## Extend

  The Stylus __@extend__ directive is inspired by (and essentially the same as) the [SASS Implementation](http://sass-lang.com/docs/yardoc/file.SASS_REFERENCE.html#extend), with few subtle differences. This feature significantly simplifies maintenance of semantic rulesets that inherit from other semantic rulesets.


### “Extending” with mixins

  Although you can use mixins to achieve a similar effect, this can lead to duplicate CSS. A typical pattern is to define several classes as shown below, then combine them on the element such as "warning message". 
  
  While this technique works just fine, it's difficult to maintain.

      .message,
      .warning {
        padding: 10px;
        border: 1px solid #eee;
      }

      .warning {
        color: #E2E21E;
      }


### Using `__@extend__`

  To produce this same output with `__@extend__`, simply pass it the desired selector.  Stylus will then append the `.warning` selector to the existing `.message` ruleset.  The `.warning` class then inherits properties from `.message`.

      .message {
        padding: 10px;
        border: 1px solid #eee;
      }

      .warning {
        @extend .message;
        color: #E2E21E;
      }


  Here's a more complex example, demonstrating how `__@extend__` cascades:
  
      red = #E33E1E
      yellow = #E2E21E

      .message
        padding: 10px
        font: 14px Helvetica
        border: 1px solid #eee

      .warning
        @extends .message
        border-color: yellow
        background: yellow + 70%

      .error
        @extends .message
        border-color: red
        background: red + 70%

      .fatal
        @extends .error
        font-weight: bold
        color: red

  Yielding the following CSS:
  
      .message,
      .warning,
      .error,
      .fatal {
        padding: 10px;
        font: 14px Helvetica;
        border: 1px solid #eee;
      }
      .warning {
        border-color: #e2e21e;
        background: #f6f6bc;
      }
      .error,
      .fatal {
        border-color: #e33e1e;
        background: #f7c5bc;
      }
      .fatal {
        font-weight: bold;
        color: #e33e1e;
      }

  Where Stylus currently differs from SASS is, SASS won't allow  `__@extend__` nested selectors:
  
     form
       button
         padding: 10px

     a.button
       @extend form button 
     Syntax error: Can't extend form button: can't extend nested selectors
             on line 6 of standard input
       Use --trace for backtrace.

   With Stylus, as long as the selectors match, it works!
   
       form
         input[type=text]
           padding: 5px
           border: 1px solid #eee
           color: #ddd

       textarea
         @extends form input[type=text]
         padding: 10px

   Yielding:
   
        form input[type=text],
        form textarea {
          padding: 5px;
          border: 1px solid #eee;
          color: #ddd;
        }
        textarea {
          padding: 10px;
        }
      
### Extending placeholder selectors

Stylus have the feature similar to the one in [Sass](http://sass-lang.com/docs/yardoc/file.SASS_REFERENCE.html#placeholders) — placeholder selectors.

Those selectors should start `$` symbol (for example, `$foo`), and are not yielded in the resulting CSS. But you still can extend them:

    $foo
      color: #FFF

    $foo2
      color: red

    .bar
      background: #000
      @extends $foo

    .baz
      @extends $foo


Yielding:

    .bar,
    .baz {
      color: #fff;
    }
    .bar {
      background: #000;
    }

Note that if the selector is not extended, it won't be in the resulting CSS, so it's a powerful way to create a library of extendable code. While you can insert code through mixins, they would insert the same code every time you use them, while extending placeholders would give you compact output.