# Sass (Syntactically Awesome StyleSheets)

* Table of contents
{:toc}

Sass is an extension of CSS
that adds power and elegance to the basic language.
It allows you to use [variables](#variables_), [nested rules](#nested_rules),
[mixins](#mixins), [inline imports](#import), and more,
all with a fully CSS-compatible syntax.
Sass helps keep large stylesheets well-organized,
and get small stylesheets up and running quickly,
particularly with the help of
[the Compass style library](http://compass-style.org).

## Features

* Fully CSS3-compatible
* Language extensions such as variables, nesting, and mixins
* Many {Sass::Script::Functions useful functions} for manipulating colors and other values
* Advanced features like [control directives](#control_directives) for libraries
* Well-formatted, customizable output
* [Firebug integration](https://addons.mozilla.org/en-US/firefox/addon/103988)

## Syntax

There are two syntaxes available for Sass.
The first, known as SCSS (Sassy CSS) and used throughout this reference,
is an extension of the syntax of CSS3.
This means that every valid CSS3 stylesheet
is a valid SCSS file with the same meaning.
In addition, SCSS understands most CSS hacks
and vendor-specific syntax, such as [IE's old `filter` syntax](http://msdn.microsoft.com/en-us/library/ms533754%28VS.85%29.aspx).
This syntax is enhanced with the Sass features described below.
Files using this syntax have the `.scss` extension.

The second and older syntax, known as the indented syntax (or sometimes just "Sass"),
provides a more concise way of writing CSS.
It uses indentation rather than brackets to indicate nesting of selectors,
and newlines rather than semicolons to separate properties.
Some people find this to be easier to read and quicker to write than SCSS.
The indented syntax has all the same features,
although some of them have slightly different syntax;
this is described in {file:INDENTED_SYNTAX.md the indented syntax reference}.
Files using this syntax have the `.sass` extension.

Either syntax can [import](#import) files written in the other.
Files can be automatically converted from one syntax to the other
using the `sass-convert` command line tool:

    # Convert Sass to SCSS
    $ sass-convert style.sass style.scss

    # Convert SCSS to Sass
    $ sass-convert style.scss style.sass

## Using Sass

Sass can be used in three ways:
as a command-line tool,
as a standalone Ruby module,
and as a plugin for any Rack-enabled framework,
including Ruby on Rails and Merb.
Sass is bundled with Haml,
so if the Haml plugin or RubyGem is installed,
Sass will already be installed as a plugin or gem, respectively.
The first step for all of these is to install the Haml gem:

    gem install haml

If you're using Windows,
you may need to [install Ruby](http://rubyinstaller.org/download.html) first.

To run Sass from the command line, just use

    sass input.scss output.css

You can also tell Sass to watch the file and update the CSS
every time the Sass file changes:

    sass --watch input.scss:output.css

If you have a directory with many Sass files,
you can also tell Sass to watch the entire directory:

    sass --watch app/sass:public/stylesheets

Use `sass --help` for full documentation.

Using Sass in Ruby code is very simple.
After installing the Haml gem,
you can use it by running `require "sass"`
and using {Sass::Engine} like so:

    engine = Sass::Engine.new("#main {background-color: #0000ff}", :syntax => :scss)
    engine.render #=> "#main { background-color: #0000ff; }\n"

### Rack/Rails/Merb Plugin

To enable Sass as a Rails plugin, run

    haml --rails path/to/rails/app

To enable Sass in Merb,
add

    dependency "merb-haml"

to `config/dependencies.rb`.

To enable Sass in a Rack application,
add

    require 'sass/plugin/rack'
    use Sass::Plugin::Rack

to `config.ru`.

Sass stylesheets don't work the same as views.
They don't contain dynamic content,
so the CSS only needs to be generated when the Sass file has been updated.
By default, `.sass` and `.scss` files are placed in public/stylesheets/sass
(this can be customized with the [`:template_location`](#template_location-option) option).
Then, whenever necessary, they're compiled into corresponding CSS files in public/stylesheets.
For instance, public/stylesheets/sass/main.scss would be compiled to public/stylesheets/main.css.

### Caching

By default, Sass caches compiled templates and [partials](#partials).
This dramatically speeds up re-compilation of large collections of Sass files,
and works best if the Sass templates are split up into separate files
that are all [`@import`](#import)ed into one large file.

Without a framework, Sass puts the cached templates in the `.sass-cache` directory.
In Rails and Merb, they go in `tmp/sass-cache`.
The directory can be customized with the [`:cache_location`](#cache_location-option) option.
If you don't want Sass to use caching at all,
set the [`:cache`](#cache-option) option to `false`.

### Options

Options can be set by setting the {Sass::Plugin#options Sass::Plugin.options} hash
in `environment.rb` in Rails or `config.ru` in Rack...

    Sass::Plugin.options[:style] = :compact

...or by setting the `Merb::Plugin.config[:sass]` hash in `init.rb` in Merb...

    Merb::Plugin.config[:sass][:style] = :compact

...or by passing an options hash to {Sass::Engine#initialize}.
All relevant options are also available via flags
to the `sass` command-line executable.
Available options are:

{#style-option} `:style`
: Sets the style of the CSS output.
  See [Output Style](#output_style).

{#syntax-option} `:syntax`
: The syntax of the input file, `:sass` for the indented syntax
  and `:scss` for the CSS-extension syntax.
  This is only useful when you're constructing {Sass::Engine} instances yourself;
  it's automatically set properly when using {Sass::Plugin}.
  Defaults to `:sass`.

{#property_syntax-option} `:property_syntax`
: Forces indented-syntax documents to use one syntax for properties.
  If the correct syntax isn't used, an error is thrown.
  `:new` forces the use of a colon or equals sign
  after the property name.
  For example: `color: #0f3`
  or `width: $main_width`.
  `:old` forces the use of a colon
  before the property name.
  For example: `:color #0f3`
  or `:width $main_width`.
  By default, either syntax is valid.
  This has no effect on SCSS documents.

{#cache-option} `:cache`
: Whether parsed Sass files should be cached,
  allowing greater speed. Defaults to true.

{#never_update-option} `:never_update`
: Whether the CSS files should never be updated,
  even if the template file changes.
  Setting this to true may give small performance gains.
  It always defaults to false.
  Only has meaning within Rack, Ruby on Rails, or Merb.

{#always_update-option} `:always_update`
: Whether the CSS files should be updated every
  time a controller is accessed,
  as opposed to only when the template has been modified.
  Defaults to false.
  Only has meaning within Rack, Ruby on Rails, or Merb.

{#always_check-option} `:always_check`
: Whether a Sass template should be checked for updates every
  time a controller is accessed,
  as opposed to only when the server starts.
  If a Sass template has been updated,
  it will be recompiled and will overwrite the corresponding CSS file.
  Defaults to false in production mode, true otherwise.
  Only has meaning within Rack, Ruby on Rails, or Merb.

{#full_exception-option} `:full_exception`
: Whether an error in the Sass code
  should cause Sass to provide a detailed description
  within the generated CSS file.
  If set to true, the error will be displayed
  along with a line number and source snippet
  both as a comment in the CSS file
  and at the top of the page (in supported browsers).
  Otherwise, an exception will be raised in the Ruby code.
  Defaults to false in production mode, true otherwise.
  Only has meaning within Rack, Ruby on Rails, or Merb.

{#template_location-option} `:template_location`
: A path to the root sass template directory for your application.
  If a hash, `:css_location` is ignored and this option designates
  a mapping between input and output directories.
  May also be given a list of 2-element lists, instead of a hash.
  Defaults to `css_location + "/sass"`.
  Only has meaning within Rack, Ruby on Rails, or Merb.
  Note that if multiple template locations are specified, all
  of them are placed in the import path, allowing you to import
  between them.

{#css_location-option} `:css_location`
: The path where CSS output should be written to.
  This option is ignored when `:template_location` is a Hash.
  Defaults to `"./public/stylesheets"`.
  Only has meaning within Rack, Ruby on Rails, or Merb.

{#cache_location-option} `:cache_location`
: The path where the cached `sassc` files should be written to.
  Defaults to `"./tmp/sass-cache"` in Rails and Merb,
  or `"./.sass-cache"` otherwise.

{#unix_newlines-option} `:unix_newlines`
: If true, use Unix-style newlines when writing files.
  Only has meaning on Windows, and only when Sass is writing the files
  (in Rack, Rails, or Merb, when using {Sass::Plugin} directly,
  or when using the command-line executable).

{#filename-option} `:filename`
: The filename of the file being rendered.
  This is used solely for reporting errors,
  and is automatically set when using Rack, Rails, or Merb.

{#line-option} `:line`
: The number of the first line of the Sass template.
  Used for reporting line numbers for errors.
  This is useful to set if the Sass template is embedded in a Ruby file.

{#load_paths-option} `:load_paths`
: An array of filesystem paths which should be searched
  for Sass templates imported with the [`@import`](#import) directive.
  This defaults to the working directory and, in Rack, Rails, or Merb,
  whatever `:template_location` is.

{#line_numbers-option} `:line_numbers`
: When set to true, causes the line number and file
  where a selector is defined to be emitted into the compiled CSS
  as a comment. Useful for debugging, especially when using imports
  and mixins.
  This option may also be called `:line_comments`.
  Automatically disabled when using the `:compressed` output style
  or the `:debug_info` option.

{#debug_info-option} `:debug_info`
: When set to true, causes the line number and file
  where a selector is defined to be emitted into the compiled CSS
  in a format that can be understood by the browser.
  Useful in conjunction with [the FireSass Firebug extension](https://addons.mozilla.org/en-US/firefox/addon/103988)
  for displaying the Sass filename and line number.
  Automatically disabled when using the `:compressed` output style.

{#custom-option} `:custom`
: An option that's available for individual applications to set
  to make data available to {Sass::Script::Functions custom Sass functions}.

{#sass2-option} `:sass2`
: Parses the document using semantics closer to that of Sass v2.
  Currently, this just means that strings in mixin arguments
  are treated as though they were in [an `=` context](#sass-script-strings).

## CSS Extensions

### Nested Rules

Sass allows CSS rules to be nested within one another.
The inner rule then only applies within the outer rule's selector.
For example:

    #main p {
      color: #00ff00;
      width: 97%;

      .redbox {
        background-color: #ff0000;
        color: #000000;
      }
    }

is compiled to:

    #main p {
      color: #00ff00;
      width: 97%; }
      #main p .redbox {
        background-color: #ff0000;
        color: #000000; }

This helps avoid repetition of parent selectors,
and makes complex CSS layouts with lots of nested selectors much simpler.
For example:

    #main {
      width: 97%;

      p, div {
        font-size: 2em;
        a { font-weight: bold; }
      }

      pre { font-size: 3em; }
    }

is compiled to:

    #main {
      width: 97%; }
      #main p, #main div {
        font-size: 2em; }
        #main p a, #main div a {
          font-weight: bold; }
      #main pre {
        font-size: 3em; }

### Referencing Parent Selectors: `&`

Sometimes it's useful to use a nested rule's parent selector
in other ways than the default.
For instance, you might want to have special styles
for when that selector is hovered over
or for when the body element has a certain class.
In these cases, you can explicitly specify where the parent selector
should be inserted using the `&` character.
For example:

    a {
      font-weight: bold;
      text-decoration: none;
      &:hover { text-decoration: underline; }
      body.firefox & { font-weight: normal; }
    }

is compiled to:

    a {
      font-weight: bold;
      text-decoration: none; }
      a:hover {
        text-decoration: underline; }
      body.firefox a {
        font-weight: normal; }

`&` will be replaced with the parent selector as it appears in the CSS.
This means that if you have a deeply nested rule,
the parent selector will be fully resolved
before the `&` is replaced.
For example:

    #main {
      color: black;
      a {
        font-weight: bold;
        &:hover { color: red; }
      }
    }

is compiled to:

    #main {
      color: black; }
      #main a {
        font-weight: bold; }
        #main a:hover {
          color: red; }

### Nested Properties

CSS has quite a few properties that are in "namespaces;"
for instance, `font-family`, `font-size`, and `font-weight`
are all in the `font` namespace.
In CSS, if you want to set a bunch of properties in the same namespace,
you have to type it out each time.
Sass provides a shortcut for this:
just write the namespace one,
then nest each of the sub-properties within it.
For example:

    .funky {
      font: {
        family: fantasy;
        size: 30em;
        weight: bold;
      }
    }

is compiled to:

    .funky {
      font-family: fantasy;
      font-size: 30em;
      font-weight: bold; }

The property namespace itself can also have a value.
For example:

    .funky {
      font: 2px/3px {
        family: fantasy;
        size: 30em;
        weight: bold;
      }
    }

is compiled to:

    .funky {
      font: 2px/3px;
        font-family: fantasy;
        font-size: 30em;
        font-weight: bold; }

## SassScript {#sassscript}

In addition to the plain CSS property syntax,
Sass supports a small set of extensions called SassScript.
SassScript allows properties to use
variables, arithmetic, and extra functions.
SassScript can be used in any property value.

SassScript can also be used to generate selectors and property names,
which is useful when writing [mixins](#mixins).
This is done via [interpolation](#interpolation_).

### Interactive Shell

You can easily experiment with SassScript using the interactive shell.
To launch the shell run the sass command-line with the `-i` option. At the
prompt, enter any legal SassScript expression to have it evaluated
and the result printed out for you:

    $ sass -i
    >> "Hello, Sassy World!"
    "Hello, Sassy World!"
    >> 1px + 1px + 1px
    3px
    >> #777 + #777
    #eeeeee
    >> #777 + #888
    white

### Variables: `$` {#variables_}

The most straightforward way to use SassScript
is to use variables.
Variables begin with dollar signs,
and are set like CSS properties:

    $width: 5em;

You can then refer to them in properties:

    #main {
      width: $width;
    }

Variables are only available within the level of nested selectors
where they're defined.
If they're defined outside of any nested selectors,
they're available everywhere.

Variables used to use the prefix character `!`;
this still works, but it's deprecated and prints a warning.
`$` is the recommended syntax.

Variables also used to be defined with `=` rather than `:`;
this still works, but it's deprecated and prints a warning.
`:` is the recommended syntax.

### Data Types

SassScript supports four main data types:

* numbers (e.g. `1.2`, `13`, `10px`)
* strings of text, with and without quotes (e.g. `"foo"`, `'bar'`, `baz`)
* colors (e.g. `blue`, `#04a3f9`, `rgba(255, 0, 0, 0.5)`)
* booleans (e.g. `true`, `false`)

SassScript also supports all other types of CSS property value,
such as Unicode ranges and `!important` declarations.
However, it has no special handling for these types.
They're treated just like unquoted strings.

#### Strings {#sass-script-strings}

CSS specifies two kinds of strings: those with quotes,
such as `"Lucida Grande"` or `'http://sass-lang.com'`,
and those without quotes, such as `sans-serif` or `bold`.
SassScript recognizes both kinds,
and in general if one kind of string is used in the Sass document,
that kind of string will be used in the resulting CSS.

There is one exception to this, though:
when using [`#{}` interpolation](#interpolation_),
quoted strings are unquoted.
This makes it easier to use e.g. selector names in [mixins](#mixins).
For example:

    @mixin firefox-message($selector) {
      body.firefox #{$selector}:before {
        content: "Hi, Firefox users!"; } }

    @include firefox-message(".header");

is compiled to:

    body.firefox .header:before {
      content: "Hi, Firefox users!"; }

It's also worth noting that when using the [deprecated `=` property syntax](#sassscript),
all strings are interpreted as unquoted,
regardless of whether or not they're written with quotes.

### Operations

All types support equality operations (`==` and `!=`).
In addition, each type has its own operations
that it has special support for.

#### Number Operations

SassScript supports the standard arithmetic operations on numbers
(`+`, `-`, `*`, `/`, `%`),
and will automatically convert between units if it can:

    p {
      width: 1in + 8pt;
    }

is compiled to:

    p {
      width: 1.111in; }

Relational operators
(`<`, `>`, `<=`, `>=`)
are also supported for numbers,
and equality operators
(`==`, `!=`)
are supported for all types.

##### Division and `/`

CSS allows `/` to appear in property values
as a way of separating numbers.
Since SassScript is an extension of the CSS property syntax,
it must support this, while also allowing `/` to be used for division.
This means that by default, if two numbers are separated by `/` in SassScript,
then they will appear that way in the resulting CSS.

However, there are three situations where the `/` will be interpreted as division.
These cover the vast majority of cases where division is actually used.
They are:

1. If the value, or any part of it, is stored in a variable.
2. If the value is surrounded by parentheses.
3. If the value is used as part of another arithmetic expression.

For example:

    p {
      font: 10px/8px;             // Plain CSS, no division
      $width: 1000px;
      width: $width/2;            // Uses a variable, does division
      height: (500px/2);          // Uses parentheses, does division
      margin-left: 5px + 8px/2px; // Uses +, does division
    }

is compiled to:

    p {
      font: 10px/8px;
      width: 500px;
      height: 250px;
      margin-left: 9px; }

#### Color Operations

All arithmetic operations are supported for color values,
where they work piecewise.
This means that the operation is performed
on the red, green, and blue components in turn.
For example:

    p {
      color: #010203 + #040506;
    }

computes `01 + 04 = 05`, `02 + 05 = 07`, and `03 + 06 = 09`,
and is compiled to:

    p {
      color: #050709; }

Often it's more useful to use {Sass::Script::Functions color functions}
than to try to use color arithmetic to achieve the same effect.

Arithmetic operations also work between numbers and colors,
also piecewise.
For example:

    p {
      color: #010203 * 2;
    }

computes `01 * 2 = 02`, `02 * 2 = 04`, and `03 * 2 = 06`,
and is compiled to:

    p {
      color: #020406; }

Note that colors with an alpha channel
(those created with the {Sass::Script::Functions#rgba rgba}
or {Sass::Script::Functions#hsla hsla} functions)
must have the same alpha value in order for color arithmetic
to be done with them.
The arithmetic doesn't affect the alpha value.
For example:

    p {
      color: rgba(255, 0, 0, 0.75) + rgba(0, 255, 0, 0.75);
    }

is compiled to:

    p {
      color: rgba(255, 255, 0, 0.75); }

The alpha channel of a color can be adjusted using the
{Sass::Script::Functions#opacify opacify} and
{Sass::Script::Functions#transparentize transparentize} functions.
For example:

    $translucent-red: rgba(255, 0, 0, 0.5);
    p {
      color: opacify($translucent-red, 80%);
      background-color: transparentize($translucent-red, 50%);
    }

is compiled to:

    p {
      color: rgba(255, 0, 0, 0.9);
      background-color: rgba(255, 0, 0, 0.25); }

#### String Operations

The `+` operation can be used to concatenate strings:

    p {
      cursor: e + -resize;
    }

is compiled to:

    p {
      cursor: e-resize; }

Note that if a quoted string is added to an unquoted string
(that is, the quoted string is to the left of the `+`),
the result is a quoted string.
Likewise, if an unquoted string is added to a quoted string
(the unquoted string is to the left of the `+`),
the result is an unquoted string.
For example:

    p:before {
      content: "Foo " + Bar;
      font-family: sans- + "serif"; }

is compiled to:

    p:before {
      content: "Foo Bar";
      font-family: sans-serif; }

By default, if two values are placed next to one another,
they are concatenated with a space:

    p {
      margin: 3px + 4px auto;
    }

is compiled to:

    p {
      margin: 7px auto; }

Within a string of text, #{} style interpolation can be used to
place dynamic values within the string:

    p:before {
      content: "I ate #{5 + 10} pies!"; }

is compiled to:

    p:before {
      content: "I ate 15 pies!"; }

#### Boolean Operations

SassScript supports `and`, `or`, and `not` operators
for boolean values.

### Parentheses

Parentheses can be used to affect the order of operations:

    p {
      width: 1em + (2em * 3);
    }

is compiled to:

    p {
      width: 7em; }

### Functions

SassScript defines some useful functions
that are called using the normal CSS function syntax:

    p {
      color: hsl(0, 100%, 50%);
    }

is compiled to:

    #main {
      color: #ff0000; }

See {Sass::Script::Functions} for a full listing of Sass functions,
as well as instructions on defining your own in Ruby.

### Interpolation: `#{}` {#interpolation_}

You can also use SassScript variables in selectors
and property names using #{} interpolation syntax:

    $name: foo;
    $attr: border;
    p.#{$name} { #{$attr}-color: blue }

is compiled to:

    p.foo {
      border-color: blue; }

### Variable Defaults: `!default`

You can assign to variables if they aren't already assigned
by adding the `!default` flag to the end of the value.
This means that if the variable has already been assigned to,
it won't be re-assigned,
but if it doesn't have a value yet, it will be given one.

For example:

    $content: "First content";
    $content: "Second content?" !default;
    $new_content: "First time reference" !default;

    #main {
      content: $content;
      new-content: $new_content;
    }

is compiled to:

    #main {
      content: First content;
      new-content: First time reference; }

## Directives

Directives allow the author to directly issue instructions to the Sass compiler.
They're prefixed with an at sign, `@`,
followed by the name of the directive,
a space, and any arguments to it -
just like CSS directives.
For example:

{.sass-ex}
    @import red.sass

{.scss-ex}
    @import "red.sass";

Some directives can also control whether or how many times
a chunk of Sass is output.
Those are documented under Control Directives.

### `@import` {#import}

The `@import` directive works in a very similar way to the CSS import directive.
It can either compile to a literal CSS `@import` directive for a CSS file,
or it can import a Sass file.
If it imports a Sass file,
not only are the rules from that file included,
but all variables in that file are made available in the current file.

Sass looks for other Sass files in the working directory,
and the Sass file directory under Rack, Rails, or Merb.
Additional search directories may be specified
using the [`:load_paths`](#load_paths-option) option.

`@import` takes a filename with or without an extension.
If an extension isn't provided,
Sass will try to find a Sass file with the given basename in the load paths.

For example,

{.sass-ex}
    @import foo.sass

{.scss-ex}
    @import "foo.sass";

or

{.sass-ex}
    @import foo

{.scss-ex}
    @import "foo";

would compile to

    .foo {
      color: #f00; }

whereas

{.sass-ex}
    @import foo.css

{.scss-ex}
    @import "foo.css";

would compile to

    @import "foo.css";

#### Partials {#partials}

If you have a Sass file that you want to import
but don't want to compile to a CSS file,
you can add an underscore to the beginning of the filename.
This will tell Sass not to compile it to a normal CSS file.
You can then refer to these files without using the underscore.

For example, you might have `_colors.sass`.
Then no `_colors.css` file would be created,
and you can do

{.sass-ex}
    @import colors.sass

{.scss-ex}
    @import colors.sass;

### `@debug`

The `@debug` directive prints the value of a SassScript expression
to standard error.
It's useful for debugging Sass files
that have complicated SassScript going on.
For example:

{.sass-ex}
    @debug 10em + 12em

{.scss-ex}
    @debug 10em + 12em;

outputs:

    Line 1 DEBUG: 22em

### `@font-face`, `@media`, etc.

Sass behaves as you'd expect for normal CSS @-directives.
For example:

{.sass-ex}
    @font-face
      font-family: "Bitstream Vera Sans"
      src: url(http://foo.bar/bvs)

{.scss-ex}
    @font-face {
      font-family: "Bitstream Vera Sans";
      src: url(http://foo.bar/bvs");
    }

compiles to:

    @font-face {
      font-family: "Bitstream Vera Sans";
      src: url(http://foo.bar/bvs); }

and

{.sass-ex}
    @media print
      #sidebar
        display: none

      #main
        background-color: white

{.scss-ex}
    @media print {
      #sidebar { display: none; }

      #main { background-color: white; }
    }

compiles to:

    @media print {
      #sidebar {
        display: none; }

      #main {
        background-color: white; } }

## Control Directives

SassScript supports basic control directives for looping and conditional evaluation.

### `@if`

The `@if` statement takes a SassScript expression
and prints the code nested beneath it if the expression returns
anything other than `false`:

{.sass-ex}
    p
      @if 1 + 1 == 2
        border: 1px solid
      @if 5 < 3
        border: 2px dotted

{.scss-ex}
    p {
      @if 1 + 1 == 2 { border: 1px solid; }
      @if 5 < 3 { border: 2px dotted; }
    }

is compiled to:

    p {
      border: 1px solid; }

The `@if` statement can be followed by several `@else if` statements
and one `@else` statement.
If the `@if` statement fails,
the `@else if` statements are tried in order
until one succeeds or the `@else` is reached.
For example:

{.sass-ex}
    $type: monster
    p
      @if $type == ocean
        color: blue
      @else if $type == matador
        color: red
      @else if $type == monster
        color: green
      @else
        color: black

{.scss-ex}
    $type: monster;
    p {
      @if $type == ocean {
        color: blue;
      } @else if $type == matador {
        color: red;
      } @else if $type == monster {
        color: green;
      } @else {
        color: black;
      }
    }

is compiled to:

    p {
      color: green; }

### `@for`

The `@for` statement has two forms:
`@for <var> from <start> to <end>` or
`@for <var> from <start> through <end>`.
`<var>` is a variable name, like `$i`,
and `<start>` and `<end>` are SassScript expressions
that should return integers.

The `@for` statement sets `<var>` to each number
from `<start>` to `<end>`,
including `<end>` if `through` is used.
For example:

{.sass-ex}
    @for $i from 1 through 3
      .item-#{$i}
        width: 2em * $i

{.scss-ex}
    @for $i from 1 through 3 {
      .item-#{$i} { width: 2em * $i; }
    }

is compiled to:

    .item-1 {
      width: 2em; }
    .item-2 {
      width: 4em; }
    .item-3 {
      width: 6em; }

### `@while`

The `@while` statement repeatedly loops over the nested
block until the statement evaluates to `false`. This can
be used to achieve more complex looping than the `@for`
statement is capable of.
For example:

{.sass-ex}
    $i: 6
    @while $i > 0
      .item-#{$i}
        width: 2em * $i
      $i: $i - 2

{.scss-ex}
    $i: 6;
    @while $i > 0 {
      .item-#{$i} { width: 2em * $i; }
      $i: $i - 2;
    }

is compiled to:

    .item-6 {
      width: 12em; }

    .item-4 {
      width: 8em; }

    .item-2 {
      width: 4em; }

## Mixins {#mixins}

Mixins enable you to define groups of CSS properties and
then include them inline in any number of selectors
throughout the document. This allows you to keep your
stylesheets DRY and also avoid placing presentation
classes in your markup.

### Defining a Mixin: `=`

To define a mixin you use a slightly modified form of selector syntax.
For example the `large-text` mixin is defined as follows:

{.sass-ex}
    =large-text
      font:
        family: Arial
        size: 20px
        weight: bold
      color: #ff0000

{.scss-ex}
    @mixin large-text {
      font: {
        family: Arial;
        size: 20px;
        weight: bold;
      }
      color: #ff0000;
    }

The initial `=` marks this as a mixin rather than a standard selector.
The CSS rules that follow won't be included until the mixin is referenced later on.
Anything you can put into a standard selector,
you can put into a mixin definition.
For example:

{.sass-ex}
    =clearfix
      display: inline-block
      &:after
        content: "."
        display: block
        height: 0
        clear: both
        visibility: hidden
      * html &
        height: 1px

{.scss-ex}
    @mixin clearfix {
      display: inline-block;
      &:after {
        content: ".";
        display: block;
        height: 0;
        clear: both;
        visibility: hidden;
      }
      * html & { height: 1px }
    }

### Mixing It In: `+`

Inlining a defined mixin is simple,
just prepend a `+` symbol to the name of a mixin defined earlier in the document.
So to inline the `large-text` defined earlier,
we include the statment `+large-text` in our selector definition thus:

{.sass-ex}
    .page-title
      +large-text
      padding: 4px
      margin-top: 10px

{.scss-ex}
    .page-title {
      @include large-text;
      padding: 4px;
      margin-top: 10px;
    }

This will produce the following CSS output:

    .page-title {
      font-family: Arial;
      font-size: 20px;
      font-weight: bold;
      color: #ff0000;
      padding: 4px;
      margin-top: 10px; }

Any number of mixins may be defined and there is no limit on
the number that can be included in a particular selector.

Mixin definitions can also include references to other mixins.
For example:

{.sass-ex}
    =compound
      +highlighted-background
      +header-text

    =highlighted-background
      background-color: #fc0
    =header-text
      font-size: 20px

{.scss-ex}
    @mixin compound {
      @include highlighted-background;
      @include header-text;
    }

    @mixin highlighted-background {
      background-color: #fc0;
    }
    @mixin header-text {
      font-size: 20px;
    }

Mixins that only define descendent selectors, can be safely mixed
into the top most level of a document.

### Arguments

Mixins can take arguments which can be used with SassScript:

{.sass-ex}
    =sexy-border($color)
      border:
        color: $color
        width: 1in
        style: dashed
    p
      +sexy-border("blue")

{.scss-ex}
    @mixin sexy-border($color) {
      border: {
        color: $color;
        width: 1in;
        style: dashed;
      }
    }

    p { @include sexy-border("blue"); }

is compiled to:

    p {
      border-color: #0000ff;
      border-width: 1in;
      border-style: dashed; }

Mixins can also specify default values for their arguments:

{.sass-ex}
    =sexy-border($color, $width: 1in)
      border:
        color: $color
        width: $width
        style: dashed
    p
      +sexy-border("blue")

{.scss-ex}
    @mixin sexy-border($color, $width: 1in) {
      border: {
        color: $color;
        width: $width;
        style: dashed;
      }
    p { @include sexy-border("blue"); }

is compiled to:

    p {
      border-color: #0000ff;
      border-width: 1in;
      border-style: dashed; }

## Comments

Sass supports two sorts of comments:
those that show up in the CSS output
and those that don't.

### CSS Comments: `/*`

"Loud" comments are just as easy as silent ones.
These comments output to the document as CSS comments,
and thus use the same opening sequence: `/*`.
For example:

{.sass-ex}
    /* A very awesome rule.
    #awesome.rule
      /* An equally awesome property.
      awesomeness: very

{.scss-ex}
    /* A very awesome rule. */
    #awesome.rule {
      /* An equally awesome property. */
      awesomeness: very;
    }

becomes

    /* A very awesome rule. */
    #awesome.rule {
      /* An equally awesome property. */
      awesomeness: very; }

You can also nest content beneath loud comments. For example:

{.sass-ex}
    #pbj
      /* This rule describes
        the styling of the element
        that represents
        a peanut butter and jelly sandwich.
      background-image: url(/images/pbj.png)
      color: red

{.scss-ex}
    #pbj {
      /* This rule describes
        the styling of the element
        that represents
        a peanut butter and jelly sandwich. */
      background-image: url(/images/pbj.png);
      color: red;
    }

becomes

    #pbj {
      /* This rule describes
       * the styling of the element
       * that represents
       * a peanut butter and jelly sandwich. */
      background-image: url(/images/pbj.png);
      color: red; }

### Sass Comments: `//`

It's simple to add "silent" comments,
which don't output anything to the CSS document,
to a Sass document.
Simply use the familiar C-style notation for a one-line comment, `//`,
at the normal indentation level and all text following it won't be output.
For example:

{.sass-ex}
    // A very awesome rule.
    #awesome.rule
      // An equally awesome property.
      awesomeness: very

{.scss-ex}
    // A very awesome rule.
    #awesome.rule {
      // An equally awesome property.
      awesomeness: very;
    }

becomes

    #awesome.rule {
      awesomeness: very; }

You can also nest text beneath a comment to comment out a whole block.
For example:

{.sass-ex}
    // A very awesome rule
    #awesome.rule
      // Don't use these properties
        color: green
        font-size: 10em
      color: red

{.scss-ex}
    // A very awesome rule
    #awesome.rule {
      // Don't use these properties
      // color: green
      // font-size: 10em
      color: red;
    }

becomes

    #awesome.rule {
      color: red; }

## Output Style

Although the default CSS style that Sass outputs is very nice,
and reflects the structure of the document in a similar way that Sass does,
sometimes it's good to have other formats available.

Sass allows you to choose between four different output styles
by setting the `:style` option.
In Rack, Rails, and Merb, this is done by setting `Sass::Plugin.options[:style]`;
otherwise, it's done by passing an options hash with `:style` set.

### `:nested`

Nested style is the default Sass style,
because it reflects the structure of the document
in much the same way Sass does.
Each property has its own line,
but the indentation isn't constant.
Each rule is indented based on how deeply it's nested.
For example:

    #main {
      color: #fff;
      background-color: #000; }
      #main p {
        width: 10em; }

    .huge {
      font-size: 10em;
      font-weight: bold;
      text-decoration: underline; }

Nested style is very useful when looking at large CSS files
for the same reason Sass is useful for making them:
it allows you to very easily grasp the structure of the file
without actually reading anything.

### `:expanded`

Expanded is the typical human-made CSS style,
with each property and rule taking up one line.
Properties are indented within the rules,
but the rules aren't indented in any special way.
For example:

    #main {
      color: #fff;
      background-color: #000;
    }
    #main p {
      width: 10em;
    }

    .huge {
      font-size: 10em;
      font-weight: bold;
      text-decoration: underline;
    }

### `:compact`

Compact style, as the name would imply,
takes up less space than Nested or Expanded.
However, it's also harder to read.
Each CSS rule takes up only one line,
with every property defined on that line.
Nested rules are placed next to each other with no newline,
while groups of rules have newlines between them.
For example:

    #main { color: #fff; background-color: #000; }
    #main p { width: 10em; }

    .huge { font-size: 10em; font-weight: bold; text-decoration: underline; }

### `:compressed`

Compressed style takes up the minimum amount of space possible,
having no whitespace except that necessary to separate selectors
and a newline at the end of the file.
It's not meant to be human-readable.
For example:

    #main{color:#fff;background-color:#000}#main p{width:10em}.huge{font-size:10em;font-weight:bold;text-decoration:underline}