# 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.
The first step for all of these is to install the Sass gem:

    gem install sass

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 Sass 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 in Rails versions before Rails 3,
add the following line to `environment.rb`:

    config.gem "sass"

For Rails 3, instead add the following line to the Gemfile:

    gem "sass"

To enable Sass in Merb,
add the following line to `config/dependencies.rb`:

    dependency "merb-haml"

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::Configuration#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` and `scss` command-line executables.
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.

{#read_cache-option} `:read_cache`
: If this is set and `:cache` is not,
  only read the Sass cache if it exists,
  don't write to it if it doesn't.

{#cache_store-option} `:cache_store`
: If this is set to an instance of a subclass of {Sass::CacheStores::Base},
  that cache store will be used to store and retrieve
  cached compilation results.
  Defaults to a {Sass::CacheStores::Filesystem} that is
  initialized using the [`:cache_location` option](#cache_location-option).

{#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.
  **Note that due to the many possible formats it can take,
  this option should only be set directly, not accessed or modified.
  Use the {Sass::Plugin::Configuration#template_location_array Sass::Plugin#template_location_array},
  {Sass::Plugin::Configuration#add_template_location Sass::Plugin#add_template_location},
  and {Sass::Plugin::Configuration#remove_template_location Sass::Plugin#remove_template_location} methods instead**.

{#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.
  If the [`:cache_store` option](#cache_location-option) is set,
  this is ignored.

{#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 or importers which should be searched
  for Sass templates imported with the [`@import`](#import) directive.
  These may be strings, `Pathname` objects, or subclasses of {Sass::Importers::Base}.
  This defaults to the working directory and, in Rack, Rails, or Merb,
  whatever `:template_location` is.

{#filesystem_importer-option} `:filesystem_importer`
: A {Sass::Importers::Base} subclass used to handle plain string load paths.
  This should import files from the filesystem.
  It should be a Class object inheriting from {Sass::Importers::Base}
  with a constructor that takes a single string argument (the load path).
  Defaults to {Sass::Importers::Filesystem}.

{#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}.

{#quiet-option} `:quiet`
: When set to true, causes warnings to be disabled.

### Syntax Selection

The Sass command-line tool will use the file extension to determine which
syntax you are using, but there's not always a filename. The `sass`
command-line program defaults to the indented syntax but you can pass the
`--scss` option to it if the input should be interpreted as SCSS syntax.
Alternatively, you can use the `scss` command-line program which is exactly
like the `sass` program but it defaults to assuming the syntax is SCSS.

### Encodings

When running on Ruby 1.9 and later, Sass is aware of the character encoding of documents
and will handle them the same way that CSS would.
By default, Sass assumes that all stylesheets are encoded
using whatever coding system your operating system defaults to.
For many users this will be `UTF-8`, the de facto standard for the web.
For some users, though, it may be a more local encoding.

If you want to use a different encoding for your stylesheet
than your operating system default,
you can use the `@charset` declaration just like in CSS.
Add `@charset "encoding-name";` at the beginning of the stylesheet
(before any whitespace or comments)
and Sass will interpret it as the given encoding.
Note that whatever encoding you use, it must be convertible to Unicode.

Sass will also respect any Unicode BOMs and non-ASCII-compatible Unicode encodings
[as specified by the CSS spec](http://www.w3.org/TR/CSS2/syndata.html#charset),
although this is *not* the recommended way
to specify the character set for a document.
Note that Sass does not support the obscure `UTF-32-2143`,
`UTF-32-3412`, `EBCDIC`, `IBM1026`, and `GSM 03.38` encodings,
since Ruby does not have support for them
and they're highly unlikely to ever be used in practice.

#### Output Encoding

In general, Sass will try to encode the output stylesheet
using the same encoding as the input stylesheet.
In order for it to do this, though, the input stylesheet must have a `@charset` declaration;
otherwise, Sass will default to encoding the output stylesheet as UTF-8.
In addition, it will add a `@charset` declaration to the output
if it's not plain ASCII.

When other stylesheets with `@charset` declarations are `@import`ed,
Sass will convert them to the same encoding as the main stylesheet.

Note that Ruby 1.8 does not have good support for character encodings,
and so Sass behaves somewhat differently when running under it than under Ruby 1.9 and later.
In Ruby 1.8, Sass simply uses the first `@charset` declaration in the stylesheet
or any of the other stylesheets it `@import`s.

## 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; }

## Comments: `/* */` and `//` {#comments}

Sass supports standard multiline CSS comments with `/* */`,
as well as single-line comments with `//`.
The multiline comments are preserved in the CSS output where possible,
while the single-line comments are removed.
For example:

    /* This comment is
     * several lines long.
     * since it uses the CSS comment syntax,
     * it will appear in the CSS output. */
    body { color: black; }

    // These comments are only one line long each.
    // They won't appear in the CSS output,
    // since they use the single-line comment syntax.
    a { color: green; }

is compiled to:

    /* This comment is
     * several lines long.
     * since it uses the CSS comment syntax,
     * it will appear in the CSS output. */
    body {
      color: black; }

    a {
      color: green; }

When the first letter of a comment is `!`, the comment will be interpolated
and always rendered into css output even in compressed output modes. This is useful for adding Copyright notices to your generated CSS.

## 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`)
* lists of values, separated by spaces or commas (e.g. `1.5em 1em 0 2em`, `Helvetica, Arial, sans-serif`)

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.

#### Lists

Lists are how Sass represents the values of CSS declarations
like `margin: 10px 15px 0 0` or `font-face: Helvetica, Arial, sans-serif`.
Lists are just a series of other values, separated by either spaces or commas.
In fact, individual values count as lists, too: they're just lists with one item.

On their own, lists don't do much,
but the {file:Sass/Script/Functions.html#list-functions Sass list functions}
make them useful.
The {Sass::Script::Functions#nth nth function} can access items in a list,
the {Sass::Script::Functions#join join function} can join multiple lists together,
and the {Sass::Script::Functions#append append function} can add items to lists.
The [`@each` rule](#each-directive) can also add styles for each item in a list.

In addition to containing simple values, lists can contain other lists.
For example, `1px 2px, 5px 6px` is a two-item list
containing the list `1px 2px` and the list `5px 6px`.
If the inner lists have the same separator as the outer list,
you'll need to use parentheses to make it clear
where the inner lists start and stop.
For example, `(1px 2px) (5px 6px)` is also a two-item list
containing the list `1px 2px` and the list `5px 6px`.
The difference is that the outer list is space-separated,
where before it was comma-separated.

When lists are turned into plain CSS, Sass doesn't add any parentheses,
since CSS doesn't understand them.
That means that `(1px 2px) (5px 6px)` and `1px 2px 5px 6px`
will look the same when they become CSS.
However, they aren't the same when they're Sass:
the first is a list containing two lists,
while the second is a list containing four numbers.

Lists can also have no items in them at all.
These lists are represented as `()`.
They can't be output directly to CSS;
if you try to do e.g. `font-family: ()`, Sass will raise an error.
If a list contains empty lists, as in `1px 2px () 3px`,
the empty list will be removed before it's turned into CSS.

### 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 `/`
{#division-and-slash}

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; }

If you want to use variables along with a plain CSS `/`,
you can use `#{}` to insert them.
For example:

    p {
      $font-size: 12px;
      $line-height: 30px;
      font: #{$font-size}/#{$line-height};
    }

is compiled to:

    p {
      font: 12px/30px;
    }

#### 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, 0.8);
      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.

#### List Operations

Lists don't support any special operations.
Instead, they're manipulated using the
{file:Sass/Script/Functions.html#list-functions list functions}.

### 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:

    p {
      color: #ff0000; }

#### Keyword Arguments

Sass functions can also be called using explicit keyword arguments.
The above example can also be written as:

    p {
      color: hsl($hue: 0, $saturation: 100%, $lightness: 50%);
    }

While this is less concise, it can make the stylesheet easier to read.
It also allows functions to present more flexible interfaces,
providing many arguments without becoming difficult to call.

Named arguments can be passed in any order, and arguments with default values can be omitted.
Since the named arguments are variable names, underscores and dashes can be used interchangeably.

See {Sass::Script::Functions} for a full listing of Sass functions and their argument names,
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; }

It's also possible to use `#{}` to put SassScript into property values.
In most cases this isn't any better than using a variable,
but using `#{}` does mean that any operations near it
will be treated as plain CSS.
For example:

    p {
      $font-size: 12px;
      $line-height: 30px;
      font: #{$font-size}/#{$line-height};
    }

is compiled to:

    p {
      font: 12px/30px;
    }

### 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"; }

## `@`-Rules and Directives {#directives}

Sass supports all CSS3 `@`-rules,
as well as some additional Sass-specific ones
known as "directives."
These have various effects in Sass, detailed below.
See also [control directives](#control-directives)
and [mixin directives](#mixins).

### `@import` {#import}

Sass extends the CSS `@import` rule
to allow it to import SCSS and Sass files.
All imported SCSS and Sass files will be merged together
into a single CSS output file.
In addition, any variables or [mixins](#mixins)
defined in imported files can be used in the main file.

Sass looks for other Sass files in the current 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,
or the `--load-path` option on the command line.

`@import` takes a filename to import.
By default, it looks for a Sass file to import directly,
but there are a few circumstances under which it will compile to a CSS `@import` rule:

* If the file's extension is `.css`.
* If the filename begins with `http://`.
* If the filename is a `url()`.
* If the `@import` has any media queries.

If none of the above conditions are met
and the extension is `.scss` or `.sass`,
then the named Sass or SCSS file will be imported.
If there is no extension,
Sass will try to find a file with that name and the `.scss` or `.sass` extension
and import it.

For example,

    @import "foo.scss";

or

    @import "foo";

would both import the file `foo.scss`,
whereas

    @import "foo.css";
    @import "foo" screen;
    @import "http://foo.com/bar";
    @import url(foo);

would all compile to

    @import "foo.css";
    @import "foo" screen;
    @import "http://foo.com/bar";
    @import url(foo);

It's also possible to import multiple files in one `@import`. For example:

    @import "rounded-corners", "text-shadow";

would import both the `rounded-corners` and the `text-shadow` files.

#### Partials {#partials}

If you have a SCSS or 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 import these files without using the underscore.

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

    @import "colors";

and `_colors.scss` would be imported.

#### Nested `@import` {#nested-import}

Although most of the time it's most useful to just have `@import`s
at the top level of the document,
it is possible to include them within CSS rules and `@media` rules.
Like a base-level `@import`, this includes the contents of the `@import`ed file.
However, the imported rules will be nested in the same place as the original `@import`.

For example, if `example.scss` contains

    .example {
      color: red;
    }

then

    #main {
      @import "example";
    }

would compile to

    #main .example {
      color: red;
    }

Directives that are only allowed at the base level of a document,
like `@mixin` or `@charset`, are not allowed in files that are `@import`ed
in a nested context.

It's not possible to nest `@import` within mixins or control directives.

### `@media` {#media}

`@media` directives in Sass behave just like they do in plain CSS,
with one extra capability: they can be nested in CSS rules.
If a `@media` directive appears within a CSS rule,
it will be bubbled up to the top level of the stylesheet,
putting all the selectors on the way inside the rule.
This makes it easy to add media-specific styles
without having to repeat selectors
or break the flow of the stylesheet.
For example:

    .sidebar {
      width: 300px;
      @media screen and (orientation: landscape) {
        width: 500px;
      }
    }

is compiled to:

    .sidebar {
      width: 300px;
    }
    @media screen and (orientation: landscape) {
      .sidebar {
        width: 500px;
      }
    }

`@media` queries can also be nested within one another.
The queries will then be combined using the `and` operator.
For example:

    @media screen {
      .sidebar {
        @media (orientation: landscape) {
          width: 500px;
        }
      }
    }

is compiled to:

    @media screen and (orientation: landscape) {
      .sidebar {
        width: 500px;
      }
    }

### `@extend` {#extend}

There are often cases when designing a page
when one class should have all the styles of another class,
as well as its own specific styles.
The most common way of handling this is to use both the more general class
and the more specific class in the HTML.
For example, suppose we have a design for a normal error
and also for a serious error. We might write our markup like so:

    <div class="error seriousError">
      Oh no! You've been hacked!
    </div>

And our styles like so:

    .error {
      border: 1px #f00;
      background-color: #fdd;
    }
    .seriousError {
      border-width: 3px;
    }

Unfortunately, this means that we have to always remember
to use `.error` with `.seriousError`.
This is a maintenance burden, leads to tricky bugs,
and can bring non-semantic style concerns into the markup.

The `@extend` directive avoids these problems
by telling Sass that one selector should inherit the styles of another selector.
For example:

    .error {
      border: 1px #f00;
      background-color: #fdd;
    }
    .seriousError {
      @extend .error;
      border-width: 3px;
    }

This means that all styles defined for `.error`
are also applied to `.seriousError`,
in addition to the styles specific to `.seriousError`.
In effect, everything with class `.seriousError` also has class `.error`.

Other rules that use `.error` will work for `.seriousError` as well.
For example, if we have special styles for errors caused by hackers:

    .error.intrusion {
      background-image: url("/image/hacked.png");
    }

Then `<div class="seriousError intrusion">`
will have the `hacked.png` background image as well.

#### How it Works

`@extend` works by inserting the extending selector (e.g. `.seriousError`)
anywhere in the stylesheet that the extended selector (.e.g `.error`) appears.
Thus the example above:

    .error {
      border: 1px #f00;
      background-color: #fdd;
    }
    .error.intrusion {
      background-image: url("/image/hacked.png");
    }
    .seriousError {
      @extend .error;
      border-width: 3px;
    }

is compiled to:

    .error, .seriousError {
      border: 1px #f00;
      background-color: #fdd; }

    .error.intrusion, .seriousError.intrusion {
      background-image: url("/image/hacked.png"); }

    .seriousError {
      border-width: 3px; }

When merging selectors, `@extend` is smart enough
to avoid unnecessary duplication,
so something like `.seriousError.seriousError` gets translated to `.seriousError`.
In addition, it won't produce selectors that can't match anything, like `#main#footer`.

#### Extending Complex Selectors

Class selectors aren't the only things that can be extended.
It's possible to extend any selector involving only a single element,
such as `.special.cool`, `a:hover`, or `a.user[href^="http://"]`.
For example:

    .hoverlink {@extend a:hover}

Just like with classes, this means that all styles defined for `a:hover`
are also applied to `.hoverlink`.
For example:

    .hoverlink {@extend a:hover}
    a:hover {text-decoration: underline}

is compiled to:

    a:hover, .hoverlink {text-decoration: underline}

Just like with `.error.intrusion` above,
any rule that uses `a:hover` will also work for `.hoverlink`,
even if they have other selectors as well.
For example:

    .hoverlink {@extend a:hover}
    .comment a.user:hover {font-weight: bold}

is compiled to:

    .comment a.user:hover, .comment .hoverlink.user {font-weight: bold}

#### Multiple Extends

A single selector can extend more than one selector.
This means that it inherits the styles of all the extended selectors.
For example:

    .error {
      border: 1px #f00;
      background-color: #fdd;
    }
    .attention {
      font-size: 3em;
      background-color: #ff0;
    }
    .seriousError {
      @extend .error;
      @extend .attention;
      border-width: 3px;
    }

is compiled to:

    .error, .seriousError {
      border: 1px #f00;
      background-color: #fdd; }

    .attention, .seriousError {
      font-size: 3em;
      background-color: #ff0; }

    .seriousError {
      border-width: 3px; }

In effect, everything with class `.seriousError`
also has class `.error` *and* class `.attention`.
Thus, the styles defined later in the document take precedence:
`.seriousError` has background color `#ff0` rather than `#fdd`,
since `.attention` is defined later than `.error`.

#### Chaining Extends

It's possible for one selector to extend another selector
that in turn extends a third.
For example:

    .error {
      border: 1px #f00;
      background-color: #fdd;
    }
    .seriousError {
      @extend .error;
      border-width: 3px;
    }
    .criticalError {
      @extend .seriousError;
      position: fixed;
      top: 10%;
      bottom: 10%;
      left: 10%;
      right: 10%;
    }

Now everything with class `.seriousError` also has class `.error`,
and everything with class `.criticalError` has class `.seriousError`
*and* class `.error`.
It's compiled to:

    .error, .seriousError, .criticalError {
      border: 1px #f00;
      background-color: #fdd; }

    .seriousError, .criticalError {
      border-width: 3px; }

    .criticalError {
      position: fixed;
      top: 10%;
      bottom: 10%;
      left: 10%;
      right: 10%; }

#### Selector Sequences

Selector sequences, such as `.foo .bar` or `.foo + .bar`, currently can't be extended.
However, it is possible for nested selectors themselves to use `@extend`.
For example:

    #fake-links .link {@extend a}

    a {
      color: blue;
      &:hover {text-decoration: underline}
    }

is compiled to

    a, #fake-links .link {
      color: blue; }
      a:hover, #fake-links .link:hover {
        text-decoration: underline; }

##### Merging Selector Sequences

Sometimes a selector sequence extends another selector that appears in another sequence.
In this case, the two sequences need to be merged.
For example:

    #admin .tabbar a {font-weight: bold}
    #demo .overview .fakelink {@extend a}

While it would technically be possible
to generate all selectors that could possibly match either sequence,
this would make the stylesheet far too large.
The simple example above, for instance, would require ten selectors.
Instead, Sass generates only selectors that are likely to be useful.

When the two sequences being merged have no selectors in common,
then two new selectors are generated:
one with the first sequence before the second,
and one with the second sequence before the first.
For example:

    #admin .tabbar a {font-weight: bold}
    #demo .overview .fakelink {@extend a}

is compiled to:

    #admin .tabbar a,
    #admin .tabbar #demo .overview .fakelink,
    #demo .overview #admin .tabbar .fakelink {
      font-weight: bold; }

If the two sequences do share some selectors,
then those selectors will be merged together
and only the differences (if any still exist) will alternate.
In this example, both sequences contain the id `#admin`,
so the resulting selectors will merge those two ids:

    #admin .tabbar a {font-weight: bold}
    #admin .overview .fakelink {@extend a}

This is compiled to:

    #admin .tabbar a,
    #admin .tabbar .overview .fakelink,
    #admin .overview .tabbar .fakelink {
      font-weight: bold; }

### `@debug`

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

    @debug 10em + 12em;

outputs:

    Line 1 DEBUG: 22em

### `@warn`

The `@warn` directive prints the value of a SassScript expression
to the standard error output stream.
It's useful for libraries that need to warn users of deprecations
or recovering from minor mixin usage mistakes.
There are two major distinctions between `@warn` and `@debug`:

1. You can turn warnings off with the `--quiet` command-line option
   or the `:quiet` Sass option.
2. A stylesheet trace will be printed out along with the message
   so that the user being warned can see where their styles caused the warning.

Usage Example:

    @mixin adjust-location($x, $y) {
      @if unitless($x) {
        @warn "Assuming #{$x} to be in pixels";
        $x: 1px * $x;
      }
      @if unitless($y) {
        @warn "Assuming #{$y} to be in pixels";
        $y: 1px * $y;
      }
      position: relative; left: $x; top: $y;
    }

## Control Directives

SassScript supports basic control directives
for including styles only under some conditions
or including the same style several times with variations.

**Note that control directives are an advanced feature,
and are not recommended in the course of day-to-day styling**.
They exist mainly for use in [mixins](#mixins),
particularly those that are part of libraries like [Compass](http://compass-style.org),
and so require substantial flexibility.

### `@if`

The `@if` directive takes a SassScript expression
and uses the styles nested beneath it if the expression returns
anything other than `false`:

    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:

    $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` directive has two forms:
`@for $var from <start> to <end>` or
`@for $var from <start> through <end>`.
`$var` can be any variable name, like `$i`,
and `<start>` and `<end>` are SassScript expressions
that should return integers.

The `@for` statement sets `$var` to each number
from `<start>` to `<end>`,
including `<end>` if `through` is used.
Then it outputs the nested styles
using that value of `$var`.
For example:

    @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; }

### `@each` {#each-directive}

The `@each` rule has the form `@each $var in <list>`.
`$var` can be any variable name, like `$length` or `$name`,
and `<list>` is a SassScript expression that returns a list.

The `@each` rule sets `$var` to each item in the list,
then outputs the styles it contains using that value of `$var`.
For example:

    @each $animal in puma, sea-slug, egret, salamander {
      .#{$animal}-icon {
        background-image: url('/images/#{$animal}.png');
      }
    }

is compiled to:

    .puma-icon {
      background-image: url('/images/puma.png'); }
    .sea-slug-icon {
      background-image: url('/images/sea-slug.png'); }
    .egret-icon {
      background-image: url('/images/egret.png'); }
    .salamander-icon {
      background-image: url('/images/salamander.png'); }

### `@while`

The `@while` directive takes a SassScript expression
and repeatedly outputs the nested styles
until the statement evaluates to `false`.
This can be used to achieve more complex looping
than the `@for` statement is capable of,
although this is rarely necessary.
For example:

    $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; }

## Mixin Directives {#mixins}

Mixins allow you to define styles
that can be re-used throughout the stylesheet
without needing to resort to non-semantic classes like `.float-left`.
Mixins can also contain full CSS rules,
and anything else allowed elsewhere in a Sass document.
They can even take [arguments](#mixin-arguments)
which allows you to produce a wide variety of styles
with very few mixins.

### Defining a Mixin: `@mixin` {#defining_a_mixin}

Mixins are defined with the `@mixin` directive.
It's followed by the name of the mixin
and optionally the [arguments](#mixin-arguments),
and a block containing the contents of the mixin.
For example, the `large-text` mixin is defined as follows:

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

Mixins may also contain selectors,
possibly mixed with properties.
The selectors can even contain [parent references](#referencing_parent_selectors_).
For example:

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

### Including a Mixin: `@include` {#including_a_mixin}

Mixins are included in the document
with the `@include` directive.
This takes the name of a mixin
and optionally [arguments to pass to it](#mixin-arguments),
and includes the styles defined by that mixin
into the current rule.
For example:

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

is compiled to:

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

Mixins may also be included outside of any rule
(that is, at the root of the document)
as long as they don't directly define any properties
or use any parent references.
For example:

    @mixin silly-links {
      a {
        color: blue;
        background-color: red;
      }
    }

    @include silly-links;

is compiled to:

    a {
      color: blue;
      background-color: red; }

Mixin definitions can also include other mixins.
For example:

    @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 {#mixin-arguments}

Mixins can take arguments SassScript values as arguments,
which are given when the mixin is included
and made available within the mixin as variables.

When defining a mixin,
the arguments are written as variable names separated by commas,
all in parentheses after the name.
Then when including the mixin,
values can be passed in in the same manner.
For example:

    @mixin sexy-border($color, $width) {
      border: {
        color: $color;
        width: $width;
        style: dashed;
      }
    }

    p { @include sexy-border(blue, 1in); }

is compiled to:

    p {
      border-color: blue;
      border-width: 1in;
      border-style: dashed; }

Mixins can also specify default values for their arguments
using the normal variable-setting syntax.
Then when the mixin is included,
if it doesn't pass in that argument,
the default value will be used instead.
For example:

    @mixin sexy-border($color, $width: 1in) {
      border: {
        color: $color;
        width: $width;
        style: dashed;
      }
    }
    p { @include sexy-border(blue); }
    h1 { @include sexy-border(blue, 2in); }

is compiled to:

    p {
      border-color: blue;
      border-width: 1in;
      border-style: dashed; }

    h1 {
      border-color: blue;
      border-width: 2in;
      border-style: dashed; }

#### Keyword Arguments

Mixins can also be included using explicit keyword arguments.
For instance, we the above example could be written as:

    p { @include sexy-border($color: blue); }
    h1 { @include sexy-border($color: blue, $width: 2in); }

While this is less concise, it can make the stylesheet easier to read.
It also allows functions to present more flexible interfaces,
providing many arguments without becoming difficult to call.

Named arguments can be passed in any order, and arguments with default values can be omitted.
Since the named arguments are variable names, underscores and dashes can be used interchangeably.

## Function Directives {#functions}

It is possible to define your own functions in sass and use them in any
value or script context. For example:

    $grid-width: 40px;
    $gutter-width: 10px;
    
    @function grid-width($n) {
      @return $n * $grid-width + ($n - 1) * $gutter-width;
    }
    
    #sidebar { width: grid-width(5); }

Becomes:

    #sidebar {
      width: 240px; }

As you can see functions can access any globally defined variables as well as
accept arguments just like a mixin. A function may have several statements
contained within it, and you must call `@return` to set the return value of
the function.

As with mixins, you can call Sass-defined functions using keyword arguments.
In the above example we could have called the function like this:

    #sidebar { width: grid-width($n: 5); }

It is recommended that you prefix your functions to avoid naming conflicts
and so that readers of your stylesheets know they are not part of Sass or CSS. For example, if you work for ACME Corp, you might have named the function above `-acme-grid-width`.

## Output Style

Although the default CSS style that Sass outputs is very nice
and reflects the structure of the document,
tastes and needs vary and so Sass supports several other styles.

Sass allows you to choose between four different output styles
by setting the [`:style` option](#style-option)
or using the `--style` command-line flag.

### `:nested`

Nested style is the default Sass style,
because it reflects the structure of the CSS styles
and the HTML document they're styling.
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:
it allows you to easily grasp the structure of the file
without actually reading anything.

### `:expanded`

Expanded is a more 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 takes up less space than Nested or Expanded.
It also draws the focus more to the selectors than to their properties.
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 separate 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 also includes some other minor compressions,
such as choosing the smallest representation for colors.
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}

## Extending Sass

Sass provides a number of advanced customizations for users with unique requirements.
Using these features requires a strong understanding of Ruby.

### Defining Custom Sass Functions

Users can define their own Sass functions using the Ruby API.
For more information, see the [source documentation](Sass/Script/Functions.html#adding_custom_functions).

### Cache Stores

Sass caches parsed documents so that they can be reused without parsing them again
unless they have changed. By default, Sass will write these cache files to a location
on the filesystem indicated by [`:cache_location`](#cache_location-option). If you
cannot write to the filesystem or need to share cache across ruby processes or machines,
then you can define your own cache store and set the[`:cache_store`
option](#cache_store-option). For details on creating your own cache store, please
see the {Sass::CacheStores::Base source documentation}.

### Custom Importers

Sass importers are in charge of taking paths passed to `@import` and finding the
appropriate Sass code for those paths. By default, this code is loaded from
the {Sass::Importers::Filesystem filesystem}, but importers could be added to load
from a database, over HTTP, or use a different file naming scheme than what Sass expects.

Each importer is in charge of a single load path (or whatever the corresponding notion
is for the backend). Importers can be placed in the {file:SASS_REFERENCE.md#load_paths-option
`:load_paths` array} alongside normal filesystem paths.

When resolving an `@import`, Sass will go through the load paths looking for an importer
that successfully imports the path. Once one is found, the imported file is used.

User-created importers must inherit from {Sass::Importers::Base}.
