Lisp Augmented Style Sheets. Compiles LASS to CSS.

About LASS

Writing CSS files comes with a lot of repetition and is generally much too verbose. With lispy syntax, shortcuts, and improvements, LASS aims to help you out in writing CSS quick and easy. LASS was largely inspired by SASS.

How To & Examples

LASS supports two modes, one being directly in your lisp code, the other in pure LASS files. Adding LASS into your code is easy:

   :background black))

    background: black;

LASS works on the following simple principles: A list is a block. The first argument in the list is a selector. The body of the list makes up the properties and sub-blocks. A property is started with a keyword that is used as the property name. Following is a bunch of property arguments until a new keyword, list, or the end is reached. A list inside a block is, again, a block with the twist that the parent block's selector is prepended to the sub-block's selector.

    :list-style none
     :margin 0 :padding 0
     :display inline-block)))))

"nav ul{
    list-style: none;

nav ul li{
    margin: 0;
    padding: 0;
    display: inline-block;

Since LASS' COMPILE-SHEET simply takes a bunch of lists as its argument, you can use the backquote and comma to integrate variables from your lisp environment:

(let ((color "#0088EE"))
     :background ,color))))

    background: #0088EE;

Alternatively however, and this is especially useful in pure LASS files, you can use the LET block to create LASS-specific bindings:

 '(:let ((color "#0088EE"))
    :background #(color))))

    background: #0088EE;

LASS' selector mechanism is very flexible and allows for some complex logic to reduce duplication:

   ((:or p blockquote)
    :margin 0 :padding 0

     :color black)

    ((:and a :hover)
     :color darkred))))

"article p, article blockquote{
    margin: 0;
    padding: 0;

article p a, article blockquote a{
    color: black;

article p a:hover, article blockquote a:hover{
    color: darkred;

But it can go even further:

    (:or article section)
    (:= data-author (:or yukari ran chen))
    (:nth-child (:or 1 2 3)))
   :display none))

    display: none;

Whoa nelly!

Some CSS properties are not fully specified yet and require browser-specific prefixes. LASS can help you with that, too:

   :linear-gradient "deg(45)" black 0% darkgray 100%
   :transform rotate -45deg))

    background: -moz-linear-gradient(deg(45), black 0%, darkgray 100%);
    background: -o-linear-gradient(deg(45), black 0%, darkgray 100%);
    background: -webkit-linear-gradient(deg(45), black 0%, darkgray 100%);
    background: -ms-linear-gradient(deg(45), black 0%, darkgray 100%);
    background: linear-gradient(deg(45), black 0%, darkgray 100%);
    -moz-transform: rotate(-45deg);
    -o-transform: rotate(-45deg);
    -webkit-transform: rotate(-45deg);
    -ms-transform: rotate(-45deg);
    transform: rotate(-45deg);

LASS also supports the various @QUERY operator blocks:

 '(:media "(max-width: 800px)"
    :margin 0)))

"@media (max-width: 800px){
        margin: 0;

By default LASS activates pretty-printing and inserts newlines and spaces where appropriate in order to make the result readable and easy to debug. However, you can also deactivate that and directly produce minified CSS:

(let ((lass:*pretty* NIL))
   '(:media "(max-width: 800px)"
      :margin 0))))

"@media (max-width: 800px){div{margin:0;}}"

As mentioned above you can write pure LASS files to compile down to a CSS file. To do that, simply use GENERATE:



Each block in a LASS sheet consists of a list containing a selector followed by one or more properties or sub-blocks.

(selector [property | block]*)


The following list contains examples for the various uses of selectors.

  • Any element
  • An element with tag-name e
  • An element with tag-name e or f
    (:or e f)
  • An e element with the :link pseudo-selector
    (:and e :link)
  • The first formatted line of an e element
    (:and e |::first-line|) or (:and e "::first-line")
  • An e element with a "warning" class
  • An e element with ID equal to warning
    |e#warning| or "e#warning"
  • An e element with a foo attribute
  • An e element whose foo attribute value is exactly equal to bar
    (:and :a (:= foo "bar"))
  • An e element whose foo attribute value is a list of whitespace-separated values, one of which is exactly equal to bar
    (:and :a (:~= foo "bar"))
  • An e element whose foo attribute has a hyphen-separated list of values beginning (from the left) with bar
    (:and :a (:/= foo "bar"))
  • An e element whose foo attribute value begins exactly with the string bar
    (:and :a (:^= foo "bar"))
  • An e element whose foo attribute value ends exactly with the string bar
    (:and :a (:$= foo "bar"))
  • An e element whose foo attribute value contains the substring bar
    (:and :a (:*= foo "bar"))
  • An e element that matches the pseudo-selector nth-child(2)
    (e (:nth-child 2))
  • An f element preceded by an e element
    (e ~ f)
  • An f element immediately precede by an e element
    (e + f)
  • An f element which is a descendant of e
    (e f)
  • An f element which is a direct descendant of e
    (e > f)

Selector Combinations

As illustrated briefly above, LASS includes two combinators for selectors, :and and :or. These combinators are combinatoric, meaning that all possible combinations are explored. Consider the following selector:

((foo (:and a .title (:or :active :hover)) (:or span div)))

Enumerating all possible answers to this combination would result in the following list

foo a.title:active span
foo a.title:active div
foo a.title:hover span
foo a.title:hover div

The number of possible combinations can quickly explode in size the more options are available. This means that for complex relations and expressions, LASS can be extremely concise. Note that combinators are available at any position in a selector, this includes the arguments of a pseudo-selector like :nth-child.


A property consists of a keyword symbol and a sequence of values. The values to a property are gathered up until either a non-value list or a new keyword is encountered. Originally it stopped as soon as a list was encountered, but this behaviour was changed and specially recognised lists are integrated to allow a more native look for certain values like colours, urls, and so on. Certain properties are specifically declared and will error if they are passed the wrong number or invalid kind of values. For most however, LASS will just blindly put things into the CSS file as you give them. It is up to you to make sure that the values are valid.

:text-style underline
:color (rgb 212 112 30)
:background (url "/foo")
:border 1px solid black

Certain properties currently still require vendor-specific declarations. LASS tries to do that automatically for you, but it also needs to know about these declarations and as such, they need to be manually added. Some of the more common ones are included in LASS by default, but if you encounter one that isn't, you are welcome to send a pull request (see Extending LASS on how to do it).


A block can contain other blocks. These sub-blocks are recursively flattened into the structure by simply prepending the selector of the parent block. Thus

(foo (bar (baz) (bam)))

Is equivalent to

(foo) ((foo bar)) ((foo bar baz)) ((foo bar bam))

Allowing this kind of nesting allows you to more closely mirror the structure present in your HTML file that you want to style. Combining this with the selector combinations, this system allows reducing code duplication a lot.

Special Blocks

In CSS3 there are special properties and blocks that are preceded by an @ symbol. The most well-known examples therefore are probably @include and @media. LASS implements all of these special blocks by a keyword symbol equivalent selector. Therefore the above two would translate to the following in LASS.

(:include (url "foo"))
(:media "(max-width: 800px)"


Often times it is useful to define variables that you can use within your style so that colours and fonts can quickly be exchanged. LASS allows you to do that too using the :let directive and by abusing the vector type. It is probably best illustrated using an example:

(:let ((foo "#0088EE"))
  ((a:active) :color #(foo)))

Extending LASS

Pretty much every part of LASS is extensible through methods. Most useful will however probably be the DEFINE-SPECIAL-PROPERTY, DEFINE-BROWSER-PROPERTY and DEFINE-SPECIAL-SELECTOR helper-macros. Here's some examples from the SPECIAL.LISP file that defines some standard special handlers:

(define-special-property font-family (&rest faces)
  (list (make-property "font-family" (format NIL "~{~a~^, ~}" (mapcar #'resolve faces)))))

(define-browser-property linear-gradient (direction &rest colors)
  (:default (property)
    (make-property "background" (format NIL "~a(~a~{, ~a ~a~})"
                                         property (resolve direction) (mapcar #'resolve colors)))))

For more control, have a look at the various COMPILE-* generic functions.

Emacs Support

LASS includes a tiny elisp file, lass.el. Add LASS' directory to your emacs LOAD-PATH and REQUIRE lass.

(add-to-list 'load-path "[path-to-lass-source-dir]/")
(require 'lass)

Once you visit a .lass file, it will automatically start in the LASS major-mode, which is a derived-mode from COMMON-LISP-MODE. Whenever you save, it will automatically try to compile the lass file to its CSS equivalent. If slime is connected, it will try to quickload LASS and evaluate GENERATE. If slime is not connected, it instead executes a shell command. In order for that to work, the lass binary must be in your path.

If your operating system is not directly supported with a binary, you can build it yourself using a build tool like Buildapp, the ASDF system BINARY-LASS and the entry-point BINARY-LASS:CMD-WRAPPER.

ASDF Integration

If you want to compile LASS files to CSS in your systems, you can now (v0.4+) do this via a lass-file component type, and :defsystem-depends-on-ing LASS.

(asdf:defsystem my-system
  :defsystem-depends-on (:lass)
  :components ((:lass-file "test-file")))

You can also specify an :output argument to a lass-file to specify what the target css file should be.

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