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Programming language: Haskell
License: BSD 3-clause "New" or "Revised" License
Tags: Language    

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LSS (Lexical Style Sheets) is a library to help writing CSS. It has two main goals / opinions:

  1. CSS should be scoped statically, not globally. Styles should only apply within the lexical context that they are declared to apply. This is VERY different from how CSS normally works, where often styles apply to a given element, regardless of where it is, and in less extreme cases, if a class-name is re-used, it can pick up styles from different parts of a site (provided they share the CSS files, of course).

  2. Styling should be written in semantic, re-usable blocks. What this means is that rather than talking about the styling of a type of paragraph (say, p.article), and the heading style that goes with it (for example, h3.article), and the way that links should appear (a.article), you should rather put all the styles for an article together within a named context.

In plain CSS, this would be accomplished with some container (often a <div>) with class article, and then styles of form .article p, .article h3, and .article a. In one of the various CSS preprocessors that supports nesting, it would be more concisely represented by something like:

   .article {
       p { ... }
       h3 { ... }
       a { ... }

LSS both restricts and expands this. Semantic blocks are not CSS selectors, and arbitrary nesting is not permitted. Our experience has been that heavy nesting actually makes organizing and re-using portions of CSS more difficult. On the other hand, semantic blocks can have parameters, to allow them to be more flexible / general.

  1. This isn't really a goal or opinion, but something that it seems bizare that CSS does not support: LSS has constants. They can be either global or inside semantic blocks, and are defined with =, with semicolons at the end of line, like:
   acme-gray = #efefef;

These can then be used anywhere a CSS identifier is legal, which is most places where you would want them.


store(highlight-color) {
  .featured {
    width: 100%;
    height: 200px;
    margin: 10px;
    border-bottom: 10px solid transparent;
  .featured:hover {
    border-bottom: 10px solid highlight-color;
    cursor: pointer;

  .item {
    width: 30%;
    height: 200px;
    display: inline-block;
    vertical-align: top;


This depends on the (unreleased as of now) library that has attoparsec parsers for the language-css library. I have a fork of that library at: https://github.com/dbp/language-css-attoparsec.

You should install that locally, or cabal sandbox add-source a checkout of it.


If you are using the Snap Web Framework, you can use the provided snaplet-lss package. It's important to modify the development reloader in the generated Main.hs file in your project so that it will reload when lss files change. The line in question looks like:


And should be changed to:

["snaplets/heist/templates", "snaplets/lss"]


There are test suites for both the core language (in package/directory lss) and the snap adaptor (in package/directory snaplet-lss).

For each, change into the corresponding directory and:

First install dependencies:

cabal install --only-dependencies --enable-tests

And then build/run the test suite with:

cabal test --show-details=streaming

If you are only changing tests, you can re-run the tests faster (and with color output) by just running the test main (which is faster because it won't check and rebuild the library):

cabal exec runghc -- test/Main.hs




Daniel Patterson ([email protected])

*Note that all licence references and agreements mentioned in the snaplet-lss README section above are relevant to that project's source code only.