At the Company Campus we use Less quite intensively when we develop websites. It allows us to have cleaner, more maintainable stylesheets. Turns out it’s quite simple to integrate it into Hakyll.

Hakyll provides unixFilter which allows to use any command-line utility during the build. So we can use it to compile .less files into .css files.

To keep things modular, we split our code into many .less files and assemble it with @import rules. The less compiler then generates a unique, compressed CSS file. Since the dependencies are declared and handled by lessc, hakyll needs a little help to know when to recompile the main less file:

    -- Tell hakyll to watch the less files
    match "assets/css/*.less" $ do
        compile getResourceBody

    -- Compile the main less file
    -- We tell hakyll it depends on all the less files,
    -- so it will recompile it when needed
    d <- makePatternDependency "assets/css/*.less"
    rulesExtraDependencies [d] $ create ["assets/css/main.css"] $ do
        route idRoute
        compile $ loadBody "assets/css/all.less"
            >>= makeItem
            >>= withItemBody 
              (unixFilter "lessc" ["-","--yui-compress","-O2"])