Compiling SCSS and JavaScript in Hakyll

Versions and tools used:

  • Hakyll

  • hjsmin

  • Sass 3.4.18

  • Stackage LTS 5.15

This seems to be an often asked question, so I thought I’d try and share the approach that I’ve arrived at after having explored a couple of solutions to the problem. If you want to see the full code in action, check out the repo for the site (linking to v1.0.0 is intended, in case the code changes later on).

Compiling and minifying JavaScript

For some reason Hakyll does not include its own JavaScript compiler, which makes little sense. Luckily there is a package called hjsmin giving us Text.Jasmine, which we will use to both compile and minify our JavaScript files.


From personal experience an earlier version of hjsmin, e.g., would throw a parse error on some files such as jQuery. This has later been fixed in, but unfortunately Stackage is using in the current LTS 5.15.

To get hjsmin working with stack, add hjsmin == 0.2.* as a dependency in the projects cabal file, and the following to the stack.yaml file in the project,

extra-deps: [ hjsmin-
            , language-javascript-

Now we are ready to construct the compiler itself, by jumping into site.hs,

import qualified Data.ByteString.Lazy.Char8 as C
import           Text.Jasmine

-- | Create a JavaScript compiler that minifies the content
compressJsCompiler :: Compiler (Item String)
compressJsCompiler = do
  let minifyJS = C.unpack . minify . C.pack . itemBody
  s <- getResourceString
  return $ itemSetBody (minifyJS s) s

The code is fairly straightforward. We use the Text.Jasmine provided function minify which has the signature ByteString -> ByteString, meaning it takes in the JavaScript code as string, and produces the result to a string.

Later on inside the main Hakyll function in site.hs, we use it as we would the other compilers,

-- | Define the rules for the site/hakyll compiler
main :: IO ()
main = hakyll $ do
  -- | Route for all JavaScript files found in the 'js' directory
  match "js/*" $ do
    route   idRoute
    compile compressJsCompiler
  -- The rest of your rules...

Compiling and minifying SCSS (Sass)

For those who aren’t aware, there are other ways to write CSS than CSS. Sass and SCSS adds a lot of niceties to CSS, such as nesting, variables and mixins, and compiles to normal CSS. You can read more about that on their website.

This time we rely on external dependencies, namely the sass tool, which can be installed with gem install sass.


There is a library called hsass, which provides a Haskell interface, but I’ve been running into problems with linking to the underlying C API. As such, I’ve opted for the external dependency for now.

Same as with the JavaScript minification, we add a compiler in site.hs,

-- | Create a SCSS compiler that transpiles the SCSS to CSS and
-- minifies it (relying on the external 'sass' tool)
compressScssCompiler :: Compiler (Item String)
compressScssCompiler = do
  fmap (fmap compressCss) $
    >>= withItemBody (unixFilter "sass" [ "-s"
                                        , "--scss"
                                        , "--compass"
                                        , "--style", "compressed"
                                        , "--load-path", "scss"

This time we have no library dependencies, and use the Hakyll provided function unixFilter to call the sass tool. An important thing is the arguments that we pass, which I’ll explain briefly:

  • -s tells sass to take its input from stdin
  • --scss tells sass to use the SCSS format
  • --compass tells sass to make compass imports available
  • --style compressed tells sass to compress the output
  • --load-path scss tells sass to look for modules in the scss directory (if we import stuff)

Much like with the JavaScript compiler, we use it in the main Hakyll function inside site.hs as such:

-- | Define the rules for the site/hakyll compiler
main :: IO ()
main = hakyll $ do
  -- | Compile the SCSS, from 'scss/app.scss', to CSS and serve it as 'app.css'
  match "scss/app.scss" $ do
   route   $ constRoute "app.css"
   compile compressScssCompiler
  -- The rest of your rules...

You can now happily compile both JavaScript and SCSS in your Hakyll project, without much hassle!