On this page:
2.1 Step 1:   Start a New Project
2.2 Step 2:   Get Rid of the Starter Stylesheet
2.3 Step 3:   Install css-expr
2.4 Step 4:   Write a Style Module
2.5 Step 5:   Add to the Workflow
2.6 Wrapping Up
2.7 Troubleshooting Notes
2.7.1 "styles.rkt does not exist"

2 Tutorial: Preprocessing CSS

The Base Workflow only does some dependency work with CSS, but does not offer minification or preprocessing. As it turns out, we can get both using css-expr. This tutorial will show you a way to integrate css-expr into your workflow.

2.1 Step 1: Start a New Project

Run polyglot start my-project. A my-project directory will appear with some starter code. You’ll notice that when you build this project using polyglot build, there are already some styles.

2.2 Step 2: Get Rid of the Starter Stylesheet

Delete the my-project/assets/styles.css file to remove all styles from the site. If you visit the page, you’ll see that it looks uglier.

2.3 Step 3: Install css-expr

Run raco pkg install css-expr, or install the same package using DrRacket. You’ll need this to write styles in the next step.

2.4 Step 4: Write a Style Module

Write the following into a styles.rkt module in your assets directory. If it is inside the assets directory, polyglot develop will monitor changes to it during a live build.


#lang racket/base
(require css-expr)
(provide css)
(define spacing '20px)
(define fg-color '|#222|)
(define site-styles (css-expr
[* #:box-sizing border-box]
  #:margin (,spacing auto)
  #:padding 0 ,spacing
  #:max-width 50ch
  #:font-size 20px
  #:line-height 1.3]
    #:margin-bottom 0]
  #:display block
  #:padding 1rem
  #:border (1px ,fg-color solid)]]))
(define css (css-expr->css site-styles))

The styles shown here are actually the same as the styles in the starter website. This module simply provides a css identifier bound to CSS code as a Racket string. You’ll notice that you can use variables and nested selectors, just like you could with other CSS preprocessors.

Fire up DrRacket or the REPL. You should see that this session works.

> (require "styles.rkt")

> css

"*{box-sizing:border-box;}body{margin:20px auto;padding:0,20px;max-width:50ch;font-size:20px;line-height:1.3;}.code-sample pre{margin-bottom:0;}.code-sample output{display:block;padding:1rem;border:1px #222 solid;}"

Nice! Save for the extra semicolons at the end of each ruleset, the CSS is minified.

2.5 Step 5: Add to the Workflow

Open your workflow module .polyglotrc.rkt and add this code to make your project generate a new styles.css on every build.


#lang racket/base
(require racket/runtime-path
(provide polyglot+%)
(define-runtime-path styles-module "./assets/styles.rkt")
(define (get-styles)
  (and (polyglot-live?)
       (> (length (dynamic-rerequire styles-module)) 0)
       (dynamic-require styles-module 'css)))
(define (generate-styles!)
  (define styles (get-styles))
  (when styles
    (display-to-file #:exists 'replace
      (assets-rel "styles.css"))))
(define polyglot+%
  (class polyglot/functional%
    (define/override (compile! #:changed c #:removed r)
      (super compile! #:changed c #:removed r))

This is a situation where instead of writing a file to a distribution, you are generating an asset before a build starts.

This example is slightly crude because the stylesheet is rebuilt from scratch every time. That’s okay for small stylesheets.

But what’s going on with get-styles? There seems to be more going on here.

(define (get-styles)
  (and (polyglot-live?)
       (> (length (dynamic-rerequire styles-module)) 0)
       (dynamic-require styles-module 'css)))

Remember that Polyglot supports live builds using polyglot develop. It would not be fun to change your styles in Racket and manually start a new build every time you want to see a change.

(polyglot-live?) tells us if we’re in an ongoing build. If we are, we use dynamic-rerequire to reload the styles module if it has changed. We check the return value of dynamic-rerequire in generate-styles! to see if a load or reload happened at all. If it did, we write the stylesheet.

2.6 Wrapping Up

This tutorial took a starter project and upgraded the workflow to use css-expr. This lets you write CSS using variables, nested selectors, and the full power of Racket. This also opens the door to sharing theme information with other code like JavaScript and SVG markup.

If your styles start getting bloated, you can always split them up so that styles.rkt requires rules from other modules and combines them together.


#lang racket/base
(require css-expr "./styles.d/typography.rkt" "./styles.d/theme.rkt")
(provide css)
(define css (css-expr->css (css-expr ,typography ,theme)))

2.7 Troubleshooting Notes

2.7.1 "styles.rkt does not exist"

Emacs users may encounter an error stating that styles.rkt does not exist during a live build. This is because some editors might temporarily delete a file before renaming an autosaved version to take its place. Polyglot will detect the deletion both the deletion and the re-addition of the file as changes that warrant a new build. Try refreshing your browser after making a style change to see if your new style rules apply despite the error. If they did, you can safely ignore it. Otherwise, try running touch on the pages you wish to preview.