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Interesting bits of life

Moldable Emacs: from a picture to a ClojureScript React-Native SVG

Too long; didn't read

A little Elisp mold to transform an SVG image into ClojureScript code for the react-native-svg library.

The problem

In my current gig I develop (among other things) a mobile app. The smart people at my company settled on transpiling ClojureScript to JS targeting React Native (which itself transpiles to iOS and Android code).

This is powerful and adds a bit of complexity when coding because most code examples for React Native development are in TypeScript. So often you need to go TypeScript -> ClojureScript in your mind. With such a pipeline you get super-polyglot though: ClojureScript, TypeScript, JavaScript, Swift and Java&Gradle (+ Bash and Make to get this working, plus a bit of Ruby to deploy, etc...).

One activity that makes me feed bored is adding new SVG images to the app. In web everything is nice and sweet: you get a file, put in the assets directory and load the image where you need it.

React Native lacks a built-in renderer for SVG. This means you cannot just add images to the bundle and load them. Instead, you need to use an extension which can render SVG tags. The extension is react-native-svg, which also offers a nice online converter https://react-svgr.com/playground/.

That makes me jealous: I need to write that in ClojureScript!

So the problem becomes: how do I translate this

<svg version="1.1"
     width="300" height="200"

  <rect width="100%" height="100%" fill="red" />

  <circle cx="150" cy="100" r="80" fill="green" />

  <text x="150" y="125" font-size="60" text-anchor="middle" fill="white">SVG</text>


into something like this?

(icon [:> svg/Rect {:width "100%" :height "100%" :fill "red"}]
      [:> svg/Circle {:cx "150" :cy "100" :r "80" :fill "green"}]
      [:> svg/Text {:x "150" :y "125" :font-size "60" :text-anchor "middle" :fill "white"}]

And there is a solution

If you squeeze your eyes (or add more than 2 images by hand), you will find that there is a pattern in the above:

  1. we skip the svg tag
  2. we change each child element adding an svg/ prefix and capitalize the tag
  3. each attribute becomes a Clojure map with keys and values
  4. each child element gets wrapped in square brackets (with the :> injecting the attributes in the react-native element)

That is definitely boring to do by hand. And since there is a pattern, we can automate! And since this is just another view of an SVG image, this calls for a moldable-emacs mold!

This is the mold code:

 :key "ToFyCljIcon"
 :given (:fn (and (me-require 'esxml) (me-require 'clojure-mode) (eq major-mode 'nxml-mode) (s-ends-with-p ".svg" (buffer-file-name))))
 :then (:fn
        (let* ((tree (libxml-parse-xml-region (point-min) (point-max))))
          (with-current-buffer buffername
            (let ((clj-string (--> tree
                                   (esxml-node-children it)
                                   (--map (concat "[:> svg/"
                                                  (s-capitalize (symbol-name (car it)))
                                                  " {"
                                                  (s-join " " (--map (format ":%s \"%s\"" (symbol-name (car it)) (cdr it)) (esxml-node-attributes it)))
                                   (s-join "\n" it)
                                   (concat "(icon " it "\n)"))))
              (insert clj-string)
              (kill-new clj-string)
              (message "added to kill ring as well")
              (setq-local self tree)))))
 :docs "You can convert an SVG image to Clojure svg."
 :examples nil)

As a reminder: a mold becomes available to you when its preconditions are met (:given clause above). In this case if the buffer is an SVG and you call moldable-emacs' me-mold only .

The core of the mold implements the list of steps we defined above.

The (libxml-parse-xml-region (point-min) (point-max)) parses the XML into an Elisp tree. And we obtain the bits we need via esxml, which is a lovely little library.

If you don't want to use moldable-emacs and just add an Elisp snippet in your Emacs, this is the core of the above:

(--> (libxml-parse-xml-region (point-min) (point-max))
     (esxml-node-children it)
     (--map (concat "[:> svg/"
                    (s-capitalize (symbol-name (car it)))
                    " {"
                    (s-join " " (--map (format ":%s \"%s\"" (symbol-name (car it)) (cdr it)) (esxml-node-attributes it)))
     (s-join "\n" it)
     (concat "(icon " it "\n)"))

Ah you may want to substitute the (icon bit with something like an Icon. That code is what I need most often at work!

Pretty cool, isn't it? This little snippet saves me a lot of boring work even if I have to add only 3 SVG icons to my ClojureScript codebase. Instead now I have just to view the SVG in a new way with moldable-emacs! It feels a quarter of a hour well spent in improving my tool of choice :)


Free yourself from boring tasks! Sharpen (mold) your tools to make them ergonomic to your life. In this case is an SVG conversion, but each of us has different needs and no time to waste ;)

Happy molding!