Where parallels cross

Interesting bits of life

Moldable Emacs: editing your file via Treesitter (or, how I fixed my CSS with a Playground)

Too long; didn't read

You can transform code with a Playground. As an example we will change my blog CSS file!

The problem

I always considered code transformations magical. I still remember how mind blowing was to realize that people have different ways to see the same thing. When I actually can see code change shape to fit those expectations: wow! It is just seeing other people ideas! Maybe this is too abstract. In my last job my team members liked code to be tidy. I think I am a tidy person, but I just got used to a different kind of tidiness during my Ph.D. Let me show an example.

def publicFn(): Int = privatePlus(1,1)

def privatePlus(a: Int, b: Int) => Int = a + b

This is what they liked: private functions at the bottom.

I would naturally write this.

def privatePlus(a: Int, b: Int) => Int = a + b

def publicFn(): Int = privatePlus(1,1)

(I guess my theorem proving practice has bent my brain: write small facts and compose them to prove bigger facts.)

Anyway, after convincing myself to rewrite my code a few times, I thought: "Why am I wasting my time?". You could say team's peace of mind, consistency, etc... But that is not the point! A machine can do that much better than me. Code transformation here we go!

How is it possible that I have still to sort my functions by hand in 2021?

It is a problem indeed

That is just the start: why do we still spend precious time in restructuring things? I propose that we should make it easy to say how we like things and let the machine do it for us. For example, what if we could say: "Mmm, enough of monolithic applications! Computer split this application into modules like so and so". Sure describing the "so and so" is not banal. But in big teams (i.e., n >= 3), you will need to explain that "so and so" anyway. Maybe somebody will be half asleep that day (because, you know, life) and miss something. Even if they were all awake, they still would have to apply your "so and so".

Do you see? Is not better to describe (i.e., program) things once?

And how could we do this in Emacs?!

And there is a solution

As usual we need to start small. We will change my blog CSS to demonstrate how to save time with code transformations. I heard that my blog style needs to improve. So say I want to change all my font sizes and invert my colors. This sounds a long boring job, doesn't it?

Well, code transformation remember? We can open any CSS file with the moldable-emacs code tree mold. This means that we can at least gather those font sizes and colors I need to change. All you need to do is to open a Playground and query things with the following code.

(-->  (me/by-type 'integer_value self)
      (--filter (s-contains-p "px" (plist-get it :text)) it))

This picks all the CSS integer values. I filter the ones that are pixels. Now say I want to change all the values in my file from pixels to the em unit of measurement. The change description looks like this.

(-->  (me/by-type 'integer_value self)
      (--filter (s-contains-p "px" (plist-get it :text)) it)
      (--map (list
              :before it
              :after (plist-put
                      (copy-list it)
                      (--> (plist-get it :text)
                           (s-replace "px" "" it)
                           (/ it 16.0)
                           (concat it "em"))))

As you can see, I map the list of pixel nodes to new nodes. So I declare a transition, a :before node and an :after. Then I run the transitions with me/change-nodes. (The translation to em is just dividing the pixel by 16 in my case and replacing px with em.)

And let's change colors too! I want to make the code blocks from a dark background to a light one. So I think that inverting the colors will bring me there.

(-->  (me/by-type 'color_value self)
      (me/transitate-node-texts it
                                (lambda (content)
                                  (--> content
                                       (color-complement it)
                                       (apply #'color-rgb-to-hex (-concat it (list 2))))))

See how easy is to collect colors with Treesitter CSS grammar identifier color_value. Also, I just came up with me/transitate-node-texts to hide the --map I showed before. The transformation is using the fabulous features of Emacs' color.el. The color-complement function find the reverse of a color. Then I transform that in a short RGB hex string (that is what (list 2) is for in there).

Now how cool is to do this? Let me show it to you!

Immediate! No boring searching for each color inverse. A few lines and zap! At least is a more interesting task!

And that is just a basic example. Imagine if you can change the code to tell a story! What if you could run a migration from an architecture to another by defining some rules like the ones I have shown? One abstraction at the time and that could become possible (I hope)!

Let's see where we get to ;)


If you have little boring tasks (or you really don't care about code organization), well transform your code! As you could see, there is already basic support in moldable-emacs. Just grab a copy and give a try. Hopefully it will save you time and bring some fun!

Happy transforming!

P.S.: once more thanks to the creators of emacs-tree-sitter and tree-sitter!