Performance optimizations in Reagent – part 2

Have I mentioned that it’s a nightmare to install React Devtools in Electron? Turns out, it’s worth it. But it’s still a pain. If you want to try, you’ll need to first install the extension in a chrome-based browser (like Brave, the one I use) and then install it by code. At Chlorine, what I did was to copy the whole extension folder to a devtools directory and then changed the script to load electron to the script below (see the BrowserWindow.addDevToolsExtension):

const path = require('path')
const {app, BrowserWindow} = require('electron')

app.on('ready', () => {
  let browser = new BrowserWindow({
    width: 900, height: 600,
    webPreferences: {nodeIntegration: true}})
  browser.loadURL(path.join('file://', __dirname, '/index.html'))

  BrowserWindow.addDevToolsExtension('./devtools')
})

Originally, the React Devtools was installed at ~/.config/BraveSoftware/Brave-Browser/Default/Extensions/fmkadmapgofadopljbjfkapdkoienihi/4.21.0_0 (tilde is my home folder). If you’re trying to install on Atom editor, you’ll have to open developer tools and issue the following script:

r = require('remote')
r.BrowserWindow.addDevToolsExtension('./devtools') // or the right directory

This command will fail, but somehow when you reload Atom, the React Devtools will be there. There’s only one feature that I use – highlight which elements were updated. And that’s where our performance tuning starts again.
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Rebirth of the idea of a Hackable Code Editor

So… Atom is in zombie state. Almost everything kinda works but not 100% works so the idea of a “Hackable Text Editor for the 21st Century” is almost dead. LightTable is buried, and it was not as hackable as Atom. And there’s VSCode – a closed-source, telemetry enabled, Microsoft controlled editor based on an open-source model that’s kinda the Webkit model – you have to fork the editor and make your changes, because they control the available APIs, the way that the editor is going, etc.

So what we have now? To be honest, not much. There’s emacs and Vim/NeoVim, but these are all text-based and have both people that love them and people that hate. It’s also not that easy to make plug-ins and configurations (one you have to learn elisp, and the other you can use a mix of multiple languages like VimL, Lua, Node.JS, Python, but they all feel “out-of-place”.

Let’s try to make a new editor? Maybe that’s insanity, so let me tell you what I have in mind:
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Implementing shadow.remote API

Since version 0.8.0 of Chlorine, there’s a new way to evaluate ClojureScript code: that’s the Shadow-CLJS Remote API. It is basically a new REPL (not nREPL, no Socket REPL) over WebSockets to try to solve problems when translating other REPLs to ClojureScript. So, to understand why these problems exist, I’ll first introduce the difference between ClojureScript and Clojure.

On Clojure, you’re always inside a JVM. This means that compilation happens on the same JVM that your REPL, and your code is running. If you practice REPL-Driven Development, even your tests are running on the same JVM. In practical terms, it means that when you fire up your REPL, you already have everything ready to run code, compile code, and evaluate forms.

On ClojureScript, the compiler is written in Clojure – that means it’s running on the JVM. So, to produce Javascript code you don’t need a Javascript environment – and that’s when things become confusing, because when exactly will you run the REPL? Let’s try from another angle: if you start the REPL on compilation time, you can’t evaluate code (because there’s no Javascript generated, nor any Javascript engine running). If you start the REPL when you run the compiled code, this REPL can become unusable if you stop the Javascript environment, and also you have to coordinate lots of state and translations between formats.
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The History of Chlorine

Before I even started with Clojure, I was analyzing LightTable – the idea of that editor was to support better integrations between the code you’re writing and the code that’s running. It was a really good experience, but the main problem I had is that it was in the very beginning, with few plug-ins and bad documentation. I tried to make the parinfer plug-in work in the editor, but it had lots of bugs and then I simply changed back to Atom. At the time, proto-repl was the best package to work with Clojure, and I made some small changes to it (so I could add some callbacks to when a new connection to nREPL was made, and other small issues) to improve my workflow.

Fast forwarding a little bit, I started my first Clojure job at Nubank. Most people were using InteliJ, but I felt that by using Atom I had a different approach on problem solving, specially those hard parts where the fast feedback of “run in the REPL and see the results in your editor, then browse over the keys” could give better insights about what’s happening. I tried to implement some features that proto-repl didn’t have at the time (and Chlorine still does not have some) like “automatically add nREPL port”, and “watch expressions” (almost the same as watch variables in a debugger). These ended up in a package called clojure-plus, that still exist today.

I also began to experiment with ClojureScript (at the time, only Figwheel was available – Figwheel-main didn’t even exist!) and found that existing tools didn’t provide the same power that I had with Clojure. It also didn’t have autocomplete, goto var definition, and so on. To ease a little bit these problems, I ended up adding on clojure-plus some CLJS support – when you tried to evaluate a .cljs file, it would try to connect to ClojureScript, reserve a REPL, then evaluate the code over there.
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Implementing a nREPL Client

Some people asked me on how did I implement the nREPL client on Chlorine. So, I’m writing this post!

nREPL is a simple protocol. It uses “sessions” that are used to isolate evaluation contexts and other things (for example, on Chlorine every connection connect to two REPLs: one “primary” and one “auxiliar” that is used to run commands like autocomplete / goto var definition, and so on). On nREPL, this isn’t necessary: you just connect to a single REPL and use two different sessions. Just for the record, because the way Chlorine works, I didn’t implement it like this (because I would have to rewrite lots of code – maybe in the future).

Now, to explain the protocol, let’s separate things in parts: the first thing I do (and I was already doing in the past) is to connect to a socket. Then, I looked at the documentation for nREPL to understand how to send and receive commands to the REPL. Now, there are some details on the way that every operation is implemented that I simply ignored because it was not necessary to understand then from the perspective of my application…
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nREPL on… Chlorine?

When I started the Chlorine project, I just thought it would be great if I could target all Clojure-like REPLs that already exist but didn’t have tooling support. At the time, this would include Lumo and Plank, mostly. Also, Shadow-CLJS and Figwheel have some “clunky run-some-code-and-transform-in-cljs” way of working that simply didn’t click with me.

Now, almost a year later, Chlorine supports Clojure, ClojureScript (Shadow-CLJS, Lumo, Plank, or even over clj), ClojureCLR, Arcadia, Babashka, Clojerl (Clojure on Erlang) and Joker (Clojure on Go, also a linter). But the reality is that working with a pure Socket REPL is really hard – a socket REPL works exactly like a regular one, printing namespaces after each code, and so on. Also, there are some strange decisions on some REPLs, mostly likely ClojureScript (that is the second most used Clojure flavor), so things are not always easy. To put things in perspective, currently Chlorine uses 3 ways to evaluate code: It uses unrepl, that only works on Clojure, or uses internal APIs of shadow-cljs (that obviously only works for shadow-cljs), and for other implementations it uses a kind of a hack – it evaluates the code, inside a trycatch, and it returns a vector where the first element is a symbol in a specific format that Chlorine will understand and then link that with the response. This “hacky way” is currently being used for every other implementation except Clojure and Shadow-CLJS. Things work (autocomplete works too), but it is not pretty and sometimes have strange results.

As a matter of fact, I was already thinking about removing UNREPL (it’s really hard to implement new features on it, and some good ideas only work in theory – for example, the ability to evaluate long strings / collections and render only a part at a time aren’t that good with lots of edge-cases) and, to do it, I though about a better, non-hacky way to evaluate things on some Socket-REPLs (that, again, would only work on some REPLs – ClojureScript REPLs will probably never support “upgradable REPLs” because of the way they work) – the only thing that I had to understand is how to implement this “upgraded REPL”…

Then, recently, Babashka added an initial support for nREPL, with an insane low amount of lines. So, I’ve tried to implement a way to evaluate code over nREPL… and it was really simple to do it, using a npm library that already did it. But implementing like this meant that the user would need to know if the host/port to connect is a Socket REPL, or a nREPL (and the user does not know – lots of tools like lein and shadow-cljs show an nREPL port to be connected).
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REPL-Tooling Clients

Chlorine, Clover and Clematis are all implementations of the same library: REPL-Tooling. In this post I will show you how to create a new implementation of it in a way that’s completely disconnected from any editors, so you can grasp the general concepts.

Suppose I want to do an implementation for an editor that doesn’t run JavaScript – so it’ll connect by some kind of socket. In this example I’m going to use WebSockets because… why not?

We’re going to create a shadow-cljs node project and add repl-tooling as a dependency. We will also had some more dependencies: mostly ws for websockets and the same react libraries that we use for reagent (react, create-react-class and react-dom) – repl-tooling still needs reagent, and probably in the future I will split it into two different libraries (one for the REPL handling and other for the visual rendering part). This supposedly is not to much of a problem because ClojureScript compiler will probably remove these parts in the dead code elimination process anyway. So, our package.json file will just be like this:

{
  "name": "ws-repl",
  "devDependencies": {
    "shadow-cljs": "^2.8.83"
  },
  "dependencies": {
    "create-react-class": "^15.6.3",
    "install": "^0.13.0",
    "react": "^16.12.0",
    "react-dom": "^16.12.0",
    "ws": "^7.2.1"
  }
}

And our shadow-cljs.edn file:

{:source-paths ["src"]

 :dependencies [[repl-tooling "0.4.0"]]
 :builds {:node {:output-to "index.js"
                 :target :node-script
                 :main ws-repl.core/main}}}

The first step is when someone connects to the WebSocket. Then, we’ll just create a connection to the client, and send a list of supported commands – for now, is just the “connect” command:
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