#+title: 💐 Making VSCode itself a Java REPL 🔁
#+author: Musa Al-hassy
#+email: alhassy@gmail.com
#+modified: 2022-09-05 Mon
#+filetags: repl-driven-development vscode emacs javascript java python ruby clojure typescript haskell lisp
#+fileimage: easy-extensibility-repl-java.gif 90% 90%
#+description: VSCode evaluates Java code wherever it sees it, by sending it to a JShell in the background, and echos the results in a friendly way!

* Abstract :ignore:

VSCode evaluates Java code wherever it sees it, by sending it to a JShell in the background, and echos the results in a
friendly way!

This is achieved with a meta-extension for VSCode that makes VSCode into a living, breathing, JS interpreter: It can
execute arbitrary JS that alters VSCode on-the-fly.  /(Inspired by using Emacs and Lisp!)/

The relevant docs show how to make a similar REPL for Python, Ruby, Clojure, Common Lisp, JavaScript, Typescript,
Haskell, and of-course Java.

* How do people usually code?

Either you,

1. Use a code editor and edit multiple lines, then jump into a console to try
   out what you wrote⏳, or

2. You use an interactive command line, and work with one line at a time
   ---continuously editing & evaluating 🔄

The first approach sucks because your code and its resulting behaviour occur in different places 😢 The second is only
realistic for small experimentation ---after all, you're in a constrained environment and don't generally have the same
features that your code editor provides 🧟‍

* If only we could have our cake, and eat it too! 🍰

With a few lines of JavaScript, to alter VSCode, we can get both approaches! No need to switch between the two anymore!
Just select some code and press =Ctrl+X Ctrl+E= ---/E for Evaluate!/ 😉

1. Install the meta-extension Easy-Extensibility.

   - Easy-Extensibility makes VSCode into a living JavaScript interpreter; that can execute arbitrary JS to alter VSCode
     on-the-fly: Just press “⌘+E” to “E”valuate code! 😲

   - You can save and reuse your code by placing it in an ~/init.js file.

   - Video Intro to Easy-Extensibility: https://youtu.be/HO2dFgisriQ

2. Make a =~/init.js= file ---or just invoke =Cmd+h Find user's init.js file, or provide a template=.

3. Stick the following code in there; this code will be run every time you open VSCode.

   - Restart or run =Cmd+h Reload ~/init.js file=.

   #+begin_src javascript
commands['Evaluate: Java'] = {
 'ctrl+x ctrl+e': E.REPL({
   command: '/usr/bin/jshell',     // Ensure this points to wherever you have JShell
   prompt: 'jshell>',
   errorMarkers: [/Error:/, /\|\s*\^-*/, /\|\s+Exception/, /\|\s+at/, /cannot find symbol/, /symbol:/],
   echo: E.overlay,   // For notification-style output, change this to:  E.message
   stdout: result => result.includes(' ==> ') ? result.replace('==>', '⮕') : `⮕ ${result} `
 })}
#+end_src

4. Now you can select any Java code and press Ctrl+X Ctrl+E to evaluate it! 🥳

   👀 Take another look at the above gif for what's possible 🔼

* Wait, how does the above JavaScript work?

Look at the docs!

image:https://raw.githubusercontent.com/alhassy/easy-extensibility/main/graphics/repl-docs.jpeg|700|600|center

| 🤯 The docs are longer than the code itself 😼 |

# Alternatively:
# +html: <script src="https://emgithub.com/embed-v2.js?target=https%3A%2F%2Fgithub.com%2Falhassy%2Feasy-extensibility%2Fblob%2Fmain%2Fvscodejs%2Findex.js%23L1404-L1438&style=base16%2Fsolarized-light&showBorder=off&showLineNumbers=off&showFileMeta=on&showFullPath=on&showCopy=on"></script>

* Technically speaking, how is VSCode /itself/ the REPL?

Let's do what math-nerds call /proof by definition-chasing:/

1. Definition: REPL, an alias for /command line/, stands for Read-Evaluate-Print-Loop

2. =Ctrl+X Ctrl+E= will echo the results in an overlay at the end of the selection ---or at the end of the line, when no explicit selection is made

3. So it retains each of the read, eval, and print parts of the Read-Evaluate-Print-Loop

4. Moreover, since the program doesn't terminate, you're still in the /loop part/ until you close VSCode

Bye! 👋 🥳