💐 Making VSCode itself a Java REPL 🔁

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 🧟‍

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.

   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} `
    })}
   

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 🔼

Look at the docs!

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! 👋 🥳