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The proposed Syntax and Semantic

These packages extend JS with a new kind of functions, the @@ functions (we lack by now of a name for this class of functions: assignable? pure?). Here is an example of declaring an assignable function:

function @@ foo(bar) {
  return bar * 2;
}

These assignable functions can be later modified using the assign expression:

foo(10) = 5;

Here is the full code for our "hello assignable functions!" left-side-plugin example:

➜ babel-npm-test git:(main) cat example.js

function @@ foo(bar) {
  return bar * 2;
}
foo(10) = 5;

console.log(foo(10)); //  5
console.log(foo(5));  // 10

You can fork the example repo ULL-ESIT-PL/babel-left-side-npm-test and test the packages in your own workspace or follow the instructions below.

Install

Here are the node and npm versions I have used to test the packages:

➜  babel-npm-test node --version
v20.5.0
➜  babel-npm-test npm --version
9.8.0

These packages use the GitHub registry instead of the npm registry. Therefore, remember to set the registry entry in your .npmrc file:

➜  babel-npm-test git:(main) ✗ cat ~/.npmrc | grep '@ull-esit-pl:'
@ull-esit-pl:registry=https://npm.pkg.github.com

or set an entry registry in your package.json file:

➜  babel-npm-test git:(main) ✗ jq '.registry' package.json 
"https://npm.pkg.github.com"

Then you can proceed to install the packages:

npm i -D @babel/cli@7.10 @ull-esit-pl/babel-plugin-left-side-plugin @ull-esit-pl/babel-plugin-left-side-support @ull-esit-pl/parser-left-side

Your package.json devDependencies section will look similar to this:

➜ babel-left-side-npm-test git:(main) ✗ jq '.devDependencies' package.json

{
  "@babel/cli": "^7.10.1",
  "@ull-esit-pl/babel-plugin-left-side-plugin": "latest",
  "@ull-esit-pl/babel-plugin-left-side-support": "latest",
  "@ull-esit-pl/parser-left-side": "latest"
}

Compiling the code

To compile the example above add a babel.config.js to your workspace folder:

➜ babel-npm-test git:(main) cat babel.config.js

module.exports = {
  "plugins": [
    "@ull-esit-pl/babel-plugin-left-side-plugin"
  ],
}

and then compile it using the installed packages:

➜  babel-npm-test npx babel  example.js

This will output the compiled code to the console:

const {
  assign,
  functionObject
} = require("@ull-esit-pl/babel-plugin-left-side-support");
const foo = functionObject(function (bar) {
  return bar * 2;
});
assign(foo, [10], 5);
console.log(foo(10));
console.log(foo(5));

If you want to save it to a file, use the -o option.

Running

You can pipe the output to node:

➜  babel-npm-test npx babel  example.js | node  -
5
10

or alternatively, use the -o option to save the output to a file and then run it:

➜  babel-left-side-npm-test git:(main) npx babel  example.js -o example.cjs
➜  babel-left-side-npm-test git:(main) ✗ node example.cjs 
5
10

References