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Adrian's else clause for Arrays and Objects

You can find Adrian's current (2024-11-07) implementation of the else clause for Arrays and Objects in the Adrian-tfg branch of the ULL-ESIT-PL/babel-tanhauhau repo.

Important

Warning: This is work in progress. These comments can be outdated.

Place yourself in the array-else directory:

➜  array-else git:(main) ✗ pwd -P
/Users/casianorodriguezleon/campus-virtual/2324/learning/babel-learning/src/array-else

Here I am using the adrianparser and adrianbabel links to compile the code:

➜  array-else git:(main) ls -l ../../node_modules/.bin/adrian*
lrwxr-xr-x@ 1 casianorodriguezleon  staff  129  5 nov 12:17 ../../node_modules/.bin/adrianbabel -> /Users/casianorodriguezleon/campus-virtual/2122/learning/compiler-learning/babel-tanhauhau-adrian/packages/babel-cli/bin/babel.js
lrwxr-xr-x@ 1 casianorodriguezleon  staff  139  5 nov 12:08 ../../node_modules/.bin/adrianparser -> /Users/casianorodriguezleon/campus-virtual/2122/learning/compiler-learning/babel-tanhauhau-adrian/packages/babel-parser/bin/babel-parser.js

[!IMPORTANT] I have noticed that if you re-build the [Adrian-tfg]((https://github.com/ULL-ESIT-PL/babel-tanhauhau/tree/Adrian-tfg/packages/babel-parser) branch the links no longer work and you have to recreate them.

Extending Arrays and Objects with an else clause

Given the following code:

➜ babel-learning git:(main) ✗ cat src/array-else/array-else.js

let a = [1, 2, 3, else x => x * x];

console.log(a[2]);  // 3
console.log(a[5]);  // 25 (porque 5 * 5 = 25)

When compiled with Adrian's parser we get an AST like:

➜  babel-learning git:(main) ✗ npx adrianparser src/array-else/array-else.js 2> /dev/null | jq '[.program.body[0].declarations[0].init.elements[] | .type ]' 
[
  "NumericLiteral",
  "NumericLiteral",
  "NumericLiteral",
  "ElseExpression"
]

To use Adrian's babel transpiler we write a babel.config.json linking to the support plugin (currently 2024-11-07 in the wrong place)

➜ array-else git:(main) ✗ cat babel.config.json

{
  "plugins": [ 
    "../../babel-tanhauhau-adrian/packages/babel-parser/defaultvector.cjs"
  ]
}

and now we can transpile the code:

➜ array-else git:(main) ✗ npx adrianbabel array-else.js

ELSE
let a = new Proxy([1, 2, 3], {
  get: function (target, prop) {
    if (prop < target.length) return target[prop];
    return (x => x * x)(prop);
  }
});
console.log(a[2]); // 3

console.log(a[5]); // 25 (porque 5 * 5 = 25)

and we can execute it (the 2>null is to avoid the console.error debug messages):

➜  array-else git:(main) ✗ npx adrianbabel array-else.js 2>/dev/null | node -
3
25

Links to sAdrian's compiler for the else clause on Arrays and Objects

martes,  5 de noviembre de 2024, 12:56:07 WEST
➜  packages git:(adrian) ✗ git lg | head -n 1
38c427bb8 - (HEAD -> adrian, origin/Adrian-tfg) Añadida la configuracion (hace 6 días Adrián Mora)

Semantics of the else clause for Arrays

Assume the following input:

➜ array-else git:(main) ✗ cat array-undefined-else.js

let a = [1, undefined, 3, else x => x * x];

console.log(a[0]);  // 1
console.log(a[1]);  // undefined
console.log(a[5]);  // 25 (porque 5 * 5 = 25)

When compiled with Adrian's parser we get:

➜  array-else git:(main) ✗ npx adrian-babel array-undefined-else.js
let a = new Proxy([1, undefined, 3], {
  get: function (target, prop) {
    if (typeof target[prop] === "function") return function (...args) {
      return target[prop].apply(target, args);
    };
    if (prop < target.length) return target[prop];
    return (x => x * x)(prop);
  }
});
console.log(a[0]); // 1

console.log(a[1]); // undefined

console.log(a[5]); // 25 (porque 5 * 5 = 25)

We notice that

  • If prop is numeric
  • Is not checking if prop is an integer
  • Is not checking if prop is negative

Family of functions supporting different semantics

Asume we want to make negative indexation available:

➜  array-else git:(main) ✗ cat array-else-negative-access.js
let a = [1, 2, 3, else x => { if (x < 0) return a[a.length+x]; else return a[x]; }];

console.log(a[2]);   // 3
console.log(a[-1]);  // undefined but a[3+-1] = a[2] = 3

Execution:

➜  array-else git:(main) ✗ node babel.js array-else-negative-access.js | node
3
undefined

Seems to be convenient to pass as second argument the object/array so that sets of functions like

const neg = (x , a) => { if (x < 0) return a[a.length+x]; else return a[x]; }

can be provided so that several common semantics are easily available (negative-indices, throw-by-default, etc.)

We can imagine a family of functions supporting different semantics like neg above, throw (throw an exception if out of bounds), wrap (wrap around), etc

Concatenation and Symbols

See a.concat(b) section for a discussion on the semantics of a.concat(b).

Issues