…and in JavaScript, using Sweet.js

This post is a continuation of my previous post, Capturing abstractions in Lisp.

Today I was playing with Sweet.JS, a JavaScript library that adds the possibility to make macros within the language.

I went through the tutorial and it is pretty straight-forward.

// delay needs to be a macro in order for the code to work correctly.
// Otherwise the invocation of delay will evaluate all its arguments eagerly.
syntax delay = function (ctx) {
    let x = ctx.next().value;
    return #`(function() { return ${x} })`;
}

// similarly for cons-stream
syntax cons_stream = function(ctx) {
    const [...args] = ctx.next().value.inner();

    let param1 = args[0];
    let param2 = args.slice(2);

    return #`([${param1}].concat(delay(${param2})))`;
}

// force just evaluates
function force(x) {
    return x();
}

function next_integer(x) {
    return cons_stream(x, next_integer(x+1));
}

function all_positive_integers() {
    return next_integer(0);
}

function integer_ref(list, j) {
    if (j == 0) {
        return list[0];
    } else {
        return integer_ref(force(list[1]), j - 1);
    }
}

var integers = all_positive_integers();
// display 100th number:
console.log(integer_ref(integers, 100));

With this code snippet, we can see that using this library we get a very powerful Lisp-like syntax 🙂