https://eloquentjavascript.net/12_language.html#i_uQzJv9I1Z6

Eloquent JavaScript 1201 Arrays.js

// Represent Egg arrays with JavaScript arrays

topScope.array = (...elements) => {
  return elements;
};

topScope.length = (array) => {
  return array.length;
};

topScope.element = (array, n) => {
  return array[n]
};

run(`
do(define(sum, fun(array,
     do(define(i, 0),
        define(sum, 0),
        while(<(i, length(array)),
          do(define(sum, +(sum, element(array, i))),
             define(i, +(i, 1)))),
        sum))),
   print(sum(array(1, 2, 3))))
`);
// → 6