topScope.array = (...values) => values;

topScope.length = array => array.length;

topScope.element = (array, i) => array[i];

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