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Last active March 21, 2017 04:05
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[RC Diary] Lazy evaluation first steps (-77)

[RC Diary] Lazy evaluation first steps (-77)

Lazy evaluation in JavaScript

So following on yesterday's white paper reading group I am trying to code lazy evaluation in JavaScript, I want to understand how it could be useful to glue modules together.

I would like to do so without any extra constructs like yield or generators.

Sadly both JavaScript REPLS in Spacemacs SPC-m-e-e do not seem to work, so I can't evaluate expressions as I'm used in ClojureScript, I will probably have to look into this.

First attempt

function thunk() {
  let args = arguments
  return function() { return args; }

function sumGenerator(prev) {
  let next = prev + 1
  return [prev, thunk(sumGenerator(next))]


which gives RangeError: Maximum call stack size exceedeed, because sumGenerator(next) is getting evaluated before the surrounding call to thunk.

I am giving it another try after having read this article on MDN; I wrote the evergreen Fibonacci number generator, using plain old JavaScript syntax.

const fibonacciGenerator = () => {
  let f1 = 0, f2 = 1

  const next = (reset) => {
    if (reset) {
      f1 = 0, f2 = 1
    const value = f1
    f1 = f2;
    f2 = value + f1;

    return {
      next: next,
      value: value

  return {
    next: next,
    value: 0

Which works as the example on MDN, without using the language features.

Project lamp

Worked again through chapter 1 and 2, introducing some small changes. Took a lot of time, I have to go carefully through every single step and see if it makes sense for me.

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