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Lazy evaluation in JavaScript
function delay(expressionAsFunction) {
var result;
var isEvaluated = false;
return function () {
if (!isEvaluated)
result = expressionAsFunction();
return result;
function force(promise) {
return promise();
function cons(car, cdr) {
return [car, cdr];
function next(n) {
return cons(n, delay(function () {return next(n + 1);}));
function head(stream) {
return stream[0];
function tail(stream) {
return force(stream[1]);
var stream = next(0);
console.log(head(tail(tail(stream)))); //==> 2
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langpavel commented May 7, 2014

Wow, LISP is better readable :-)

But good example ;-)

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gavin-romig-koch commented Jun 9, 2015

Except that you never set isEvaluated, so expressionAsFunction is called every time the promise is forced, not just the first time.

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