Lazy evaluation in JavaScript

lazy.js

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(stream);
console.log(head(tail(tail(stream))));  //==> 2

Wow, LISP is better readable :-)

But good example ;-)

Wow, LISP is better readable :-)

But good example ;-)

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

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