JavaScript generators: fibonacci, natural numbers and sieve of Erasthosthenes for primes.

fibPrimeGenerators.js

/* Infinite stream of numbers! 
Format: `[number, suspendedStream]` where suspendedStream is really just a
function that, upon calling, returns another `[number, suspendedStream]`.
*/

function head(a) {
  return a[0];
}

function tail(a) {
  return a[1]();
}

function take(n, a) {
  if (n === 0) return [];
  return [head(a)].concat(take(n - 1, tail(a)));
}

function fib(a, b) {
  return [a, function() { return fib(b, a + b); }];
}

function one() {
  return [1, function() { return one(); }];
}

function nat(a) {
  return [a, function() { return nat(a + 1); }];
}

function filter(a, fn) {
  if (fn(head(a))) {
    return [head(a), function() { return filter(tail(a), fn); }];
  } else {
    return filter(tail(a), fn);
  }
}

function sieve(a) {
  var x = head(a);
  var discarder = function(item) { return item % x !== 0; };
  return [x, function() {
    return sieve(filter(tail(a), discarder));
  }];
}

console.log(take(10, one())); // [ 1, 1, 1, 1, 1, 1, 1, 1, 1, 1 ].
console.log(take(10, fib(0, 1))); // [ 0, 1, 1, 2, 3, 5, 8, 13, 21, 34 ].
console.log(take(10, sieve(nat(2)))); // [ 2, 3, 5, 7, 11, 13, 17, 19, 23, 29 ].