Hassan Hassan - 1 year ago 110
Javascript Question

Javascript lazy evaluation fibonacci function

I asked a question before about using lazy evaluation in Scala. I was trying to write the following Haskell function in Scala:

fib a b = c : (fib b c)
where c = a+b

The answer to that question was that I couldn't use Lists, but should rather use Streams. Now I'm trying to do the same thing in Javascript. I translated the function, and tried it on this site:

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

var res = fib(0,1).slice(0,10);


But I get the following error:

RangeError: Maximum call stack size exceeded

Does Javascript have a way to do this?

Answer Source

You could reify the thunk (read: "not yet evaluated function which continues the computation") that the lazy computation is using.

var fib = function (a, b) {
  var c = a + b
  return { "this": c, "next": function () { return fib(b, c) } }

Such that

> var x = fib(1,1)
> x.this
> x = x.next()
> x.this

In some sense this is an exact translation*, where my return object represents a single Haskell (:) "cons" cell. From here it'd be relatively easy to write a "take" function to convert this javascript "lazy list" into a javascript strict array.

Here's one version.

var take = function(n, cons) {

    var res = []
    var mem = cons

    for (var i = 0; i < n; i++) {
      mem = mem.next()

    return res

Such that

> take(10, fib(1,1))
[2, 3, 5, 8, 13, 21, 34, 55, 89, 144]

(*) Technically even the "this" value ought to be wrapped in a thunk, but I took the head-strict notion of lists which is usually pretty close to everyone's intuition.

Recommended from our users: Dynamic Network Monitoring from WhatsUp Gold from IPSwitch. Free Download