Hassan Hassan - 8 days ago 6
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);

console.log(res);


But I get the following error:

RangeError: Maximum call stack size exceeded


Does Javascript have a way to do this?

Answer

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
2
> x = x.next()
> x.this
3

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++) {
      res.push(mem.this)
      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.

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