[danjoc]

So we start out with an attempt to build an anonymous recursive function. That's cool I suppose. But doing it in a language like javascript (or java)? It turns out that tail recursion == stack overflow. Not daunted, the plucky author decides that we can solve this particular issue by creating a closure over stack variables and put them on the heap instead!

You know, if you want to use Haskell, use Haskell. Trying to shoehorn this stuff into languages designed for loops instead of recursion just means impressionable JS developers are going to see this and rush out to implement it in some project to prove how smart they are. Then everyone suffers. The implementation is slower because it goes to heap. The memory usage explodes because it goes to heap.

Purely for illustration of a Y combinator, this is great. But there should be a big red warning somewhere on the page as a reminder of the downsides of this approach.

[braythwayt]

We actually don’t start out with an attempt to build anonymous recursion.

That is the goal of the Y Combinator in the Lambda Calculus and Combinatory Logic, but here the initial goal was to decouple an anonymous function from itself so that we could decorate it.

[danjoc]

Going out to main memory is orders of magnitude slower than hitting cache. Even a stack overflow would be preferable to debug vs an out of memory error. At least with the stack overflow, the stack trace pinpoints the problem.

Like I said, I have no problem with this article as an illustration of a ycombinator for JS devs. The problem is it does not warn them sufficiently of the very major drawbacks in using it. This is why we have Gate's law; Developers using inefficient constructs because they're neato.

[braythwayt]

Well, given that the post links to two different discussion forums, I attest that it’s not my responsibility to outline every single consideration of every single technique, nor is it my responsibility to include the entire syllabus of computer science to explain the background.

Instead, I outsource that to people like you. You voice your concerns here, and people can read them and make up their own minds.

Speaking of making up their own minds, please explain in more detail how the trampoline’s memory consumption grows unboundedly, to the point where it could cause an out-of-memory error. That would be most interesting for the community to ponder.

[danjoc]

Why don't we just illustrate the problem using the code from the article?

https://jsfiddle.net/q59Ljeu7/

A loop is not just fewer LoC and easier, it is orders of magnitude faster than the ycombinator decoupled trampoline. The first alert is nearly instant. The second one takes almost a minute. My chrome dev tools are hard locked waiting for the ycombinator to finish.

This is not something I would encourage JS devs to use in practice.

[braythwayt]

Well, refactoring isEven or exponentiation to a loop is faster. But wait, you know what's even faster? Using built-in math primitives.

I wrote an entire article about refactoring tail-recursion to iteration and linked to it in TFA. I've also written a Scheme interpreter that can perform this optimization for certain functions on-the-fly. But arguing that the code is slow is missing the point by a country mile, and then some. The article is not arguing that the code is fast, or that you should always do this. I find it amazing that every time a programming technique is discussed, people want to worry about whether it belongs in production.

Is there no recreational programming any more? Or is it all about shipping the next app to deliver food by electric scooter? My world would be depressingly boring if the only things I thought about were the things I use at PagerDuty every day.

Now as to what I asked you about:

What you said was that this implementation leaks memory in such a way that using the trampoline it trades a stack overflow for an out-of-memory error.

I wish to understand that problem. Specifically, how does the decoupled trampoline leak memory?

[empath75]

You don't even use the y combinator in haskell. It's an academic exercise that's mostly relevant in lambda calculus.