jfmengels1

Custom kernel never was a feature the language had. It was an implementation detail that people discovered they could hack, but it was never meant as a feature.

Oh ok. I didn't mean to "reject" the solution, since that's what I see done most of the time in practice, because of the current limitations of the language regarding stack safety.

For foldr, I mean that specific implementation - which fully prioritizes performance and uses several levels of unwrapping, etc. - is hard to reuse for functions we wish to maintain. Using List.foldr is fine (when applicable, which is not always), but copy-pasting its implementation to adapt it to our need isn't, from a maintenance point of view.

On the topic of continuation-passing style, I don't personally believe that the solution I've highlighted uses CPS. At least from my shallow understanding, CPS uses functions, which my solution doesn't. CPS can easily emulate TRMC, but TRMC works differently (and is more performant though is applicable to less situations).

My understanding of CPS is maybe shallow, so I could be missing some understanding, but to me they're different and therefore not part of my explanation.

> Continuations provide the structure to hold your "holes" until they can be filled in

That's one mental model to view it, but there isn't actually any "real" hole to fill with CPS, whereas with TRMC there is (at least for data construction).

The mapHelper version that was rejected changed the order. The one that did `List.reverse` had the correct order, and this one is equivalent to the List.foldr solution.

The only difference is List.foldr's implementation is more performant (up to a certain number of elements as you say). If this is not what you asked, then I did not understand the question.

I think it could yes if you had a keyword for adding the guarantee.

If you didn't have a keyword, then all functions would have to be made stack-safe, which is I think a lot to ask the users in some cases, especially when the language doesn't give you all the tools for it (I think CPS does not work for instance in Elm, currently investigating that a bit).

If you have a keyword, then it would be as restrictive as you'd want it to be I imagine?

Factorial was taken as an example (in this post) exactly because it is such a common textbook example, which also happens to be bad example in the sense that it is not stack-safe.

As for walking a tree, I agree, but this optimization does not work well with trees shrug, making lists a nicer example in my opinion.

As a teaching method, I feel that factorial is a relatively good example because it is fairly easy to grok in your head or on paper. Recursing through a tree adds a lot of mental overhead, so I feel like it could be a follow-up example to factorial, but it's probably too hard as an initial example. Also, switching to a stack-safe function using recursion is again much easier for factorial than it would be for traversing a tree.

Potentially yes. The question then arise: does it ever make sense not to add that keyword, and should you get linter/compiler warnings when it's not used? In which case, there will be little gain. My mind is not yet made up on this to be honest. I feel like it would be valuable, but it feels weird when I think about it some more.

This advice was for custom rules for a team, so it's about enforcing what the team has already agreed upon and understood the trade-offs, not anything the tool decided upon on its own and imposes on the team.