✔ Performance of `List.walk` vs destructured recursion · beginners

Stream: beginners

Hello,

out of curiosity, are there any performance implications on iterating a list with List.walk vs tail-call recursion with the .. as rest pattern?

List.walk(some_list, {}, |state, item|
    ...
)

some_func = |some_list|
    [] -> ...
    [item, .. as rest] ->
        ...
        some_func(rest)

I see that List.walk calls get_unsafe underneath to avoid a bounds check. Can I assume the list destructuring also avoids bounds checks?

And are there any other performance implications to be aware of between the two?

Thanks.

Raw tail recursion is likely slower

Not by a lot, but slower

Llvm might be smart enough to remove the bounds check, but it is not guaranteed.

On top of that, rest is a seamless slice. While those are fast to make, it is still more work than just incrementing an offset.

Overall, probably not something to worry about, but is a minor optimization that could see gains in a very hot loop with essentially no body.

I see,

so rest would be like me manually doing a rest = List.drop_first(some_list, idx) inside a List.walk_with_index or something similar?

No doubt the .. as rest is convenient for a lot of code

Yeah, rest is equivalent to drop first on the original list.

Which the cost would be incrementing a pointer and decrementing a length.

With a bit of extra orchestration logic

Instead of walk with index where the cost is just an int increment with no extra logic.

hchac has marked this topic as resolved.

Last updated: Jul 26 2025 at 12:14 UTC