Kilian Vounckx (Dec 09 2024 at 18:34): Kilian Vounckx (Dec 09 2024 at 18:34):Is there a function for inserting something into the middle of a list? I can't seem to find it. Otherwise, I feel like it should be there
Anthony Bullard (Dec 09 2024 at 18:35):List.replace ?
Anthony Bullard (Dec 09 2024 at 18:35):https://www.roc-lang.org/builtins/List#replace
Kilian Vounckx (Dec 09 2024 at 18:35):No I mean putting it in the middle and shifting everything above. Something like List.insert [0, 1, 2, 3] 2 42 == [0, 1, 42, 2, 3]
Kilian Vounckx (Dec 09 2024 at 18:36):I know it isn't good for performance, but if order matters, it can be the only way to do something
Anthony Bullard (Dec 09 2024 at 18:40):No, I think you'd have to implement yourself
listInsert = \list, item, index ->
start = List.sublist {start: At 0, end: Length index}
end = List.sublist {start: After index}
l = List.withCapacity ((List.len list) + 1)
List.concat l start
|> LIst.append item
|> List.concat end
Or something like that
Anthony Bullard (Dec 09 2024 at 18:41):And def won't be the best perf
Anthony Bullard (Dec 09 2024 at 18:42):Don't know a way to get Roc to do a memcpy straight from the original list to the new list, maybe LLVM will get rid of the interstitial list
Kilian Vounckx (Dec 09 2024 at 18:43):In pure roc, I'd pop to the end and use List.swap in repeatedly to put it in place. It would have good performance I think, since no new memory is allocated
Kilian Vounckx (Dec 09 2024 at 18:44):But I really think it should be a builtin
Anthony Bullard (Dec 09 2024 at 18:45):Oh, yeah, like List.reserve list 1 and then do the swaps? I think if the list is created AND this all happens in a single function it will mutate in place
Kilian Vounckx (Dec 09 2024 at 18:47):something like this
insert : List a, U64, a -> List a
insert = \list, index, value ->
pushed = List.append list value
List.range { start: At (List.len list), end: At index }
|> List.walk pushed \acc, i ->
List.swap acc i (i + 1)
probably an of-by-1 error or something. Just wrote it real quick. The range list has to be allocated though
Anthony Bullard (Dec 09 2024 at 18:50):If you are going to do that, I think my implementation would be faster. You should do both, measure and compare. And let me know
Anthony Bullard (Dec 09 2024 at 18:50):Sublist is fast since it's mostly just creating a new stack-allocated struct
Kilian Vounckx (Dec 09 2024 at 18:51):It should just be a builtin though :sweat_smile:, then the discussion doesn't matter. I want to use it for AOC, so there is probably another way. But it feels missing
Anthony Bullard (Dec 09 2024 at 18:51):Mine'll be just three new stack-allocated structs, a heap allocation, and two memcpys
Anthony Bullard (Dec 09 2024 at 18:51):I agree :smile:
Anthony Bullard (Dec 09 2024 at 18:52):If you are doing day 9, I have a different idea
Anthony Bullard (Dec 09 2024 at 18:52):But I only have 8 minutes left on lunch
Kilian Vounckx (Dec 09 2024 at 18:53):Enjoy!
Once there is some more feedback on the feature request, I think I might take a stab at implementing it. It's been a while since I have done a contribution
Kilian Vounckx (Dec 09 2024 at 18:55):Assuming everyone agrees this should be a builtin, what should happen if the index is out of bounds? Leave the list unchanged, push the value to the end, or return a Result? (Or another option I can't think of?)
Anton (Dec 09 2024 at 19:00):Can you put something in the #ideas channel?
Kilian Vounckx (Dec 09 2024 at 19:03):Oh yeah, this was the wrong channel al along. I created a new topic there. Someone with more zulip-fu knowledge is welcome to move this if they want, but I don't feel it's necessary
Last updated: Jul 26 2025 at 12:14 UTC