Johan Lövgren (Dec 13 2023 at 11:29): Johan Lövgren (Dec 13 2023 at 11:29):A few months ago I had the understanding that in general it was better to "do things" at the end of a List instead of at the start. So appending a value is much more performant than prepending a value for example. But now I have seen some discussions about "seamless slices" changing this. But I can't quite wrap my head around what that means. Is it still better to append than to prepend for example? I would like to learn more here :smile:
Brian Carroll (Dec 13 2023 at 11:56):The "seamless slice" feature allows us to share data between lists. For example the sublist function returns a seamless slice. If you ask for elements 5-10 of a list, it will not create a whole new memory allocation for it and copy the contents across. Instead, it just returns a structure that refers to part of the original list. It contains metadata that says "Hey I'm really a slice, not a real List. I refer to the elements of this other list at address 0xabc123456, starting at offset 5 with length 6"
If you then modify that list in any way, making it different from the original list, we have to create a new one and copy the contents. There's no way around it.
Brian Carroll (Dec 13 2023 at 11:56):But if you don't modify it, it's way more efficient to just share the data instead of allocating and copying.
Brian Carroll (Dec 13 2023 at 11:58):So the situation has gotten more complex for sure.
BUT if you are trying to decide "should I append or prepend", then definitely append rather than prepend! Prepending is never better. It is either the same (if you were doing a copy anyway) or worse, but never better.
Johan Lövgren (Dec 13 2023 at 16:05):Thanks for the great explanation! That makes a lot of sense.
Last updated: Jul 26 2025 at 12:14 UTC