Richard Feldman (Nov 04 2023 at 20:26): Richard Feldman (Nov 04 2023 at 20:26):I'd like to add a builtin List function with this type:
List elem -> Result (elem, List elem) [ListWasEmpty]
so it gives you back the first element in the list followed by the rest of the list after it (as a seamless slice).
I specifically want this for parsers - it's a nice way to use a single conditional to advance to the next element in the list - although it may have other use cases.
Richard Feldman (Nov 04 2023 at 20:26):I definitely do not want to introduce special pattern matching syntax for this like Elm and Haskell have.
Richard Feldman (Nov 04 2023 at 20:26):anyone have thoughts on what a good name would be for this function?
Richard Feldman (Nov 04 2023 at 20:26):for reference, here are the current List operations
Agus Zubiaga (Nov 04 2023 at 20:33):It’s similar to String.uncons in Elm, but that’s a pretty bad name
Agus Zubiaga (Nov 04 2023 at 20:34):Too bad List.splitFirst is already taken
Isaac Van Doren (Nov 04 2023 at 20:42):What about List.splitHead or List.headAndTail?
Richard Feldman (Nov 04 2023 at 20:44):popFirst and popLast are maybe interesting
Isaac Van Doren (Nov 04 2023 at 20:45):I do like that those names work for the first and last elements
Brendan Hansknecht (Nov 04 2023 at 21:16):Don't we already have pattern matching syntax for this?
Brendan Hansknecht (Nov 04 2023 at 21:16):[head, .. as rest]
Richard Feldman (Nov 04 2023 at 21:16)::face_with_raised_eyebrow: does that work?
Brendan Hansknecht (Nov 04 2023 at 21:16):It should
Brendan Hansknecht (Nov 04 2023 at 21:17):I know it had a bug at one point
Brendan Hansknecht (Nov 04 2023 at 21:17):Not sure if it got fixed
Isaac Van Doren (Nov 04 2023 at 21:19):There is this issue for reference https://github.com/roc-lang/roc/issues/5187
Richard Feldman (Nov 04 2023 at 21:23):huh, cool! Okay, then fixing the bug seems like a good solution to this :big_smile:
Notification Bot (Nov 04 2023 at 21:23):Richard Feldman has marked this topic as resolved.
Last updated: Jun 16 2026 at 16:19 UTC