Hi folks!
I'd like to propose a List.rotate/4 function (and maybe some convenience wrappers for it as well). Rotate is one of those algorithms that, once I learned about it, I started seeing it everywhere. It's somewhat complicated to grasp (it takes 4 parameters, after all), but in situations where you need it, it's still *much* simpler than the equivalent imperative version. The classic use case is this: Suppose you have a list of to-do items, which the user has ordered by priority: 1. Apply to college 2. Brush the dog 3. Change the car's oil 4. Deliver flowers 5. Exchange gifts A "rotate" or "slide" occurs when the user selects some number of elements and drags them to a new place in the list. Let's say they selected items 3 & 4 from the preceding and dragged them above item 2. When they release the mouse, the new order should be: 1. Apply to college 2. Change the car's oil 3. Deliver flowers 4. Brush the dog 5. Exchange gifts Doing this without the named algorithm requires 3 splits (one at the insertion point, one at the start of the selected range, and one at the end of the selected range). It's easy to get the index math wrong, and it's even harder for readers of your code to grasp what's going on. Adding this as a "vocabulary" algorithm would help a lot, I feel. A number of other languages <https://twitter.com/code_report/status/1419900906062204939?s=20> have a rotate algorithm, though it's still somewhat uncommon. I found Dave Abrahams' comments <https://forums.swift.org/t/proposal-implement-a-rotate-algorithm-equivalent-to-std-rotate-in-c/491/2> valuable when this was discussed for inclusion in Swift. I've put together a first draft of an implementation <https://github.com/s3cur3/elixir-xutil/blob/main/lib/x_util/list.ex>, plus some basic tests <https://github.com/s3cur3/elixir-xutil/blob/main/test/list_test.exs>. If this is something folks decide we want, it might also be worth considering a few variants (also implemented in the file linked above): - slide/4: Syntactic sugar over rotate, but it's useful in practice because usages of rotate often need to be able to move a chunk of the list either or backward. (Most usages I've seen in the wild end up doing an if insertion_idx < range_start . . . else . . .) - slide_one/3, where the second argument is the index of the single element to move and the last argument the target index. This would just be syntactic sugar over slide/4. (In my experience, about half the times I use this algorithm, the conceptual requirements guarantee that it'll only act on a single element.) -- You received this message because you are subscribed to the Google Groups "elixir-lang-core" group. To unsubscribe from this group and stop receiving emails from it, send an email to elixir-lang-core+unsubscr...@googlegroups.com. To view this discussion on the web visit https://groups.google.com/d/msgid/elixir-lang-core/01c63660-6a11-4e69-bdcb-6659579ef683n%40googlegroups.com.
