Can Gleam lists be indexed backwards from their last element?

I need to separate strings containing different numbers of - delimiters. I want the fourth item counting backwards, ignoring any results that have too few pieces

 let segments = string.split("2-33-444-5555-66666-7777-8888", on: "-")
 let chosen = segments[-4] // <- This syntax is not valid.

According to this guide (https://tour.gleam.run/everything/#basics-lists):

Finding a list's size or retrieving entries somewhere beyond its beginning costs a lot, so people seldom do it. Algorithms which access sequences by index are uncommon in Gleam, and for those cases lists would be an unsuitable structure to choose.

but there's no suggestion for another data structure. How do I retrieve the separated pieces counting backwards?

The conversation continues 2

Follow the reasoning

liesl15 Oct

Start by reversing it:

 let segments = list.reverse(segments)

then take element number 4

 let chosen = case list.drop(segments, 3) {
   [fourth, ..] -> fourth
   [] -> ""
 }

I haven't tried running this locally, but wouldn't this work?

Here's an approach that's more idiomatic:

 pub fn fourth_from_tail(entries: String) -> Result(String, Nil) {
   case list.reverse(string.split(entries, on: "-")) {
     // Matches if there are at least 4 segments, binding the 4th from the end to `picked`
     [_, _, _, picked, ..] -> Ok(picked)
     // Discards strings with fewer than 4 segments
     _ -> Error(Nil) 
   }
 }

tidepool15 Oct

I've managed to wrap the [-4] shorthand syntax inside a function:

 fn item_from_back(entries: List(a), position: Int) -> Result(a, Nil) {
   let size = list.length(entries)
   case size >= position {
     True -> list.drop(entries, size - position) |> list.first
     False -> Error(Nil)
   }
 }

 let segments = string.split("2-33-444-5555-66666-7777-8888", on: "-")
 case item_from_back(segments, 4) {
     Ok(piece) -> io.println("piece: " <> piece)
     _ -> io.println("No piece found!")

 }

Can Gleam lists be indexed backwards from their last element?

The conversation continues 2

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