playground

import gleam/int
import gleam/list
import gleam/string

pub fn group_by_odds_and_evens(numbers: List(Int)) -> String {
  let #(xs, ys) = list.fold(numbers, #([], []), folder)

  "Odds: " <> format_list(xs) <> ", Evens: " <> format_list(ys)
}

fn folder(acc: #(List(Int), List(Int)), x: Int) {
  case is_even(x) {
    True -> #(acc.0, [x, ..acc.1])
    False -> #([x, ..acc.0], acc.1)
  }
}

fn format_list(xs: List(Int)) -> String {
  xs |> list.reverse |> list.map(int.to_string) |> string.join(",")
}

fn is_even(x: Int) -> Bool {
  x % 2 == 0
}