lib.rs

use arma_rs::{arma, Extension};

#[arma]
fn init() -> Extension {
  Extension::build()
    .command("side_select", side_select)
    .finish()
}

fn side_select(side: String) -> Vec<String> {
  match side.as_str() {
    "WEST" => vec!["B_Placeholder1", "B_Placeholder2"],
    "EAST" => vec!["O_Placeholder1", "O_Placeholder2"],
    _ => vec!["I_Placeholder1", "I_Placeholder2"],
  }.into_iter().map(|s| s.to_string()).collect()
}

side_select.sqf

("ext" callExtension ["side_select", [str _side]]) params ["_classes", "_code"];

if (_code == 0) then {
  private _unitList = parseSimpleArray _classes;
} else {
  WARN_1("Error encountered determining side classes: %1",_code);
};