OCaml `List.group_by`

List.ml

module List = struct
  include Stdlib.List 
            
  let group_by (projection : 'a -> 'key) (xs : 'a list) : ('key * 'a list) list =
    let pairs = xs |> List.map (fun x -> projection x, x) in
    
    let rec aux acc = function
      | [] -> acc |> List.rev
      | ((k, _) :: _) as xs ->
          let ys, zs = xs |> List.partition (fun t -> fst t = k) in
          aux ((k, ys |> List.map snd) :: acc) zs
    in
    aux [] pairs
end