OCaml bounded LRU cache on an association list

module Lru = struct
  type 'a t = { capacity : int; mutable entries : (string * 'a) list }

  let create capacity =
    if capacity <= 0 then invalid_arg "Lru.create: capacity must be positive";
    { capacity; entries = [] }

  let get t key =
    match List.assoc_opt key t.entries with
    | None -> None
    | Some value ->
      t.entries <- (key, value) :: List.remove_assoc key t.entries;
      Some value

  let put t key value =
    let entries = (key, value) :: List.remove_assoc key t.entries in
    if List.length entries > t.capacity then
      t.entries <- List.filteri (fun i _ -> i < t.capacity) entries
    else t.entries <- entries
  let size t = List.length t.entries
end