How to get code completion in IEx programmatically?

Hi all,

I’m new to Elixir. I’m trying to use IEx in Emacs. Right now the code completion isn’t supported, and I don’t know how to get the completion from Elixir/IEx programmatically, I tried IEx.Autocomplete.expand "String." but it doesn’t work (I expect it to return a list of completions so I can use it from Emacs.):

iex(11)> IEx.Autocomplete.expand "String."
** (FunctionClauseError) no function clause matching in IEx.Autocomplete.expand/2

    The following arguments were given to IEx.Autocomplete.expand/2:

        # 1
        "String."

        # 2
        IEx.Server

    Attempted function clauses (showing 2 out of 2):

        def expand([], server)
        def expand([h | t] = expr, server)

    (iex) lib/iex/autocomplete.ex:6: IEx.Autocomplete.expand/2

Emacs setup:

(define-derived-mode inferior-elixir-mode comint-mode "Inferior Elixir"
  "Major mode for Elixir inferior process."
  (setq comint-prompt-regexp (rx bol (or "iex" "...") "(" (1+ num) ") ")))

(defun run-elixir ()
  (interactive)
  (with-current-buffer (make-comint-in-buffer "Elixir" "*Elixir*" "iex" nil)
    (inferior-elixir-mode)
    (display-buffer (current-buffer))))