Increment A to B or ZZ to AAA - only works on capitol ASCII letters

string_extensions.ex

defmodule Tallyj.StringExtensions do

  @moduledoc """
  This module provides a function that will take a uppercase ASCII letter and
  return the next logical value. Wrapping back to A when Z is provided.

  Examples:

  next("A")
  => "B"

  next("QA")
  => "QB"

  next("QZ")
  => "RA"
  """
  def next(string) when is_binary(string) do
    val = next(string |> String.to_char_list |> Enum.reverse, [])
    to_string(val |> Enum.reverse)
  end

  defp next([letter | rest], acc) do
    new_letter = next_letter(letter)
    next({new_letter, Enum.count(rest)}, rest, acc)
  end

  defp next([], acc) do
    acc
  end

  defp next({65, 0}, rest, acc), do: next(rest, [[65, 65] | acc])
  defp next({65, _}, rest, acc), do: next(rest, [65 | acc])
  defp next({ltr, _}, rest, acc), do: [[ltr | acc] | rest]

  defp next_letter(value) when value >= 90, do: 65
  defp next_letter(value), do: value + 1

end

I tried with this version to be a little more pattern match with functions instead of a case. Logically still the same but implementation is quite different?