deep_merge.ex

defmodule DeepMerge do
  def deep_merge(m1, m2) do
    Map.new(m1, fn {k, v1} -> 
      case Map.get(m2, k) do
        nil -> {k, v1}
        v2 -> {k, deep_merge_values(v1, v2)}
      end
    end)
  end
  
  defp deep_merge_values(v1, v2) when is_map(v1) and is_map(v2), do: deep_merge(v1, v2)
  
  defp deep_merge_values(v1, v2) when is_list(v1) and is_list(v2), do: v1 ++ v2

  defp deep_merge_values(_v1, v2), do: v2
end