Example:

defmodule Nav do
  def a,
    do: key(:a, [])

  def c,
    do: key(:c, [])

  def some,
    do: key(:some, [])

  defp key(key, default) do
    fn
      :get, data, next ->
        next.(Keyword.get(data, key, default))

      :get_and_update, data, next ->
        value = Keyword.get(data, key, default)

      case next.(value) do
        {old, update} ->
          {old, Keyword.put(data, key, update)}

        :pop ->
          {value, Keyword.delete(data, key)}
      end
    end
  end
end

thing = []
path = [Nav.a(), Nav.c(), Nav.some()]
new_thing = put_in(thing, path, "value")
IO.puts("#{inspect(new_thing)}")

update = &{&1, "more " <> &1}
{old_value, more_thing} = get_and_update_in(new_thing, path, update)
IO.puts("#{inspect(old_value)} #{inspect(more_thing)}")

result = get_in(more_thing, path)
IO.puts("#{inspect(result)}")

{deleted_value, pop_thing} = pop_in(more_thing, path)
IO.puts("#{inspect(deleted_value)} #{inspect(pop_thing)}")

$ elixir nav.exs
[a: [c: [some: "value"]]]
"value" [a: [c: [some: "more value"]]]
"more value"
"more value" [a: [c: []]]