My favourite approach to this so far, that I would like other libraries to copy,...

My favourite approach to this so far, that I would like other libraries to copy, is Elixir’s Access protocol, which gives you e.g.:

    foo = %{key: [[1, 2], [3, 4], [5, 6]]}

    path = [:key, Access.all, Access.at(0)]

    get_in foo, path
    # => [1, 3, 5]

    update_in foo, path, &(&1 * 10)
    # => %{key: [[10, 2], [30, 4], [50, 6]]}

    foo
    |> put_in([:key, Access.all], “foo”)
    |> put_in([:new_key], “bar”)
    # => %{key: [“foo”, “foo”, “foo”], new_key: “bar”}

That third form is essentially the equivalent of building up a complex object through a series of mutations—but entirely functional.