@Rory You can use a nested reduce calls. The actual code would be different in each specific use case. For example if you want to take soonest date from such a nested data then you do not need to collect all elements and simply work on one single data, for example:

defmodule ToDo do
  defmodule List do
    defstruct ~w[items name]a

    defmodule Item do
      defstruct ~w[content date]a
    end
  end
end

defmodule Example do
  def get_data do
    [
      %ToDo.List{
        items: [
          %ToDo.List.Item{content: "Do X", date: ~D[2022-12-15]},
          %ToDo.List.Item{content: "Do Y", date: ~D[2022-12-16]},
          %ToDo.List.Item{content: "Do Z", date: ~D[2022-12-17]}
        ],
        name: "List A"
      },
      %ToDo.List{
        items: [
          %ToDo.List.Item{content: "Do X", date: ~D[2022-12-01]},
          %ToDo.List.Item{content: "Do Y", date: ~D[2022-12-02]},
          %ToDo.List.Item{content: "Do Z", date: ~D[2022-12-03]}
        ],
        name: "List B"
      },
      %ToDo.List{
        items: [
          %ToDo.List.Item{content: "Do X", date: ~D[2023-01-01]},
          %ToDo.List.Item{content: "Do Y", date: ~D[2023-01-02]},
          %ToDo.List.Item{content: "Do Z", date: ~D[2023-01-03]}
        ],
        name: "List C"
      }
    ]
  end

  def get_first_date(data) when is_list(data) do
    Enum.reduce(data, nil, fn %ToDo.List{items: items}, acc ->
      Enum.reduce(items || [], acc, fn
        %ToDo.List.Item{date: nil}, acc ->
          acc

        %ToDo.List.Item{date: date}, nil ->
          date

        %ToDo.List.Item{date: date}, acc ->
          if Date.compare(date, acc) == :lt, do: date, else: acc
      end)
    end)
  end
end

Example.get_data()
|> Example.get_first_date()
|> Date.diff(Date.utc_today())
|> then(&IO.puts("You have still #{&1} free days!"))
# You have still 20 free days!

Look how simple it’s to change. If we would like to take the opposite date then we all we need to do is to change 2 letters i.e. :lt to :gt (when comparing date with acc). If you want to return multiple dates in this example then all you need to do is to change a default acc from nil to [] (empty list) and replace two last nested reduce clauses to:

        %ToDo.List.Item{date: date}, acc ->
          [date | acc]

With nested reduce or alternatively simple pattern matching the whole list is iterated only once. See Enum.reduce/3 for more information.