create "default lists" doc for dwyl/mvp#401

src/mvp/16a-default-lists.md

@@ -0,0 +1,73 @@
+# Default `lists`
+
+We want `people` to use our `App` 
+for a wide variety of areas/projects in their life+work.
+Therefore we are creating several `default` `lists` 
+for them when they first authenticate.
+See: [dwyl/mvp#401](https://github.com/dwyl/mvp/issues/401)
+for the discussion.
+
+> **Note**: This section is "parked" here in the `book` for future reference.
+The `default lists` feature is not yet integrated into the `MVP`.
+Once we have an idea of which `default lists` we want to work on
+in terms of interface/experience,
+we can easily enable it and guide people through the workflow.
+
+
+### Test creating "Default" `lists`
+
+Let's write a quick test to
+confirm that our `default lists` are created.
+
+```elixir
+test "create_default_lists/1 creates the default lists" do
+  # Should have no lists:
+  person_id = 3
+  lists = App.List.get_lists_for_person(person_id)
+  assert length(lists) == 0
+  # Create the default lists for this person_id:
+  assert List.create_default_lists(person_id) |> length() == 9
+end
+```
+
+> **Note**: at present we have **9** `default lists`
+This is just a _suggested_ collection of _generic_ `lists`. 
+We could easily have fewer or more.
+Please discuss! 
+[dwyl/mvp#401](https://github.com/dwyl/mvp/issues/401)
+
+### Define `create_default_lists/1`
+
+```elixir
+@default_lists ~w(All Goals Fitness Meals Recipes Reading Shopping Today Todo)
+
+def create_default_lists(person_id) do
+  # Check if the "All" list exists for the person_id
+  lists = get_lists_for_person(person_id)
+  # Extract just the list.text (name) from the person's lists:
+  list_names = Enum.reduce(lists, [], fn l, acc -> [l.text | acc] end)
+  # Quick check for length of lists:
+  if length(list_names) < length(@default_lists) do
+    create_list_if_not_exists(list_names, person_id)
+    # Re-fetch the list of lists for the person_id
+    get_lists_for_person(person_id)
+  else
+    # Return the list we got above
+    lists
+  end
+end
+
+@doc """
+`create_list_if_not_exists/1` create the default "All" list
+for the `person_id` if it does not already exist.
+"""
+def create_list_if_not_exists(list_names, person_id) do
+  Enum.each(@default_lists, fn name ->
+    # Create the list if it does not already exists
+    unless Enum.member?(list_names, name) do
+      %{name: name, person_id: person_id, status: 2}
+      |> List.create_list()
+    end
+  end)
+end
+```