First commit

chapter4/todo_builder.ex

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+defmodule TodoList do
+  defstruct auto_id: 1, entries: %{}
+
+  def new(), do: %TodoList{}
+
+  def new(entries \\ []) do
+    Enum.reduce(
+      entries,
+      %TodoList{},
+      fn entry, todo_list_acc ->
+        add_entry(todo_list_acc, entry)
+      end
+      # Alternative definition
+      # &add_entry(&2, &1)
+    )
+  end
+
+  def add_entry(todo_list, entry) do
+    entry = Map.put(entry, :id, todo_list.auto_id)
+
+    new_entries =
+      Map.put(
+        todo_list.entries,
+        todo_list.auto_id,
+        entry
+      )
+
+    %TodoList{todo_list | entries: new_entries, auto_id: todo_list.auto_id + 1}
+  end
+
+  def entries(todo_list, date) do
+    todo_list.entries
+    |> Stream.filter(fn {_, entry} -> entry.date == date end)
+    |> Enum.map(fn {_, entry} -> entry end)
+  end
+
+  def update_entry(todo_list, %{} = new_entry) do
+    update_entry(todo_list, new_entry.id, fn _ -> new_entry end)
+  end
+
+  def update_entry(todo_list, entry_id, updater_fun) do
+    case Map.fetch(todo_list.entries, entry_id) do
+      :error ->
+        todo_list
+
+      {:ok, old_entry} ->
+        old_entry_id = old_entry.id
+        # Make sure that the result of the updater is a map and the
+        # id remains unchanged.
+        new_entry = %{id: ^old_entry_id} = updater_fun.(old_entry)
+        new_entries = Map.put(todo_list.entries, new_entry.id, new_entry)
+        %TodoList{todo_list | entries: new_entries}
+    end
+  end
+
+  def delete_entry(todo_list, entry_id) do
+    %TodoList{todo_list | entries: Map.delete(todo_list.entries, entry_id)}
+  end
+end