First commit

chapter4/todo_map_entry.ex

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+defmodule MultiDict do
+  def new(), do: %{}
+
+  def add(dict, key, value) do
+    Map.update(
+      dict,
+      key,
+      [value],
+      &[value | &1]
+    )
+  end
+
+  def get(dict, key) do
+    Map.get(dict, key, [])
+  end
+end
+
+defmodule TodoList do
+  def new(), do: MultiDict.new()
+
+  def add_entry(todo_list, entry) do
+    MultiDict.add(todo_list, entry.date, entry)
+  end
+
+  def entries(todo_list, date) do
+    MultiDict.get(todo_list, date)
+  end
+end