Default values and required values

getting-started/structs.markdown

@@ -113,3 +113,27 @@ iex> Map.keys(john)
 ```
 
 Structs alongside protocols provide one of the most important features for Elixir developers: data polymorphism. That's what we will explore in the next chapter.
+
+## Default values and required keys
+
+If you don't specify a default key value when defining a struct, `nil` will be assumed:
+
+```iex
+iex> defmodule Product do
+...>   defstruct [:name]
+...> end
+iex> %Product{}
+%Product{name: nil}
+```
+
+You can also enforce that certain keys have to be specified when creating the struct:
+
+```iex
+iex> defmodule Car do
+...>   @enforce_keys [:make]
+...>   defstruct [:model, :make]
+...> end
+iex> %Car{}
+** (ArgumentError) the following keys must also be given when building struct Car: [:make]
+    expanding struct: Car.__struct__/1
+```