til/elixir/requiring-keys-for-structs.md

Requiring Keys For Structs

When defining a struct in Elixir, we may want to ensure that certain values are provided. We can require that certain keys are used in the creation of a struct with the @enforce_keys attribute. Here is an example of a Name struct.

defmodule Name do
  @enforce_keys [:first, :last]
  defstruct [:first, :middle, :last]
end

With this defined, we can create a struct that uses all of the keys.

> jack = %Name{first: "Jack", middle: "Francis", last: "Donaghy"}
%Name{first: "Jack", last: "Donaghy", middle: "Francis"}

We can also ignore :middle and just provide the required keys.

> liz = %Name{first: "Liz", last: "Lemon"}
%Name{first: "Liz", last: "Lemon", middle: nil}

We cannot, however, omit any of the keys specified in @enforce_keys. If we do omit any of them, Elixir will raise an error.

> tracy = %Name{first: "Tracy"}
** (ArgumentError) the following keys must also be given when building struct Name: [:last]
    expanding struct: Name.__struct__/1
    iex:6: (file)