Add Requiring Keys For Structs as an elixir til

elixir/requiring-keys-for-structs.md

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+# Requiring Keys For Structs
+
+When defining a
+[`struct`](http://elixir-lang.org/getting-started/structs.html) 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`](https://hexdocs.pm/elixir/Kernel.html#defstruct/1)
+attribute. Here is an example of a `Name` struct.
+
+```elixir
+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.
+
+```elixir
+> 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.
+
+```elixir
+> 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.
+
+``` elixir
+> 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)
+```