Add Requiring Keys For Structs as an elixir til

Author: jbranchaud

README.md

@@ -7,7 +7,7 @@ variety of languages and technologies. These are things that don't really
 warrant a full blog post. These are mostly things I learn by pairing with
 smart people at [Hashrocket](http://hashrocket.com/).
 
-_489 TILs and counting..._
+_490 TILs and counting..._
 
 ---
 
@@ -100,6 +100,7 @@ _489 TILs and counting..._
 - [Pattern Matching In Anonymous Functions](elixir/pattern-matching-in-anonymous-functions.md)
 - [Quitting IEx](elixir/quitting-iex.md)
 - [Replace Duplicates In A Keyword List](elixir/replace-duplicates-in-a-keyword-list.md)
+- [Requiring Keys For Structs](elixir/requiring-keys-for-structs.md)
 - [Reversing A List](elixir/reversing-a-list.md)
 - [Reversing A List - Part 2](elixir/reversing-a-list-part-2.md)
 - [Root Directory Of A Project](elixir/root-directory-of-a-project.md)

elixir/requiring-keys-for-structs.md

@@ -0,0 +1,39 @@
+# 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)
+```