Add Passing Around And Using Modules as an elixir til

README.md

@@ -10,7 +10,7 @@ smart people at [Hashrocket](http://hashrocket.com/).
 For a steady stream of TILs from a variety of rocketeers, checkout
 [til.hashrocket.com](https://til.hashrocket.com/).
 
-_796 TILs and counting..._
+_797 TILs and counting..._
 
 ---
 
@@ -145,6 +145,7 @@ _796 TILs and counting..._
 - [List Functions For A Module](elixir/list-functions-for-a-module.md)
 - [Listing Files In IEx](elixir/listing-files-in-iex.md)
 - [Match On A Map In A With Construct](elixir/match-on-a-map-in-a-with-construct.md)
+- [Passing Around And Using Modules](elixir/passing-around-and-using-modules.md)
 - [Pattern Matching In Anonymous Functions](elixir/pattern-matching-in-anonymous-functions.md)
 - [Quitting IEx](elixir/quitting-iex.md)
 - [Range Into List Using Comprehensions](elixir/range-into-list-using-comprehensions.md)

elixir/passing-around-and-using-modules.md

@@ -0,0 +1,38 @@
+# Passing Around And Using Modules
+
+A module is a bag of functions. When we define a module, we are tying it to
+an atom. If we pass around the atom that references this module, then we can
+use it to call its functions.
+
+For example, consider two types of greetings:
+
+```elixir
+defmodule Hello do
+  def get_greeting do
+    "Hello, World!"
+  end
+end
+
+defmodule Hola do
+  def get_greeting do
+    "Hola, Mundo!"
+  end
+end
+```
+
+And a generic greeting module that accepts a language module:
+
+```elixir
+defmodule Greeting do
+  def say_hello(language_module) do
+    language_module.get_greeting
+    |> IO.puts
+  end
+end
+
+Greeting.say_hello(Hello) # => "Hello, World!"
+Greeting.say_hello(Hola) # => "Hola, Mundo!"
+```
+
+The module reference that we pass in to `Greeting.say_hello` can be used to
+invoke the `get_greeting` function.