Describe how Elixir names functions. Addresses elixirschool#583. (eli…

…xirschool#599)

* Describe how Elixir names functions. Addresses elixirschool#583.

* Accept rewording on arity from @ybur-yug

lessons/basics/collections.md

@@ -40,6 +40,9 @@ iex> [1, 2] ++ [3, 4, 1]
 [1, 2, 3, 4, 1]
 ```
 
+A side note about the name (`++/2`) format used above: In Elixir (and Erlang, upon which Elixir is built), a function or operator name has two components: the name you give it (here `++`) and its _arity_. Arity is a core part of speaking about Elixir (and Erlang) code. It is the number of arguments a given function takes (two in this case). Arity and the given name are combined with a slash. We'll talk more about this later; this knowledge will help you understand the notation for now.
+
+
 ### List Subtraction
 
 Support for subtraction is provided via the `--/2` operator; it's safe to subtract a missing value:

lessons/basics/functions.md

@@ -93,6 +93,29 @@ iex> Length.of [1, 2, 3]
 3
 ```
 
+### Function Naming and Arity
+
+We mentioned earlier that functions are named by the combination of given name and arity (number of arguments). This means you can do things like this:
+
+```elixir
+defmodule Greeter2 do
+ def hello(), do: "Hello, anonymous person!" # hello/0
+ def hello(name), do: "Hello, " <> name # hello/1
+ def hello(name1, name2), do: "Hello, #{name1} and #{name2}"
+ # hello/2
+end
+
+iex> Greeter2.hello()
+"Hello, anonymous person!"
+iex> Greeter2.hello("Fred")
+"Hello, Fred"
+iex> Greeter2.hello("Fred", "Jane")
+"Hello, Fred and Jane"
+```
+
+We've listed the function names in comments above. The first implementation takes no arguments, so it is known as `hello/0`; the second takes one argument so it is known as `hello/1`, and so on. Unlike function overloads in some other languages, these are thought of as _different_ functions from each other. (Pattern matching, described just a moment ago, applies only when multiple definitions are provided for function definitions with the _same_ number of arguments.)
+
+
 ### Private Functions
 
 When we don't want other modules accessing a specific function we can make the function private. Private functions can only be called from within their own Module. We define them in Elixir with `defp`: