In Elixir, named functions (defined with the def macro) can accept arguments and sometimes, it is convenient to assume them as optional by defining a default value or expression. Default values for function arguments are defined using \\ after the argument name.
Note
If we define a foo(a, b, c \\ 0) function and c has a default value, although the function can be invoked as foo(1,3) with arity 2, the function foo/3 is executed, in this case, as foo(1,3,0). We don't explicitly pass a value for c but it will take the defined value, in this case, 0.
Load the Defaults file module inside IEx and open the file defining the module (defaults.ex) inside your favorite code editor:
> iex defaults.ex [Defaults]