Arity is the number of arguments a function receives. In Elixir, a function is identified by its module, name, and arity, so Rectangle.area/1 and Rectangle.area/2 are distinct functions. section 2.3.3
Different arities commonly provide a simple interface over a more general implementation: a lower-arity function delegates to a higher-arity function with default arguments. The \\ operator generates these arities automatically. Because arity distinguishes functions, Elixir does not support functions accepting a variable number of arguments. section 2.3.3
Delegating across arities
Rectangle.area(5) calls area/1 and returns 25; Rectangle.area(5, 6) calls area/2 and returns 30. Likewise, def add(a, b \\ 0), do: a + b provides add/1 and add/2. section 2.3.3
Listing 2.2 Functions with the same name but different arities (arity_demo.ex)
defmodule Rectangle do def area(a), do: area(a, a) #1 def area(a, b), do: a * b #2 end
Inspect a compact implementation where area/1 delegates to area/2. section 2.3.3
