Behaviours are a list of functions specifications that another module can implement. They are similar to interfaces in other languages.

If you’ve used the use keyword when writing GenServers or in the Phoenix framework, you’ve already used behaviours.

Here’s an example behaviour:

defmodule Parser do
  @callback parse(String.t) :: any
  @callback extensions() :: [String.t]

And a module that implements it:

defmodule JSONParser do
  @behaviour Parser

  def parse(str), do: # ... parse JSON
  def extensions, do: ["json"]

The @behaviour module attribute above indicates that this module is expected to define every function defined in the Parser module. Missing functions will result in undefined behaviour function compilation errors.

Modules can have multiple @behaviour attributes.