A function decorator is a "@decorate" annotation that is put just before a function definition. It can be used to add extra functionality to Elixir functions. The runtime overhead of a function decorator is zero, as it is executed on compile time.

Examples of function decorators include: loggers, instrumentation (timing), precondition checks, et cetera.

Some people think function decorators are a bad idea, as they can perform magic stuff on your functions (side effects!). Personally, I think they are just another form of metaprogramming, one of Elixir's selling points. But use decorators wisely, and always study the decorator code itself, so you know what it is doing.

Decorators are always marked with the @decorate literal, so that it's clear in the code that decorators are being used.

Add :decorator to your list of dependencies in mix.exs:

def deps do
  [
    {:decorator, "~> 1.2"}
  ]
end

You can now define your function decorators.

Function decorators are macros which you put just before defining a function. It looks like this:

defmodule MyModule do
  use PrintDecorator

  @decorate print()
  def square(a) do
    a * a
  end
end

Now whenever you call MyModule.square(), you'll see the message: Function called: square in the console.

Defining the decorator is pretty easy. Create a module in which you use the Decorator.Define module, passing in the decorator name and arity, or more than one if you want.

The following declares the above @print decorator which prints a message every time the decorated function is called:

defmodule PrintDecorator do
  use Decorator.Define, [print: 0]

  def print(body, context) do
    quote do
      IO.puts("Function called: " <> Atom.to_string(unquote(context.name)))
      unquote(body)
    end
  end

end

The arguments to the decorator function (the def print(...)) are the function's body (the abstract syntax tree (AST)), as well as a context argument which holds information like the function's name, defining module, arity and the arguments AST.

Decorators can have compile-time arguments passed into the decorator macros.

For instance, you could let the print function only print when a certain logging level has been set:

@decorate print(:debug)
def foo() do
...

In this case, you specify the arity 1 for the decorator:

defmodule PrintDecorator do
  use Decorator.Define, [print: 1]

And then your print/3 decorator function gets the level passed in as the first argument:

def print(level, body, context) do
# ...
end

A shortcut to decorate all functions in a module is to use the @decorate_all attribute, as shown below. It is important to note that the @decorate_all attribute only affects the function clauses below its definition.

defmodule MyApp.APIController
  use MyBackend.LoggerDecorator

  @decorate_all log_request()

  def index(_conn, params) do
    # ...
  end

  def detail(_conn, params) do
    # ...
  end

In this example, the log_request() decorator is applied to both index/2 and detail/2.

If you have a function with multiple clauses, and only decorate one clause, you will notice that you get compiler warnings about unused variables and other things. For functions with multiple clauses the general advice is this: You should create an empty function head, and call the decorator on that head, like this:

defmodule DecoratorFunctionHead do
  use DecoratorFunctionHead.PrintDecorator

  @decorate print()
  def hello(first_name, last_name \\ nil)

  def hello(:world, _last_name) do
    :world
  end

  def hello(first_name, last_name) do
    "Hello #{first_name} #{last_name}"
  end
end

Besides the function body AST, the decorator function also gets a context argument passed in. This context holds information about the function being decorated, namely its module, function name, arity, function kind, and arguments as a list of AST nodes.

The print decorator can print its function name like this:

def print(body, context) do
  Logger.debug("Function #{context.name}/#{context.arity} with kind #{context.kind} called in module #{context.module}!")
end

Even more advanced, you can use the function arguments in the decorator. To create an is_authorized decorator which performs some checks on the Phoenix %Conn{} structure, you can create a decorator function like this:

def is_authorized(body, %{args: [conn, _params]}) do
  quote do
    if unquote(conn).assigns.user do
      unquote(body)
    else
      unquote(conn)
      |> send_resp(401, "unauthorized")
      |> halt()
    end
  end
end

A "hidden" function __decorated_functions__/0 is added to any module that decorates functions and returns a list of all decorated functions within that module.

This can be useful for testing purposes since you don't have to assert the decorated behaviors of the decorated functions. So long as your decorator function is well tested, you can just assert the expected decorators are present for the given function.

The function returns a map with the function name and arguments as the key and a list of tuples with the decorator. This permits multiple decorators to be asserted with a single function call so that adding new decorators will cause the assertion to fail.

For example, given the following module:

  defmodule NewModule do
    use Decorator.Define, [new_decorator: 1, another_decorator: 2]

    @decorate new_decorator("one")
    def func(%StructOne{} = msg) do
      msg
    end

    @decorate new_decorator("two")
    @decorate another_decorator("a", "b")
    @decorate new_decorator("b")
    def func(%StructTwo{c: 2} = _msg) do
      :ok
    end
  end

You can assert the decorated functions like so:

  test "Module with decorated functions are returned by `__decorated_functions__()" do
    assert %{
             {:func, ["%StructOne{} = msg"]} => [
               {DecoratorTest.Fixture.NewDecorator, :new_decorator, ["one"]}
             ],
             {:func, ["%StructTwo{c: 2} = _msg"]} => [
               {DecoratorTest.Fixture.NewDecorator, :new_decorator, ["two"]},
               {DecoratorTest.Fixture.AnotherDecorator, :another_decorator, ["a", "b"]},
               {DecoratorTest.Fixture.NewDecorator, :new_decorator, ["b"]}
             ]
           } == NewModule.__decorated_functions__()
  end

Obviously, any changes to the parameters of the decorated function will cause the simple assertion to fail, but that's intentional, as the decorator may be dependent on the parameters of the decorated function.

Copyright (c) 2016 Arjan Scherpenisse

This library is MIT licensed. See the LICENSE for details.