Elixir function decorators

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 remarks in advance

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.

Installation

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

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

You can now define your function decorators.

Usage

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 AST), as well as a context argument which holds information like the function's name, defining module, arity and the arguments AST.

Compile-time arguments

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

Decorating all functions in a module

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.

Functions with multiple clauses

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

Decorator context

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, 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} 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