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, "~> 0.0"}]
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

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: