Elixir function decorators

A function decorator is an "@" annotation that sits in front of 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.

A remark 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.

Note When using decorators without arguments, Elixir warns you with a message warning: module attribute @some_decorator in code block has no effect as it is never returned. This is unfortunate but cannot be prevented, as this warning is emitted in a very early stage of compilation.

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

@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 defines a print() decorator which prints a message every time the 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

Note that print() here is a function, not a macro! The actual macro has arity 0, and will be imported in the caller module. The decorator module's print() function gets called when the actual function is being defined.

The arguments to the decorator function 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:

@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() decorator function gets the level passed in as the first argument:

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