I have come to realize that there are a few hook methods I really don’t like in Ruby. Or actually, it’s not the hook methods I have a problem with – it’s the way much code is written using them. The specific hooks that seems to cause the most trouble for me is included, extended, append_features and extend_features. Let me first reiterate – I don’t dislike the methods per se. The power they give the language is incredible and should not be underestimated. What I dislike is the way it makes things un-obvious when reading code that depends on them.
Let’s take a silly example:
module Ruby;end module Slippers def self.included(other) other.send :include, Ruby end end class Judy include Slippers end p Judy.included_modules
Since all this code is in the same place, you can see what will happen when someone include Slippers. And really, in this case the side effect isn’t entirely dire. But imagine that this was part of a slightly larger code base. Like for example Rails. And the modules were spread over the code base. And the included hook did a few more things with your class. No way of knowing what – except reading the code – and the Ruby idiom is that include will add some methods and constants to your class and that is it. Anything else is going outside what the core message of that statement is.
One of the most common things you see with the included hook is something like this:
module Slippers module ClassMethods end def self.included(other) other.send :extend, ClassMethods end end class Judy include Slippers end
This will add some class methods to the class that includes this module. DataMapper does this in the public API, for example. It’s very neat, you only have to include one thing and you get stuff on both your instances and your classes. Except that’s not what include does. Not really. So say you’re debugging someone’s code and happen upon an include statement. You generally don’t check what it’s doing until you’ve exhausted most other options.
So what’s wrong with having a public API like this?
module Slippers module ClassMethods;end end class Judy include Slippers extend Slippers::ClassMethods end
where you explicitly include the Slipper module and then extend the class methods. This is more obvious code, it doesn’t hide anything behind your expectations, and it also might give me the possibility to choose. What if I want most of the DataMapper instance methods, but really don’t want to have finders on my class? Maybe I want to have a repository pattern? In that case I’ll have to explicitly remove all class methods introduced by that include, because there is no way of choosing if I want to have the class methods or not.
So that’s another benefit of dividing the extending out from the included hook. And finally, what about all those other things that people do in those hooks? Well, you don’t really need it. Make it part of the public API too! Instead of this:
module Slippers def self.included(other) do_funky_madness_on other end end class Judy include Slippers end
make it explicit, like this:
module Slippers;end class Judy include Slippers Slippers.do_funky_madness_on self end
This is really just good design. It makes the functionality explicit, it makes it possible for the user to choose what he wants without doing monkey patching. And it makes the code easier to read. Yeah, I know, this will mean more lines of code. Booo hooo! I know that Ruby people are generally obsessed with making their libraries as easy to use as possible, but it feels like it sometimes goes totally absurd and people stop thinking about readability, maintainability and all those other things. And really, Ruby is such a good language that a few more lines of code like this still won’t make a huge impact on the total lines of code.
I’m not saying I haven’t done this, of course. But hopefully I’ll get better at it. And I’m not saying not to use these methods at all – I’m just saying that you should use them with caution and taste.