Detecting Ruby mixin being included - ruby

I have a question similar to this one. I'm writing an app that has an addon system. There's an Addon mixin module, which detects when it's included and registers the new addon automatically:
module Addon
def self.included(receiver)
addon = receiver.new # Create an instance of the addon
(snip) # Other stuff to register the addon
addon.on_register # Tell the instance it was registered
end
end
Here is an example of how the mixin would be used:
class MyAddon
def on_register
puts "My addon was registered"
end
include Addon # ** note that this is at the end of the class **
end
As noted above, this implementation requires that the include be at the bottom of the class. Otherwise on_register isn't defined at the time that self.included gets called.
My concern is that an addon developer might accidentally put the include at the top, causing the addon not to work. Or that there might be a derived class or something that would extend the MyAddon class after it was already included.
Is there a better way to go about this?

After distilling the other answers and some other info I found, I wanted to document the answer that ended up working for me.
As this question explains, you can't detect at include() time when a class is "done" being defined. So, relying on the 'included' callback to create the objects was not a very robust solution.
The solution instead was to discover all of the addons and instantiate them after everything had been loaded. The only constraint placed on addon developers was that their code had to share a common top-level module namespace.
I still don't know if this is the best way to do it, but it definitely works better than what I started with.
Here is the code that searches the module for addons. It starts at a passed-in namespace and searches recursively for classes that include the addon class:
def find_and_instantiate(mod, addons)
consts = mod.constants
consts.each do |c|
sym = mod.const_get(c)
if (sym.class == Module)
find_and_instantiate(sym, addons)
else
if (sym.class == Class && sym.include?(Addon))
addons << sym.new(#container)
end
end
end
end

The best I can come up with is to notify the user of your module that it needs to be included after the #on_register method has been declared:
module Addon
def self.included(receiver)
raise "include #{self.name} after the #on_register method is defined" unless receiver.method_defined? :on_register
receiver.new.send(:on_register)
end
end
This is less than ideal but it will prevent aggravating mistakes until you discover a better way of doing this.

Sorry, but I have to hand this to you. So you want to hear your code is clever, pal. Yes, it is, it is too clever. But if you want to be concerned about other developers, you will have to stop being clever and start being normal.
Ruby is an OO language. The basis of OOP is that I define my object structure and their methods first, and then, when everything is defined, I instantiate my world ideally with a single call MyApp.new. That way, it does not matter what I have defined first. 'include' calls serve to establish the module inheritance hierarchy, nothing else.
But no, you do not want to do OOP. You want to abuse the included hook and instantiate receiver (!) in it, and call its instance methods. That's what they call obfuscated code. In other words, you go from OOP back to the old good goto style directive programming. Ruby allows you to obfuscate, not to limit your abilities, but with great power great responsibility, you yourself must exercise moderation.
You will yoursef have to refactor your code to make it normal. We cannot do that for you, because we don't know what do you want to achieve. Who knows, maybe you idea is worthy of a genius. Too bad we don't know it from your question. Some addon system you have. I'm asking you: Do you need it? Isn't good old mixin system good enough for you? Are you sure you are not programming Java in Ruby? And, then, is your app going to be greater than Rails? If yes, go ahead with the addon system. If not – stop playing around and invest your time in writing the algos that actually do what your app is suppose to do rather than creating inextricable addon systems in advance. If you are working in a company - which I doubt - make sure you let your boss read this.

Related

Organizing monkey patches

I read a blog post that recommends namespacing your monkey patches so they can be easily viewed and included.
For example:
module CoreExtensions
module DateTime
module BusinessDays
def weekday?
!sunday? && !saturday?
end
end
end
end
Would go in the: lib/core_extensions/class_name/group.rb file.
It can be included in the DateTime class with the Module#include instance method (which a class inherits because a Class is a Module)
# Actually monkey-patch DateTime
DateTime.include CoreExtensions::DateTime::BusinessDays
My question is where do the include statements go? Is there a convention?
For example:
I have the following monkey patches:
# http://www.justinweiss.com/articles/3-ways-to-monkey-patch-without-making-a-mess/
module CoreExtensions
module String
module Cases
def snakecase
return self if self !~ /[A-Z]+.*/
# http://rubular.com/r/afGWPWLRBB
underscored = gsub(/(.)([A-Z])/, '\1_\2')
underscored.downcase
end
def camelcase
return self if self !~ /_/ && self =~ /[A-Z]+.*/
split('_').map{ |e| e.capitalize }.join
end
end
end
end
That live inside the lib/core_extensions/string/cases.rb file.
Where should I put my String.include CoreExtensions::String::Cases statement?
Also to be clear this is just a ruby project, does that make a difference?
I've tried putting it inside lib/devify.rb
require 'devify/version'
require 'devify/some_dir'
require 'devify/scaffold'
require 'devify/tree_cloner'
require 'devify/renderer'
require 'devify/project'
require 'devify/tasks/some_task'
require 'devify/tasks/bootstrap'
require 'core_extensions/string/cases'
module Devify
String.include CoreExtensions::String::Cases
end
This works and it makes sense why it works. It's because my entire app lives inside the Devify module or namespace.
This way is also good because I'm not polluting the global namespace correct? Because I'm only monkey patching Strings that live inside Devify?
Just not sure not if this is the right way to go about it.
It doesn't matter where you put the include call.
Calling String.include will always monkey patch the one String class that is used by all the strings in the entire object space. So best put the instruction at the top level as to not mislead readers of the code.
Monkey patching is always global.
It is a powerful feature and can be used for good.
If you are authoring a gem be aware that you're sharing a global namespace with others. The same is also true for top-level modules and even the gem name though. Shared namespaces are just a reality of shared code.
If you are looking for lexically scoped monkey patches look into the new refinement feature that was introduce with Ruby 2.
Refinements are an idea taken from Smalltalk's class boxes. Refinements are not without their own issues though, for example they lack support for introspection and reflection. Thus essentially making them stealth and unfit for production use.
If you are looking to limit the monkey patches to some string object only, consider either subclassing String or calling extend on an instance.
Although ruby offers many ways of changing the content of a class or a method dynamically, the monkey patching can lead to big problems and strange bugs. I read this post (http://www.virtuouscode.com/2008/02/23/why-monkeypatching-is-destroying-ruby/) about why it´s a bad idea to use monkey-patching.
In summary, many things that he says make sense. When you create a monkey-patching, you are assuming that it will only works at that project, and, maybe you can create collisions and unprevisible side-effects when more libraries with similar purposes are put together.
There are cases where the benefits of the monkey-patching were awesome, like ActiveSupport way of dealing with dates manipulation by monkey-patching the Fixnum class with the ago or from_now methods, or as the method to_json. However, monkey patching should be avoided.
The point is: Ruby is an object-oriented language, and you can achieve your objectives using object composition, or any other patterns. Monkey-patching, at some way, leads you in the opposite direction of object oriented philosophy, since you add more responsibilities to an pre-existent class and increases it's public interface to serve a new funcionallity.
Besides, it's not explicit the behavior of that class and the public methods available. You cannot know, by looking at the class definition, what it makes and what is it's role at the system, and how it interact with other objects. It makes a simple task much more harder at the end.
Monkey patching makes everything much more smaller and simpler, apparently, but avoiding it makes your code much more maintanable, easier to debug, read and test, and much more elegant, since it is compliant to the "OOP" patterns.

Creating a simple ruby gem that allows the user to call a defined method?

I've been looking for some tutorials on creating ruby gems, but all seem to be overly complicated. Essentially, all I want is to make a gem that (trivial example alert) allows the user to place a method add_one(x) in their code, once the gem is installed.
A lot of tutorials explain how you can call the gem's methods with Classname.method(), but that's really not what I want. RyanB has a good example here, but again I can't get it to work - mainly because of the 3 lines of code at the bottom of uniquify.rb that read.
class ActiveRecord::Base
include Uniquify
end
In my case, my main module is called AddOne, so how would I go about doing this? Again, sorry for the trivial example, I'm just looking to get started so that my gems will allow the user to call plain ol' methods, without specifying Modules or Classes. In Ryan's example, he's able to simple call uniquify() in the code.
In the example given, he can call uniquify as a method because he is calling it inside a class which includes ActiveRecord::Base. (He calls uniquify inside the class Product < ActiveRecord::Base...end block) There isn't any real magic here.
To my understanding, what you desire is easily achievable, but isn't a good idea, 'best practice' considered. You can define anything you want in your gem. For instance, I could package a file called say_hello.rb inside a gem called salutations.
say_hello.rb
def say_hello
puts "Hello everybody"
end
By requiring the packaged gem salutations I would gain access to anything defined therein. However, it might lead to unexpected consequences. Do what you need to do, but just understand that it might not be the best idea. Hence, you build things into classes or modules to contain them, and to prevent conflicts with other code. Good luck.

Do rubyists reference accessors from mixins?

Is it considered bad practice to reference accessors on the extended object from within a mixin method? A simplistic example:
module WindInstrument
def play
mouthpiece.blow #requires a mouthpiece
end
end
class Saxophone
attr_reader :mouthpiece
def initialize
#mouthpiece = Mouthpiece.new
end
include WindInstrument
end
Saxophone.new.play
In this case, I would actually just move the requirement for a mouthpiece directly to the WindInstrument module, but what about in a more complex scenario, where it really makes sense for the accessor to live on the extended object? Is this just an issue of an inappropriate separation of concerns?
Mixins feel useful for adding encapsulated behavior that doesn't require knowledge of the extended object's state. In fact, my gut tells me that a mixin shouldn't have knowledge of any state, whatsoever. If it needs knowledge of state, I would typically fall back to one of two choices:
Put the state in a class, and add it through composition, instead of through the inheritance hierarchy. My issue with this is that I know rubyists out there are creating mixins that access state, which makes for more readable, if less intuitive (to me) design.
Pass the mouthpiece as a parameter to the module. Even I can tell that this seems to muddy the design, and feels like an abomination in the ruby worldview.
Does this code bother anyone else? I know there are a lot of smart people using ruby out there, so I assume the problem is mine. What am I missing? Do I just need to chill out? What would you do?
I think it's the same as in monkey-patching: it's ok to do it, but you have to make sure that there are no alternatives first (ie. You can't modify the classes using your interface) and second, you must be very explicit about it (make sure that the docs, the comments and the interface mention that this our that method is required and will be invoked) and throw a useful error message if it isn't
Ruby's accessors are interfaces, not implementation.
For example, if you call person.height_in_feet=, you don't know what units the height is actually implemented in as an instance variable. It could be metres, feet, or cubits.
One real-life example of mixins using an accessor is the Enumerable module. Although I don't include this module in any classes I create, I'm happy with what it does. It gives you convenient methods like map and each_with_index, while staying DRY - there's only one implementation of what objects you'd access with all of the methods of the "mixee", and there's only one definition of what map does, for any object that uses Enumerable.

What are some good examples of Mixins and or Traits?

I was reading up on Ruby, and learned about its mixins pattern, but couldn't think of many useful mixin functionality (because I'm not used to thinking that way most likely). So I was wondering what would be good examples of useful Mixin functionality?
Thanks
Edit: A bit of background. I'm Coming from C++, and other Object languages, but my doubt here is that Ruby says it's not inheriting mixins, but I keep seeing mixins as Multiple inheritance, so I fear I'm trying to categorize them too soon into my comfort zone, and not really grok what a mixin is.
They are usually used to add some form of standard functionality to a class, without having to redefine it all. You can probably think of them a bit like interfaces in Java, but instead of just defining a list of methods that need to be implemented, many of them will actually be implemented by including the module.
There are a few examples in the standard library:
Singleton - A module that can be mixed into any class to make it a singleton. The initialize method is made private, and an instance method added, which ensures that there is only ever one instance of that class in your application.
Comparable - If you include this module in a class, defining the <=> method, which compares the current instance with another object and says which is greater, is enough to provide <, <=, ==, >=, >, and between? methods.
Enumerable - By mixing in this module, and defining an each method, you get support for all the other related methods such as collect, inject, select, and reject. If it's also got the <=> method, then it will also support sort, min, and max.
DataMapper is also an interesting example of what can be done with a simple include statement, taking a standard class, and adding the ability to persist it to a data store.
Well the usual example I think is Persistence
module Persistence
def load sFileName
puts "load code to read #{sFileName} contents into my_data"
end
def save sFileName
puts "Uber code to persist #{#my_data} to #{sFileName}"
end
end
class BrandNewClass
include Persistence
attr :my_data
def data=(someData)
#my_data = someData
end
end
b = BrandNewClass.new
b.data = "My pwd"
b.save "MyFile.secret"
b.load "MyFile.secret"
Imagine the module is written by a Ruby ninja, which persists the state of your class to a file.
Now suppose I write a brand new class, I can reuse the functionality of persistence by mixing it in by saying include ModuleILike. You can even include modules at runtime. I get load and save methods for free by just mixing it in. These methods are just like the ones that you wrote yourself for your class. Code/Behavior/Functionality-reuse without inheritance!
So what you're doing is including methods to the method table for your class (not literally correct but close).
In ruby, the reason that Mixins aren't multiple-inheritance is that combining mixin methods is a one time thing. This wouldn't be such a big issue, except that Ruby's modules and classes are open to modification. This means that if you mixin a module to your class, then add a method to the module, the method will not be available to your class; where if you did it in the opposite order, it would.
It's like ordering an ice-cream cone. If you get chocolate sprinkles and toffee bits as your mixins, and walk away with your cone, what kind of ice cream cone you have won't change if someone adds multicolored sprinkles to the chocolate sprinkles bin back at the ice-cream shop. Your class, the ice cream cone, isn't modified when the mixin module, the bin of sprinkles is. The next person to use that mixin module will see the changes.
When you include a module in ruby, it calls Module#append_features on that module, which add a copy of that module's methods to the includer one time.
Multiple inheritance, as I understand it, is more like delegation. If your class doesn't know how to do something, it asks its parents. In an open-class environment, a class's parents may have been modified after the class was created.
It's like a RL parent-child relationship. Your mother might have learned how to juggle after you were born, but if someone asks you to juggle and you ask her to either: show you how (copy it when you need it) or do it for you (pure delegation), then she'll be able at that point, even though you were created before her ability to juggle was.
It's possible that you could modify a ruby module 'include' to act more like multiple inheritance by modifying Module#append_features to keep a list of includers, and then to update them using the method_added callback, but this would be a big shift from standard Ruby, and could cause major issues when working with others code. You might be better creating a Module#inherit method that called include and handled delegation as well.
As for a real world example, Enumerable is awesome. If you define #each and include Enumerable in your class, then that gives you access to a whole host of iterators, without you having to code each and every one.
It is largely used as one might use multiple inheritance in C++ or implementing interfaces in Java/C#. I'm not sure where your experience lies, but if you have done those things before, mixins are how you would do them in Ruby. It's a systemized way of injecting functionality into classes.

Mocking constructors in Ruby

I'm a Java-developer toying with Ruby, and loving it. I have understood that because of Ruby's metaprogramming facilities my unit-tests become much cleaner and I don't need nasty mocking frameworks. I have a class which needs the File class's services and in my test I don't want to touch my real filesystem. In Java I would use some virtual file system for easier "seams" to pass fake-objects in but in Ruby that's obviously overkill. What I come up seems already really nice compared to the Java-world. In my class under test I have an optional constructor parameter:
def initialize(file_class=File)
When I need to open files within my class, I can then do this:
#file_class.open(filename)
And the call goes to either the real File-class, or in case of my unit-test, it goes to a fake-class which doesn't touch the filesystem. I know there must be a better way to do this with metaprogramming?
Mocha (http://mocha.rubyforge.org/) is a very good mocking library for ruby. Depending on what you're actually wanting to test (i.e. if you want to just fake out the File.new call to avoid the file system dependency or if you want to verify that the correct arguments are passed into File.new) you could do something like this:
require 'mocha'
mock_file_obj = mock("My Mock File") do
stubs(:some_instance_method).returns("foo")
end
File.stubs(:new).with(is_a(String)).returns(mock_file_obj)
In the case you've outlined, I'd suggest that what you're doing seems fine. I know that it's a technique that James Mead (the author of Mocha) has advocated. There's no need to do metaprogramming just for the sake of it. Here's what James has to say about it (and a long list of other techniques you could try)
This is a particularly difficult challenge for me. With the help I received on this question, and some extra work on my behalf, here's the solution I arrived at.
# lib/real_thing.rb
class RealThing
def initialize a, b, c
# ...
end
end
# test/test_real_thing.rb
class TestRealThing < MiniTest::Unit::TestCase
class Fake < RealThing; end
def test_real_thing_initializer
fake = mock()
Fake.expects(:new).with(1, 2, 3).returns(fake)
assert_equal fake, Fake.new(1, 2, 3)
end
end

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