Ruby, Rails and something else
Let’ say you need to debug this piece of code:
very_cool_method(egg, sausage)
What would you do? I guess something along the lines of:
spam = very_cool_method(egg, sausage) p spam
Well, Ruby 1.9 let you do something cooler instead:
very_cool_method(egg, sausage).tap{ |s| p s }
The cool thing about tap is that you can use it to eavesdrop method chains:
very_cool_method(egg, sausage).tap{ |s| p s }.now_make_sandwich_from(bacon)
Hooray! 
Simply type in rails console:
ActiveRecord::Base.logger = Logger.new(STDOUT)
If you want to install gems quickly without waiting for ri and rdoc documentation files, run this command:
echo "gem: --no-ri --no-rdoc" >> ~/.gemrc
Your new gems won’t include documentation at all. Hooray!
Have you ever run a
rails generate controller UsersController
and seen your app folder filled with useless files?
create app/controllers/users_controller_controller.rb
invoke erb
create app/views/users_controller
invoke test_unit
create test/functional/users_controller_controller_test.rb
invoke helper
create app/helpers/users_controller_helper.rb
invoke test_unit
create test/unit/helpers/users_controller_helper_test.rb
invoke assets
create app/assets/javascripts/users_controller.js.coffee
invoke scss
create app/assets/stylesheets/users_controller.css.scss
Eww, that’s gross.
To undo that rails generate, simply run:
rails destroy controller UserController
and rails will destroy all those ugly files for you!
remove app/controllers/users_controller_controller.rb
invoke erb
remove app/views/users_controller
invoke test_unit
remove test/functional/users_controller_controller_test.rb
invoke helper
remove app/helpers/users_controller_helper.rb
invoke test_unit
remove test/unit/helpers/users_controller_helper_test.rb
invoke assets
remove app/assets/javascripts/users_controller.js.coffee
invoke scss
remove app/assets/stylesheets/users_controller.css.scss
String.instance_eval { def foo; p "bar"; end } String.foo = "bar"
String.class_eval { def foo; p "bar"; end } "sausage".foo = "bar"
So, using instance_eval adds a class method.
On the other hand, using class_eval adds a instance method.
Why?
class_eval is a method of the Module class, meaning that the receiver will be a module or a class. The block you pass to class_eval is evaluated in the context of that class. Defining a method with the standard def keyword within a class defines an instance method, and that’s exactly what happens here.
instance_eval, on the other hand, is a method of the Object class, meaning that the receiver will be an object. The block you pass to instance_eval is evaluated in the context of that object. That means that String.instance_eval is evaluated in the context of the String object. Remember that a class name is simply a constant which points to an instance of the class Class. Because of this fact, defining a method in the context of Class instance referenced by String creates a class method for the String class.