Circumventing Ruby's scope gates when defining class methods - ruby

Question

I have a class A, which I would like to anonymously extend and add a class method to the child class. E.g.:
class A
end
Class.new A do
def self.new_class_method
puts 'I am a class method'
end
end.new_class_method
=> I am a class method
The above example works well, unless you want to access some variables outside of the def self.new_class_method block. E,g,
greeting = 'hello'
Class.new A do
def self.new_class_method
puts greeting + ' I am a class method'
end
end.new_class_method
=> NameError: undefined local variable or method `greeting'
I am using Ruby 1.8.7, which is sad because I believe Ruby 1.9+ contains an analog to define_method which adds a class method. Does anyone have a work around for 1.8.7?

Answer 1

I have tested the below in Ruby 1.8.7 :-
greeting = 'hello'
class A
end
Class.new A do
meta_klass = class << self; self ;end
meta_klass.send(:define_method, :new_class_method) do
puts greeting + ' I am a class method'
end
end.new_class_method
# >> hello I am a class method
As Ruby 1.8.7 doesn't support Object#singleton_class, I used meta_klass = class << self; self ;end. This method is available since 1.9.2, I think.

Answer 2

You can also use extend() to pry open an object's singleton class. Calling extend(module) adds the methods in the module to the calling object's(i.e. the receiver's) singleton class. So if you call extend(module) when self=A, i.e. inside class A, then the module's methods will be inserted into A's singleton class, and the methods in A's singleton class are also known as class methods of A:
class A
end
greeting = "hello"
Class.new(A) do
extend(
Module.new do
define_method(:greet) do
puts greeting
end
end
)
end.greet
--output:--
hello
And you can rewrite that like this (although then it's not as tricky):
class A
end
greeting = "hello"
Class.new(A) do
m = Module.new do
define_method(:greet) do
puts greeting
end
end
extend(m)
end.greet
...which isn't much different than:
class A
end
greeting = "hello"
m = Module.new do
define_method(:greet) do
puts greeting
end
end
Class.new(A) do
extend(m)
end.greet
...which moves the closure out of the class, and doesn't seem very tricky at all because it only opens up two scope gates instead of three.
Also note, extend() is a public method, so it doesn't require the trickery of a private method, i.e. where you can't specify an explicit receiver, so you have to create a context in which self is the object you want to call the private method on. In other words, you can specify an explicit receiver for extend(). How about the class that is returned by Class.new(A)?
class A
end
greeting = "hello"
Class.new(A).extend(
Module.new do
define_method(:greet) do
puts greeting
end
end
).greet
--output:--
hello
Hey, tacking on ".greet" there works! Uh oh, that has the makings of a one liner:
class A
end
greeting = "hello"
Class.new(A).extend(Module.new {define_method(:greet) {puts greeting} }).greet
--output:--
hello
Yeech!

Answer 3

More generally,
class A
end
class Object
def meta_def name, &blk
(class << self; self; end).instance_eval { define_method name, &blk }
end
end
greeting = 'hello'
Class.new A do
meta_def :new_class_method do
puts greeting + ' I am a class method'
end
end.new_class_method
#=> hello I am a class method
If you find this useful, don't thank me, thank some lucky stiff (which I saw mentioned by Jay Fields).

Answer 4

Not sure if this will solve your problem, but changing Greeting to uppercase (making it a constant) would work...
class A
end
Greeting = 'hello'
Class.new A do
def self.new_class_method
puts Greeting + ' I am a class method'
end
end.new_class_method