Define singleton methods on initialization using instance variables

Question

I'm trying to optimize some code and I want to instead on checking a value on every method call just define the method to respond with the checking already pre-calculate, because this checking doesn't change on the whole live of the instance.

I decided to define different versions of the method for every instance created. More or less this way:

class TestingSingletonMethodsWithVariable
  METHODS = %w(a b c d)

  def initialize(favorite_method)
    class << self
      METHODS.each do |method_name|
        if( favorite_method == method_name )
          define_method method_name do
            puts "#{method_name} its my favorite method"
          end
        else
          define_method method_name do
            puts "#{method_name} its not my favorite method"
          end
        end
      end
    end
  end
end

t = TestingSingletonMethodsWithVariable.new('b')
t.a
t.b
t.c
t.d

# $ ruby test/testing_singleton_methods_with_variable.rb 
# test/testing_singleton_methods_with_variable.rb:7:in `initialize': undefined local variable or method `favorite_method' for #<Class:#<TestingSingletonMethodsWithVariable:0x1001a77b8>> (NameError)
#   from test/testing_singleton_methods_with_variable.rb:6:in `each'
#   from test/testing_singleton_methods_with_variable.rb:6:in `initialize'
#   from test/testing_singleton_methods_with_variable.rb:21:in `new'
#   from test/testing_singleton_methods_with_variable.rb:21

What is happening is that something weird is happening with the variables: the variables declares out-side the class << self block are not visible for the variables inside.

Any one can explain me how can I do the behavior I'm looking for?

Thanks

I think there's some mixup with your example. In the subject line you mention instance variables, but there are no instance variables anywhere in the code you posted. — Jörg W Mittag
– Jörg W Mittag, Commented Nov 16, 2010 at 17:56

Jörg W Mittag · Accepted Answer · 2013-10-30 19:56:19Z

9

In Ruby, only blocks can be closures, class bodies (as well as module and method bodies) cannot be closures. Or to put it another way: only blocks create a new nested lexical scope, all others (module bodies, class bodies, method bodies and script bodies) create new top-level scopes.

So, you will need a block. Normally, this would mean using some form of eval, but here you can just use define_singleton_method instead:

class TestingSingletonMethodsWithVariable
  METHODS = %w(a b c d)

  def initialize(favorite_method)
    METHODS.each do |method_name|
      if favorite_method == method_name
        define_singleton_method method_name do
          puts "#{method_name} its my favorite method"
        end
      else
        define_singleton_method method_name do
          puts "#{method_name} its not my favorite method"
        end
      end
    end
  end
end

t = TestingSingletonMethodsWithVariable.new('b')
t.a
t.b
t.c
t.d

edited Oct 30, 2013 at 19:56

answered Nov 16, 2010 at 17:55

Jörg W Mittag

371k79 gold badges457 silver badges666 bronze badges

Sign up to request clarification or add additional context in comments.

2 Comments

fguillen Over a year ago

Thanks very much @Jörg, I am gonna accept the answer of @Chubas just because in it are your both answers and I think is important to people with my same problem to read both answers.

Chris Cirefice Over a year ago

I would suggest changing the accepted answer to this one - I doubt many people are pre-1.9 still, so the workaround is not necessary :)

Chubas · Accepted Answer · 2010-11-16 19:08:43Z

1

Adding to Jörg's answer: define_singleton_method is Ruby 1.9+. If you want to run it in pre 1.9, the following works:

class Object
  def metaclass
    class << self; self; end
  end
end
class TestingSingletonMethodsWithVariable
  METHODS = %w(a b c d)

  def initialize(favorite_method)
    METHODS.each do |method_name|
      if( favorite_method == method_name )
        metaclass.send(:define_method, method_name, Proc.new do
          puts "#{method_name} its my favorite method"
        end)
      else
        metaclass.send(:define_method, method_name, Proc.new do
          puts "#{method_name} its not my favorite method"
        end)
      end
    end
  end
end

t = TestingSingletonMethodsWithVariable.new('b')
t.a
t.b
t.c
t.d

answered Nov 16, 2010 at 19:08

Chubas

18.1k4 gold badges51 silver badges48 bronze badges

1 Comment

fguillen Over a year ago

Please read the @Jörg answer because in it is the Ruby 1.9+ solution.

Collectives™ on Stack Overflow

Define singleton methods on initialization using instance variables

2 Answers 2

2 Comments

1 Comment

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

2 Comments

1 Comment

Your Answer

Sign up or log in

Post as a guest

Related