Instance_eval: why the class of subclass is superclass

Question

    def singleton_class
       class << self
         self
       end
    end

    class Human
      proc = lambda { puts 'proc says my class is ' + self.name.to_s }

      singleton_class.instance_eval do
        define_method(:lab)  do 
          proc.call
        end
      end
    end

    class Developer < Human
    end

    Human.lab # class is Human
    Developer.lab # class is Human ; oops

Following solution works.

def singleton_class
  class << self
    self
  end
end

class Human
  proc =  lambda { puts 'proc says my class is ' + self.name.to_s }
  singleton_class.instance_eval do
    define_method(:lab) do
      self.instance_eval &proc
    end
  end
end

class Developer < Human
end

Human.lab # class is Human
Developer.lab # class is Human ; oops

Why Developer.lab is reporting that it is Human ? And what can be done so that proc reports Developer when Developer.lab is invoked.

Answer 1

The closure is capturing self in the context where it is defined - just like closures should do. So, when it is called, it will use the references to the context it captured. A closure is not the ideal tool to define the intended functionality. Instead of proc.call, the content of the "define_method" invocation should be "puts 'proc says my class is ' + name.to_s"

Answer 2

It's subtle, but it boils down to simply calling the block (in which case it acts as a normal closure, and self corresponds to where it was defined, ie in Human ), or using it (directly) as a block for a method definition or instance_eval :

def singleton_class
   class << self
     self
   end
end


class Human

  PROC = proc { puts 'proc says my class is ' + self.name.to_s }

  singleton_class.instance_eval do
    define_method(:lab)  do 
      PROC.call
    end
    define_method(:lab2, &PROC.method(:call))

    define_method(:lab3)  do 
      instance_eval(&PROC)
    end
    define_method(:lab4, &PROC) 
  end
end

class Developer < Human
end

Human::PROC.call  # => "class is Human"  (original closure)
Developer.lab     # Same as previous line, so "class is Human"  (original closure)
Developer.lab2    # ditto

Developer.instance_eval(&Human::PROC)  # => "class is Developer"  (instance_eval changes sets a different scope)
Developer.lab3    # Same as previous line, so "class is Developer"
Developer.lab4    # ditto

Answer 3

I have to think a little bit about exactly why this works, but for the moment it does work:

class Human
  proc = -> { name }

  define_singleton_method(:lab, &proc)
end

class Developer < Human; end

require 'test/unit'
class TestClosures < Test::Unit::TestCase
  def test_that_the_human_class_is_named_human
    assert_equal 'Human', Human.lab
  end
  def test_that_the_developer_class_is_named_developer
    assert_equal 'Developer', Developer.lab
  end
end