trololol.rb

module Kernel
  def int(mid)
    meth = instance_method(mid)
    define_method(mid) { |*args, &bk|
      val = meth.bind(self).call(*args, &bk)
      raise TypeError, "#{mid} did not return an Integer" unless val.is_a? Integer
      val
    }
    mid
  end
end

class Foo
  public int def foo
    10
  end
end

Foo.new.foo