instance_exec + blocks

gistfile1.rb

class X
  def self.morph(name, &blk)
    define_method(name) do |*args, &user_blk|
      instance_exec(*args, &blk)
    end
  end

  morph :xyz do |a, b, &blk|
    puts "HELLO, WORLD!"
    blk.call(a, b)
  end
end

X.new.xyz(5, 10) {|a, b| puts "YAY! a = #{a}; b = #{b}"}