Ruby Metaprogramming

ruby_meta1.rb

'hello'.hello_world # NoMethodError: undefined method `hello_world' for "hello":String

class String
  def hello_world
    p 'Hello ' + self 
  end
end

'hello'.hello_world # "Hello hello" 

class String < Integer; end # TypeError: superclass mismatch for class String

ruby_meta2.rb

class Fixnum
  def +(arg)
    self * arg
  end
end

2 + 4 # 8

module OddNum
  refine Fixnum do
    def -(arg)
      self * arg
    end
  end
  
  def self.minus(arg1, arg2)
    arg1 * arg2
  end
end

OddNum.minus(3,4) # 12

3 - 4 # -1

ruby_meta3.rb

class A
  def a
    p 'a'
  end
  
  private 
  
  def b(arg)
    p arg
  end
end

A.new.send :a # "a"
A.new.send :b, 'hello' # "hello"

class A
  def self.create_new_method(name, &block)
    define_method name, &block
  end
end

A.create_new_method(:print_x_times) do |str, times| 
  p str * times
end

A.new.print_x_times('hello ', 3) # "hello hello hello "

ruby_meta4.rb

class A
  def a
    p 'a_a'
  end
  
  def b
    p 'b_a'
  end
end

class B < A
  def a
    p 'b_a'
  end
  
  def b 
    p 'b_b'
  end
end

B.send :undef_method, :a
B.new.a # NoMethodError: undefined method `a' for #<B:0x007fbce9183af0>
A.new.a # "a_a"

B.send :remove_method, :b 
B.new.b # "b_a"

ruby_meta5.rb

A = Class.new do
  define_method :a do
    p 'A a'
  end
  
  def b 
    p 'A b'
  end
end

A.new.a # "A a"
A.new.b # "A b"

self.class.send :remove_const, :A
A.new # NameError: uninitialized constant A

ruby_meta6.rb

class A
  def method_missing (method_name, *args, &block)
    name = method_name.to_s
    if valid_missing_method_name? name
      string_to_print = extract_print_string name
      p string_to_print
    else
      super
    end
  end
  
  def extract_print_string(name)
    name.match(missing_method_regex)[1]
  end
  
  def valid_missing_method_name?(name)
    name.to_s =~ missing_method_regex
  end
  
  def missing_method_regex
    /^print_(\w+)$/
  end
end

A.new.print_hello # "hello"
A.new.hello_print # NoMethodError: undefined method `hello_print' for #<A:0x007ff21a10f860>

ruby_meta7.rb

class A
  def a
    p 'a'
  end
  
  alias :b :a
end

A.new.b # "a"

A.send :remove_method, :a
A.new.a # undefined method `a' for #<A:0x007f9d1992b900>
A.new.b # "a"