An extremely brief intro to Metaprogramming in Ruby. Given at a Women Who Code Meetup. Feel free to contact me at i@mindlace.net if you do have questions.

1. Metaprogramming
what’s up with that

2. Yo, Dawg - I heard you like programming
so I got you some meta
so you can program your programs

3. Using Open Classes is Meta
class Integer
def hearts
self.times do
print "u2764 "
end
end
end
irb(main):003:0> require "./meta.rb"
=> true
irb(main):004:0> 3.hearts
❤ ❤ ❤ => 3
irb(main):005:0> 1.hearts
❤ => 1

4. Objects and Classes
Object stores only
variables and a
reference to the Class
Class stores only
methods (both instance
and class methods)
class = :Animal
@says = “bark”
def speak
puts @says
end

5. A class is an object!
you can change variables on an object
you can change methods on a class
in ruby, you can do this whenever you want
if your code changes other code, especially
when running, that’s metaprogramming

6. Text
Can be dangerous!
But ruby 2.0 makes it safer

7. Refine is safer (ruby 2)
module MadHacks
refine Integer do
def hearts
self.times do
print "u2764 "
end
end
end
end
!
irb(main):001:0> require "./meta.rb"
=> true
irb(main):002:0> 1.hearts
NoMethodError: undefined method `hearts' for 1:Fixnum
from (irb):2
from /Users/mindlace/.rbenv/versions/2.0.0-p247/bin/irb:12:in `<main>'
irb(main):003:0> using MadHacks
=> main
irb(main):004:0> 1.hearts
❤ => 1

8. Lots of other techniques
Dynamic methods (add methods when they are named)
Method missing (respond to any method name)
And more!
Ruby makes it very easy to metaprogram
With power comes responsibility

9. Questions?