budapest ruby meetup, 10/2013 Tamas Tompa

1. Basics of Metaprogramming
Tamás Tompa (Digital Natives)
tamas.tompa@digitalnatives.hu
Budapest Ruby Meetup 2013
02/10/2013
Thursday, October 3, 13

2. “Metaprogramming is writing code
that writes code.”
Thursday, October 3, 13

3. what for?
§ examples
§ writing a wrapper
§ DSL (domain specific language)
§ keep your code DRY
§ extend core ruby classes
Thursday, October 3, 13

4. runtime
§ static vs dynamic metaprogramming
§ everything is object (even classes)
§ code introspection
Thursday, October 3, 13

5. well known example
§ ActiveRecord::Base
§ reads the schema at runtime
§ discovers fields
§ conventions
Thursday, October 3, 13

6. monkey patching
§ monkey patch is a way to extend or
modify the run-time code of dynamic
languages without altering the original
source code
§ can be dangerous
§ always carefully check the existing
methods
Thursday, October 3, 13

7. basics
obj = MyClass.new
obj.class # => MyClass
"hello".class # => String
String.class # => Class
obj.instance_variables # => [:@v]
obj.methods.grep(/my/) # => [:my_method]
Class.instance_methods(false)
Thursday, October 3, 13

8. basics
# ignore inherited methods
Class.instance_methods(false)
String.superclass # => Object
Object.superclass # => BasicObject
BasicObject.superclass # => nil
# module and kernel
D.ancestors # => [D, C, M, Object,
Kernel, BasicObject]
Thursday, October 3, 13

9. “Classes themselves are nothing
but objects.”
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10. object model
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11. dynamic methods
class MyClass
define_method :my_method do |my_arg|
my_arg * 3
end
end
obj = MyClass.new
obj.my_method(2) # => 6
obj.send(:my_method, 2) # => 6
Thursday, October 3, 13

12. generating methods
class Computer
def initialize(computer_id, data_source)
end
def self.define_component(name)
define_method(name) do
info = @data_source.send "get_#{name}_info" , @id
price = @data_source.send "get_#{name}_price" , @id
end
end
define_component :mouse
define_component :cpu
end
Thursday, October 3, 13

13. method missing
class Lawyer
def method_missing(method, *args)
puts "You called: #{method}(#{args.join(', ')})"
puts "(You also passed it a block)" if block_given?
end
end
bob = Lawyer.new
bob.talk_simple('a' , 'b' ) do
# a block
end
You called: talk_simple(a, b)
(You also passed it a block)
Thursday, October 3, 13

14. http://pragprog.com/book/ppmetr/metaprogramming-ruby
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15. We’re hiring!
http://digitalnatives.hu/jobs
Thursday, October 3, 13