basics of metaprogramming in ruby

Basics of MetaprogrammingTamás Tompa (Digital Natives)

tamas.tompa@digitalnatives.hu

Budapest Ruby Meetup 2013

02/10/2013

Thursday, October 3, 13

“Metaprogramming is writing code that writes code.”

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what for?§ examples

§ writing a wrapper§ DSL (domain specific language)§ keep your code DRY§ extend core ruby classes

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runtime§ static vs dynamic metaprogramming§ everything is object (even classes)§ code introspection

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well known example§ ActiveRecord::Base

§ reads the schema at runtime§ discovers fields§ conventions

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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

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basicsobj = MyClass.newobj.class # => MyClass

"hello".class # => StringString.class # => Class

obj.instance_variables # => [:@v]

obj.methods.grep(/my/) # => [:my_method]

Class.instance_methods(false)

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basics# ignore inherited methodsClass.instance_methods(false)

String.superclass # => ObjectObject.superclass # => BasicObjectBasicObject.superclass # => nil

# module and kernelD.ancestors # => [D, C, M, Object, Kernel, BasicObject]

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“Classes themselves are nothing but objects.”

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object model

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dynamic methodsclass 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

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generating methodsclass 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

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method missingclass 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)

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http://pragprog.com/book/ppmetr/metaprogramming-ruby

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We’re hiring!http://digitalnatives.hu/jobs

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