Dynamic Ruby for Nubies

DYNAMIC RUBY FOR NUBIES

Nuby Hoedown MMX
September 2nd, 2010

Kevin W. Gisi | http://www.kevingisi.com | kevin@kevingisi.com | @gisikw

LANGUAGE


WHAT IS A DSL?

• Domain-Speciﬁc Language


CUCUMBER IS A DSL
Feature: Password recovery
As a forgetful user
I want to be able to recover my password
So that I can get back into the site when I lose my password!

Scenario: Recovering password
Given I have signed up as a user
    When I go to the home page
     And I click "Recover your password"
     And I fill in "Email" with "user@example.com"
     And my email address is "user@example.com"
     And I press "Reset my password"
    Then I should receive an email
    When I open the email
     And I click the first link in the email
     And I fill in "Password" with "simpsonsrock!"
     And I fill in "Password Confirmation" with "simpsonsrock!"
     And I press "Update password"
    Then I should be on the dashboard page
     And I should see "Your password has been reset"


REAL LANGUAGES

• Rule-based

• Pattern-matching

• Very dull to read


DSL EVALUATION (RUBY)

• Example: turn the monitor background color green



• English: turn the monitor background color green

• Ruby: turn(“monitor”, “background”, “#00FF00”)



• Example: turn the monitor background color green

• Ruby: turn( the( monitor( background( color( green) ) ) ) )


DSL EVALUATION
turn the monitor background color green

class DSLObject

  def method_missing(name,*args)
    return name.to_s
  end

end

# @dsl_object.green => "green"


DSL EVALUATION
turn the monitor background color “green”
class DSLObject

  def color(name)
    case name
      when "green" : "#00FF00"
      when "blue" : "#0000FF"
      when "red" : "#FF0000"
    end
  end

    return name.to_s
  end

end

# @dsl_object.color green => NameError: undefined local variable
or method `green' for #<Object:0x1001c8288>


PROBLEM!

• How do we execute the code inside this object?

• @dsl_object.color green

• “green” is evaluated within the current scope, so unless it’s a
variable, it doesn’t exist

• (what we really mean is @dsl_object.color @dsl_object.green)


BAD OBJECT-ORIENTATION


BETTER OBJECT-ORIENTATION


STATIC DISPATCH


DYNAMIC DISPATCH


CONTEXT-SWITCHING


RUBY HAS DYNAMIC DISPATCH

• Ruby uses message-passing

• @dsl_object.send(“col
or green”)

• Hey, some guy named Irb
says “color green”


WRONG AGAIN!
• Hmmm, do I have a method
named “color green”? Nope

• Do my ancestors have a
method “color green”?
Nope

• Ooh! I’ll call
method_missing(“color
green”)

• @dsl_object.send(“col
or green”) #=> “color

CHANGING SCOPE
@dsl_object.instance_eval do
  # Everything in this block
  # will treat @dsl_object
  # as self

  color green
end

# This is equivalent
@dsl_object.instance_eval("color green")


DSL EVALUATION
turn the monitor background “#00FF00”
class DSLObject

  def color(*args)
    case args[0]
      when "green" then "#00FF00"
      when "blue" then "#0000FF"
      when "red" then "#FF0000"
    end
  end

    return *args.flatten.unshift(name.to_s)
  end
end

# @dsl_object.instance_eval("background color green") =>
["background","#00FF00"]


DSL EVALUATION
turn the monitor [“background”, “#00FF00”]
class DSLObject

  def color(*args)
    case args[0]
    end
  end

  end

end

# @dsl_object.instance_eval("monitor background color green") =>
["monitor","background","#00FF00"]


DSL EVALUATION
turn the [“monitor”,“background”, “#00FF00”]
class DSLObject

  def color(*args)
    case args[0]
    end
  end

  def the(*args)
    return *args
  end

  end

end

# @dsl_object.instance_eval("the monitor background color green") =>
["monitor","background","#00FF00"]


DSL EVALUATION
turn [“monitor”,“background”, “#00FF00”]

Now we’re in normal programming territory


CONTRIVED EXAMPLES

• Example: turn the beat around

• With our DSL #=> turn [“beat”,“around”]

• Valid syntax, bad semantics


SYNTAX LIMITATIONS

• Example: Set course for the Hoth system

• Can this be valid syntax?


SYNTAX LIMITATIONS

• Example: Set course for the Hoth system

• Set, Hoth => Constant undeﬁned

• for => reserved word

• system => already deﬁned (inherited from Object)


BLANK SLATE
class DSLObject < BasicObject

  end

  def const_missing(name)
    eval(name.to_s_downcase)
  end

end

# @dsl_object.instance_eval("Set course fer the Hoth system")
# => ["Set","course","fer","the","Hoth","system"]


TEXT ADVENTURE GAMES


DOCUMENTATION


ENVIRONMENTS ARE HARD
game = TextAdventureGame.new("My First Text Adventure")

hallway = Location.new("A dark and quiet hallway")
kitchen = Location.new("An abandoned kitchen")
hall = Location.new("A banquet hall...strangely
deserted")

game.rooms << hallway
game.rooms << kitchen
game.rooms << hall

hallway.north = kitchen
kitchen.south = hallway

kitchen.north = hall
hall.south = kitchen


TEXT ADVENTURE WITH BLOCKS
World.new("My First Text Adventure") do

  location "hallway" do
    description "A dark and quiet hallway"
    north "kitchen"
  end

  location "kitchen" do
    description "An abandoned kitchen"
    south "hallway"
  end

  start "hallway"

end


PYTHONY TEXT ADVENTURE

world "My First Text Adventure"

  location "hallway"
    description "A dark and quiet hallway"
    north "kitchen"

  location "kitchen"
    description "An abandoned kitchen"
    south "hallway"

  start "hallway"


INTERPRETATION


BLOCKS

• do..end

• Delayed execution

• Changes the execution scope

• Is a real argument


CONTEXTS

• instance_eval

• class_eval

• eval


WORLD
require 'game'

class World

  attr_accessor :name, :locations, :player

  def initialize(name,&block)
    @name = name
    @locations = {}
    @player = Player.new

    instance_eval &block
  end

  def location(name,&block)
    @locations[name] = Location.new(name,&block)
  end

  def start(location)
    @player.location = @locations[location]
  end

end


LOCATION
class Location

  attr_accessor :name, :description, :exits

  def initialize(name,&block)
    @name = name
    @exits = {}

    instance_eval &block
  end

  def description(prose)
    @description = prose
  end

  ["north","south","east","west"].each do |direction|
    class_eval <<-END
def #{direction}(location)
@exits["#{direction}"] = location
end
END
  end

end


PLAYER
class Player

attr_accessor :location

end


HANDLING USER INPUT

• Preprocess the input

• Provide a context to execute

• Manipulate the environment


PREPROCESS THE INPUT
require 'world'
require 'location'
require 'player'
require 'runner'

class Game

  def self.run(world)
    runner = Runner.new(world)
    puts "Welcome to the text adventure game!"
    print "> "
    until (input = gets.chomp) == "exit"
      runner.__execute(input.downcase)
      print "> "
    end
    puts "Thanks for playing"
  end

end


PROVIDE AN EXECUTION CONTEXT
class Runner < BasicObject

  attr_accessor :__world, :__handled

  def initialize(world)
    @__world = world
  end

  def __handle!
    @__handled = true
  end

  def __execute(string)
    @__handled = false
    instance_eval(string)
    __puts "I don't understand" unless @__handled
  end

  def __puts(message)
    $stdout.puts(message)
  end

  end

  def look(*args)
    __puts @__world.player.location.instance_eval(:@description)
    __handle!
  end

end

METAPROGRAMMING

• method_missing

• dynamically writing code

• evaluating code in different contexts


COMMENTS? QUESTIONS?

• Metaprogramming Ruby - Pragmatic Bookshelf

• kevin@kevingisi.com

• Thank you!


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