Solutions to Metaprogramming In Ruby

Exercise 1

$results = []

$results << class A
  def initialize
    @a = 11
    @@a = 22
    a = 33
  end
  @a = 1
  @@a = 2
  a = 3
end

$results << A.instance_variable_get(:@a)
$results << A.class_variable_get(:@@a)

class B < A
  def initialize
    $results << super
  end
end


A.new
$results <<  A.new.instance_variable_get(:@a)
$results <<  A.class_variable_get(:@@a)

B.new


puts $results.sort!

Exercise 2

class I
  def like arg
    puts "%s %s %s" % [self.class, self.class.instance_methods(false), arg]
  end
end

c, m, arg = gets.split  # input 'I like metaprogramming.'

# write your code here to display 'I like metaprogramming.'
$old_stdout = $stdout
class StdOut
  def puts(string)
    ["[", "]", ":"].each do |bad|
      string.gsub!(bad, '')
    end
    $old_stdout.puts string.to_s
  end

  def write

  end
end
$stdout = StdOut.new


Object.const_get(c).new.send(m,arg)

Exercise 3

animal = 'dog'

# define singleton speak() method
def animal.speak
  puts 'Dog says Woof!'
end

animal.speak  #=> 'Dog says Woof!'

Exercise 4

the_binding = class A
  @@a = 1
  @a = 2
  a = 3

  binding

end

p eval('[@@a, @a, a]', the_binding)   # Replace '*****' to your code

Exercise 5

MaskedString = Class.new(String)

MaskedString.class_eval do
  define_method(:tr_vowel) do
    tr 'aeiou', '*'
  end

  define_singleton_method(:tr_vowel) do |str|
    str.tr 'aeiou', '*'
  end
end

s = MaskedString.new('hello')
p s.tr_vowel

p MaskedString.tr_vowel('goodbye')

Exercise 6

class Dog
  MSGS = {:dance => 'is dancing', :poo => 'is a smelly doggy!', :laugh => 'finds this hilarious!'}

  def initialize(name)
    @name = name
  end

  def method_missing(method)
    "#{@name.capitalize} doesn't know #{method}"
  end

  def can(name, &block)
    if block.nil?
      block = -> { "#{@name} #{MSGS[name]}" }
    end
    self.define_singleton_method(name, &block)
  end
end

lassie = Dog.new('Lassie')
lassie.can(:dance)
lassie.can(:poo)
lassie.can(:sing) { "#{@name} can sing!"}
stimpy = Dog.new('Stimpy')
stimpy.can(:dance)
stimpy.can(:cry){"#{@name} cried AHHHH"}
fido = Dog.new('Fido')
fido.can(:poo)

p lassie.dance
p lassie.poo
p lassie.laugh
p lassie.sing
puts
p fido.dance
p fido.poo
p fido.laugh
puts
p stimpy.dance
p stimpy.poo
p stimpy.laugh
p stimpy.cry