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def call_block(&block)  # NOTE: block is actually an instance of Proc
  block.call 1, 2
end
call_block { |a,b| puts "Hello #{a} and #{b}" } # "Hello 1 and 2"

def yield_params
  yield 1, 2
end
yield_params { |a,b| puts "#{a} and #{b}" } # "1 and 2"

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class MyClass
  def self.class_method # creates a class method. Always public
    "class_method"
  end      
  def method1     # default is 'public'
    "method1"
  end
  protected       # subsequent methods will be 'protected'
    def method2   # will be 'protected'
      "method2"

yangsu

class MyClass
  # define method 1 through 4 without and encapulation directives ...
  
  public    :method1, :method4
  protected :method2
  private   :method3
end

a = MyClass.new

yangsu

class Person
  attr :age
  attr_reader :age
  # gets translated into:
  def age
    @age
  end

  attr_writer :age
  # gets translated into:

yangsu

class Base
  def initialize()
    @x = 10
  end
end
d = Base.new
puts d.x # => undefined method `x' for ... (NoMethodError)
puts d.instance_variable_get :@x # => 10

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module T
  @@foo = 'bar'

  def self.set(x)
    @@foo = x
  end

  def self.get
    @@foo
  end

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class Base
  def initialize()
    @x = 10
  end
end

class Derived < Base
  def x
    @x = 20
  end

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module ToFile
  def filename
    "object_#{self.object_id}.txt"
  end
  def to_f
    File.open(filename, 'w') {|f| f.write(to_s)}
  end
end
class Person
  include ToFile

yangsu

'begin' <=> 'end' # => -1
'same' <=> 'same' # => 0

yangsu

class SizeMatters
  include Comparable
  attr :str
  def <=>(anOther)
    str.size <=> anOther.str.size
  end
  def initialize(str)
    @str = str
  end
  def inspect