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# ==============================
# Spell: Class Instance Variable 
# ==============================

# Store class-level state in an instance variable of the Class object.

class C
  @my_class_instance_variable = "some value"

  def self.class_attribute

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# ==================
# Spell: Class Macro
# ==================

# Use a class method in a class definition.

class C; end

class << C
  def my_macro(arg)

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# =================
# Spell: Clean Room
# =================

# Use an object as an environment in which to evaluate a block.

class CleanRoom
  def a_useful_method(x); x * 2; end
end

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# =====================
# Spell: String of Code
# =====================

# Evaluate a string of Ruby code.

my_string_of_code = "1 + 1"
eval(my_string_of_code) # => 2

# For more information: http://www.pragprog.com/titles/ppmetr/metaprogramming-ruby

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# =====================
# Spell: Code Processor
# =====================

# Process Strings of Code (http://gist.github.com/535047) from an external source.

File.readlines("file_containing_lines_of_ruby.txt").each do |line|
  puts "#{line.chomp} ==> #{eval(line)}"
end

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# ====================
# Spell: Context Probe
# ====================

# Execute a block to access information in an object’s context.

class C
  def initialize
    @x = "a private instance variable"
  end

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# ==========================
# Spell: Deferred Evaluation
# ==========================

# Store a piece of code and its context in a proc or lambda for evaluation later.

class C
  def store(&block)
    @my_code_capsule = block
  end

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# =======================
# Spell: Dynamic Dispatch
# =======================

# Decide which method to call at runtime.

method_to_call = :reverse
obj = "abc"

obj.send(method_to_call) # => "cba"

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# =====================
# Spell: Dynamic Method
# =====================

# Decide how to define a method at runtime.

class C
end

C.class_eval do

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# ====================
# Spell: Dynamic Proxy
# ====================

# Forward to another object any messages that don’t match a method.

class MyDynamicProxy
  def initialize(target)
    @target = target
  end