From: Ruby Quiz <james@...>
Date: 2006-02-17T22:56:51+09:00
Subject: [QUIZ] metakoans.rb (#67)

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by ara.t.howard

[ Editors note:  This quiz file can be downloaded:

	http://rubyquiz.com/metakoans.rb

Partial solutions are welcome.  --JEG2 ]

	#
	# metakoans.rb is an arduous set of exercises designed to stretch
	# meta-programming muscle.  the focus is on a single method 'attribute' which
	# behaves much like the built-in 'attr', but whose properties require delving
	# deep into the depths of meta-ruby.  usage of the 'attribute' method follows
	# the general form of
	#   
	#   class C
	#     attribute 'a'
	#   end
	#   
	#   o = C::new
	#   o.a = 42  # setter - sets @a
	#   o.a       # getter - gets @a 
	#   o.a?      # query  - true if @a
	#   
	# but reaches much farther than the standard 'attr' method as you will see
	# shortly.
	#   
	# your path, should you choose to follow it, is to write a single file
	# 'knowledge.rb' implementing all functionality required by the koans below.
	# as a student of meta-programming your course will be guided by a guru whose
	# wisdom and pithy sayings will assist you on your journey.
	#   
	# a successful student will eventually be able to do this   
	#   
	#   harp:~ > ruby metakoans.rb knowledge.rb
	#   koan_1 has expanded your awareness
	#   koan_2 has expanded your awareness
	#   koan_3 has expanded your awareness
	#   koan_4 has expanded your awareness
	#   koan_5 has expanded your awareness
	#   koan_6 has expanded your awareness
	#   koan_7 has expanded your awareness
	#   koan_8 has expanded your awareness
	#   koan_9 has expanded your awareness
	#   mountains are again merely mountains
	#   
	
	
	module MetaKoans
	#
	# 'attribute' must provide getter, setter, and query to instances
	#
	  def koan_1
	    c = Class::new {
	      attribute 'a'
	    }
	
	    o = c::new
	
	    assert{ not o.a? }
	    assert{ o.a = 42 }
	    assert{ o.a == 42 }
	    assert{ o.a? }
	  end
	#
	# 'attribute' must provide getter, setter, and query to classes 
	#
	  def koan_2
	    c = Class::new {
	      class << self
	        attribute 'a'
	      end
	    }
	
	    assert{ not c.a? }
	    assert{ c.a = 42 }
	    assert{ c.a == 42 }
	    assert{ c.a? }
	  end
	#
	# 'attribute' must provide getter, setter, and query to modules at module
	# level
	#
	  def koan_3
	    m = Module::new {
	      class << self
	        attribute 'a'
	      end
	    }
	
	    assert{ not m.a? }
	    assert{ m.a = 42 }
	    assert{ m.a == 42 }
	    assert{ m.a? }
	  end
	#
	# 'attribute' must provide getter, setter, and query to modules which operate
	# correctly when they are included by or extend objects
	#
	  def koan_4
	    m = Module::new {
	      attribute 'a'
	    }
	
	    c = Class::new {
	      include m
	      extend m
	    }
	
	    o = c::new
	
	    assert{ not o.a? }
	    assert{ o.a = 42 }
	    assert{ o.a == 42 }
	    assert{ o.a? }
	
	    assert{ not c.a? }
	    assert{ c.a = 42 }
	    assert{ c.a == 42 }
	    assert{ c.a? }
	  end
	#
	# 'attribute' must provide getter, setter, and query to singleton objects 
	#
	  def koan_5
	    o = Object::new
	
	    class << o
	      attribute 'a'
	    end
	
	    assert{ not o.a? }
	    assert{ o.a = 42 }
	    assert{ o.a == 42 }
	    assert{ o.a? }
	  end
	#
	# 'attribute' must provide a method for providing a default value as hash
	#
	  def koan_6
	    c = Class::new {
	      attribute 'a' => 42
	    }
	
	    o = c::new
	
	    assert{ o.a == 42 }
	    assert{ o.a? }
	    assert{ (o.a = nil) == nil }
	    assert{ not o.a? }
	  end
	#
	# 'attribute' must provide a method for providing a default value as block
	# which is evaluated at instance level 
	#
	  def koan_7
	    c = Class::new {
	      attribute('a'){ fortytwo }
	      def fortytwo
	        42
	      end
	    }
	
	    o = c::new
	
	    assert{ o.a == 42 }
	    assert{ o.a? }
	    assert{ (o.a = nil) == nil }
	    assert{ not o.a? }
	  end
	#
	# 'attribute' must provide inheritance of default values at both class and
	# instance levels
	#
	  def koan_8
	    b = Class::new {
	      class << self
	        attribute 'a' => 42
	        attribute('b'){ a }
	      end
	      attribute 'a' => 42
	      attribute('b'){ a }
	    }
	
	    c = Class::new b
	
	    assert{ c.a == 42 }
	    assert{ c.a? }
	    assert{ (c.a = nil) == nil }
	    assert{ not c.a? }
	
	    o = c::new
	
	    assert{ o.a == 42 }
	    assert{ o.a? }
	    assert{ (o.a = nil) == nil }
	    assert{ not o.a? }
	  end
	#
	# into the void 
	#
	  def koan_9
	    b = Class::new {
	      class << self
	        attribute 'a' => 42
	        attribute('b'){ a }
	      end
	      include Module::new {
	        attribute 'a' => 42
	        attribute('b'){ a }
	      }
	    }
	
	    c = Class::new b
	
	    assert{ c.a == 42 }
	    assert{ c.a? }
	    assert{ c.a = 'forty-two' }
	    assert{ c.a == 'forty-two' }
	    assert{ b.a == 42 }
	
	    o = c::new
	
	    assert{ o.a == 42 }
	    assert{ o.a? }
	    assert{ (o.a = nil) == nil }
	    assert{ not o.a? }
	  end
	
	  def assert() 
	    bool = yield
	    abort "assert{ #{ caller.first[%r/^.*(?=:)/] } } #=> #{ bool.inspect }" unless bool 
	  end
	end
	
	
	class MetaStudent
	  def initialize knowledge
	    require knowledge
	  end
	  def ponder koan
	    begin
	      send koan
	      true
	    rescue => e
	      STDERR.puts %Q[#{ e.message } (#{ e.class })\n#{ e.backtrace.join 10.chr }]
	      false
	    end
	  end
	end
	
	
	class MetaGuru
	  require "singleton"
	  include Singleton
	
	  def enlighten student
	    student.extend MetaKoans
	
	    koans = student.methods.grep(%r/koan/).sort
	
	    attainment = nil
	
	    koans.each do |koan| 
	      awakened = student.ponder koan
	      if awakened
	        puts "#{ koan } has expanded your awareness"
	        attainment = koan
	      else
	        puts "#{ koan } still requires meditation"
	        break
	      end
	    end
	
	    puts(
	      case attainment
	        when nil 
	          "mountains are merely mountains"
	        when 'koan_1', 'koan_2'
	          "learn the rules so you know how to break them properly"
	        when 'koan_3', 'koan_4'
	          "remember that silence is sometimes the best answer"
	        when 'koan_5', 'koan_6'
	          "sleep is the best meditation"
	        when 'koan_7'
	          "when you lose, don't lose the lesson"
	        when 'koan_8'
	          "things are not what they appear to be: nor are they otherwise"
	        else
	          "mountains are again merely mountains"
	      end
	    )
	  end
	  def self::method_missing m, *a, &b
	    instance.send m, *a, &b
	  end
	end
	
	
	
	knowledge = ARGV.shift or abort "#{ $0 } knowledge.rb"
	student = MetaStudent::new knowledge 
	MetaGuru.enlighten student