metakoans.rb

#
# 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