samnang
9/29/2011 - 9:29 AM
metakoans.rb (#67)
metakoans.rb (#67)
#
# 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