Metaprogramming and module question


#1

Hi

Yesterday , i’d made an example for fun, after read [1]. that let me
introduce pre o post behavior to an method with this easy sintax

class Foo
pre(:my_method) { #some_code}
end

but i ve a proble when i try to move the pre and post method to a
module. Can anyone explainme why the pre methods is undefined in the
class ?

thanks in advance.

[1] :
http://split-s.blogspot.com/2006/02/design-by-contract-for-ruby.html

My code :

Pre and Post method implemented in Song class

class Song
attr_accessor :title
def play
puts “#{@title}”
end
end

class Song
def self.pre (name_method, &condition)
old_method = instance_method(name_method)

define_method(name_method)  { |*args|
   condition.call
   old_method.bind(self).call(*args)
}

end

def self.post (name_method, &condition)
old_method = instance_method(name_method)

define_method(name_method)  { |*args|
   condition.call
}

end
end

class Song
pre (:play ) { puts “Playing …” }
end

song = Song.new
song.title= “Hola song”
song.play


Als module implementation : not work undefined method `pre’ for
Song:Class (NoMethodError)

class Song
attr_accessor :title
def play
puts “#{@title}”
end
end

module MetaTest

def self.included(mod)
puts “and now MetaTest has been used by #{mod.inspect}…”
end

def self.pre (name_method, &condition)
old_method = instance_method(name_method)

define_method(name_method)  { |*args|
   condition.call
   old_method.bind(self).call(*args)
}

end

def self.post (name_method, &condition)
old_method = instance_method(name_method)

define_method(name_method)  { |*args|
   condition.call
}

end
end

class Song
include MetaTest
end

class Song
pre (:play ) { puts “Playing …” }
end

song = Song.new
song.title= “Hola caracola”
song.play


Pedro Del G.

Email : removed_email_address@domain.invalid


#2

On Mar 6, 2:24 pm, “Pedro Del G.” removed_email_address@domain.invalid
wrote:

but i ve a proble when i try to move the pre and post method to a
module. Can anyone explainme why the pre methods is undefined in the
class ?
[snip]

Including a Module into a Class only links in the ‘instance’ methods
of that module as instance methods of the class. Module methods (def
self.foo) are not inherited as ‘class’ methods.

See http://phrogz.net/RubyLibs/RubyMethodLookupFlow.png for a visual
explanation.

The easiest thing to do is make those methods instance methods of the
module, and extend your Song class with them:

module Foo
def bar; puts “#bar called on #{self}”; end
end

class Cat
include Foo
end

class Dog
extend Foo
end

Cat.bar rescue puts “No Cat.bar method!”
Cat.new.bar rescue puts “No Cat#bar method!”
Dog.bar rescue puts “No Dog.bar method!”
Dog.new.bar rescue puts “No Dog#bar method!”

#=> No Cat.bar method!
#=> #bar called on #Cat:0x28347b0
#=> #bar called on Dog
#=> No Dog#bar method!


#3

See http://phrogz.net/RubyLibs/RubyMethodLookupFlow.png for a visual
explanation.

Good, i think i get it. Good example

Pedro Del G.

Email : removed_email_address@domain.invalid


#4

On 3/6/07, Phrogz removed_email_address@domain.invalid wrote:

See http://phrogz.net/RubyLibs/RubyMethodLookupFlow.png for a visual
explanation.

The easiest thing to do is make those methods instance methods of the
module, and extend your Song class with them

And a good way to do this automatically and make including the
MetaTest module do what the OP expects is something like:

module MetaTest

def self.included(mod)
puts “and now MetaTest has been used by #{mod.inspect}…”
mod.extend(ClassMethods)
end

module ClassMethods

def pre (name_method, &condition)
  old_method = instance_method(name_method)

  define_method(name_method)  { |*args|

condition.call
old_method.bind(self).call(*args)
}
end

def post (name_method, &condition)
  old_method = instance_method(name_method)

  define_method(name_method)  { |*args|

condition.call
}
end
end
end


Rick DeNatale

My blog on Ruby
http://talklikeaduck.denhaven2.com/