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comp.lang.ruby

Metaprogramming and module question

Pedro Del Gallego

3/6/2007 9:24:00 PM

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...

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 Gallego

Email : pedro.delgallego@gmail.com
3 Answers

Gavin Kistner

3/6/2007 9:33:00 PM

0

On Mar 6, 2:24 pm, "Pedro Del Gallego" <pedro.delgall...@gmail.com>
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/RubyMethodLook... 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!

Pedro Del Gallego

3/6/2007 9:58:00 PM

0

> See http://phrogz.net/RubyLibs/RubyMethodLook... for a visual
> explanation.

Good, i think i get it. Good example

--
-------------------------------------
Pedro Del Gallego

Email : pedro.delgallego@gmail.com

Rick DeNatale

3/6/2007 9:59:00 PM

0

On 3/6/07, Phrogz <gavin@refinery.com> wrote:
> On Mar 6, 2:24 pm, "Pedro Del Gallego" <pedro.delgall...@gmail.com>
> 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/RubyMethodLook... 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.denh...