From: Philipp Meier <meier@...>
Date: 2002-06-30T02:10:15+09:00
Subject: Re: Mixin using append_features and class<<self do not work together


--Q68bSM7Ycu6FN28Q
Content-Type: text/plain; charset=iso-8859-1
Content-Disposition: inline
Content-Transfer-Encoding: quoted-printable

On Sat, Jun 22, 2002 at 04:23:24AM +0900, David Alan Black wrote:

> > I have a Mixin like this:
> >
> > Module B
> >   def B.append_features(aClass)
> >     def aClass.foo
> >       "foo"
> >     end
> >   end
> > end
> >
> > o =3D Object.new
> > o.instance_eval<<-"end_eval"
> >   include B
> > end_eval
> >
> > But o.foo does not exist!?
> >
> > Is there a way to use append_features together with class<<self ?
>=20
> See Bob's answer -- but also, I'm wondering whether there might be
> simpler ways to do what you want to do.

[...]

> Do you actually need #append_features for what you want to do?  (Not a
> rhetorical question -- it is at least as likely that I am missing the
> point as that my simplifications are appropriate :-)

To be honest I don't remember the exact reason why I uses
append_features beside it was the only thing that worked out.

But I will try to give you a overview of the things I'm doing. As I did
not found any persistance framework for ruby I'm currently developing
one for my own. One of the most important features to me is that the
Object that should persisted can be easily declared. It came to my mind
to use a way like the attr_read function: attr_persist. This is declared
in a Mixim Module which simply declares a function that call a
Persistors write_attr or read_attr method:

Code extract:

module Persist

  require 'singleton'

 class HashPersistor
   =20
    include Singleton

    include Enumerable

    @allvalues =3D {}

    def write(obj, attr, value)
      @allvalues =3D {} unless @allvalues !=3D nil
      @allvalues[obj] =3D {} unless @allvalues.has_key? obj
      @allvalues[obj][attr] =3D value
    end
   =20
    def read(obj, attr)
      @allvalues[obj][attr]
    end

    def each
      @allvalues.keys
    end
  end

  def Persist.append_features(includingClass)
    super
    def includingClass.persist(*ids)
      for id in ids
	ev=3D<<-"end_eval"
	  attr_accessor :persistor

	  @@persistor =3D Persist::HashPersistor.instance=20

	  def #{id.id2name}(*args, &block)
	    persistor.read(self, "#{id.id2name}")
	  end

	  def #{id.id2name}=3D(*args, &block)
	    persistor.write(self, "#{id.id2name}", *args)
  	  end

        end_eval
      end

      ev+=3D<<-"end_eval"
	def find_all
	    persistor.each.to_a
	  end
      end_eval
   =20
      module_eval ev
      end
    end

  end
end


I came to a problem when I'd implemented a dbiPersistor when I used to
restore an instance from the database. I need to declare all the
accessors to the persisted class fields at runtime. Sth. like the
following did not work:

o =3D Object.new
o.instance_eval<<-"env_eval"
class << self
	include Persist
end
end_val

attrs.each(|attr| o.instance_eval("persist :${attr}"));

I hope this gave you some insight in my motivation.

-billy.

--=20
Meisterbohne       S=F6flinger Stra=DFe 100          Tel: +49-731-399 499-0
   eL=F6sungen       89077 Ulm                     Fax: +49-731-399 499-9

--Q68bSM7Ycu6FN28Q
Content-Type: application/pgp-signature
Content-Disposition: inline

-----BEGIN PGP SIGNATURE-----
Version: GnuPG v1.0.7 (GNU/Linux)

iD8DBQE9HeqjfBriNoqItSYRAm66AKCU5WUwRwVaQsza34qMhaYqHlkX4gCfaTAm
vMeRsjzqUqfd5HT+lfm0MyI=
=xbSw
-----END PGP SIGNATURE-----

--Q68bSM7Ycu6FN28Q--