From: Robert Dober <robert.dober@...>
Date: 2007-08-24T20:23:05+09:00
Subject: Re: Dynamic methods

On 8/24/07, Marcin Miel�y�ski <lopx@gazeta.pl> wrote:> Trans wrote:>> > Also, keep in mind Ruby classes are open. You can add real methods to> > them on the fly.> >> >   class Example> >   end> >> >   foo = "bar"> >> >   Example.class_eval %{> >     def #{foo}> >       "#{foo}"> >     end> >   }> >>> It's better not to use string evals:>> Example.class_eval do>      define_method foo do>          "#{foo}">      end> end>> lopex
You are right for sure here Marcin, however are you aware that the twoare not exactly the same? In case you get bitten by define_methodmaybe Ara's recent post about instance_eval{ def ... } might behelpful,the following is a "String-eval-free" way to define a method on aclass *without* being exposed to the closure of the currentenvironment.----------------------- 8< ----------------------
value = 222 ## evil closure variable shadowing A#value
class A  class << self    def value; 1120 end # just to show that Ara's method works  end  def value; 101010 end # the value we really wantend
A.class_eval %{  def tom; value end}  ## works but we use the threaded String eval :(
A.class_eval do  define_method :marcin do value end # value is the closure of course :(end ## nice, but does not work :(
class A  class << self    alias_method :old_new, :new    def new *args, &block      o = old_new( *args, &block )      o.instance_eval do        def ara; value end      end      o    end  endend ## this does the trick, simple concise, trivial :( well we wish it were.## and there are issues with the alias it is a potential redefeinition disaster

a = A.newp [:tom, a.tom]p [:marcin, a.marcin]p [:ara, a.ara]

CheersRobert
-- I'm an atheist and that's it. I believe there's nothing we can knowexcept that we should be kind to each other and do what we can forother people.-- Katharine Hepburn