Forum: Ruby can I tighten this up with block or eval?

Announcement (2017-05-07): www.ruby-forum.com is now read-only since I unfortunately do not have the time to support and maintain the forum any more. Please see rubyonrails.org/community and ruby-lang.org/en/community for other Rails- und Ruby-related community platforms.
Fbb4d027695dfdf76bf448b15d7e306a?d=identicon&s=25 matt neuburg (Guest)
on 2009-03-15 18:21
(Received via mailing list)
I have here a class called ClassMaker whose greeter method returns an
instance of anonymous class containing a hello method:

class ClassMaker
  def initialize(s); @howdy = s; end
  def greeter
    c = Class.new do
      def initialize(s); @hello = s; end
      def hello
        puts @hello # this is the nub of the matter
      end
    end
    c.new(@howdy)
  end
end

ClassMaker.new("gday").greeter.hello

I would like the anonymous class's hello method to access whatever the
value of my ClassMaker instance's @howdy ivar may be at the moment
greeter is called. I am presently doing this, as you can see, by giving
the anonymous class an initialize method and passing the @howdy ivar's
value into an ivar of the anonymous class's instance.

This works but seems heavy-handed. I feel I should be able to do it
without the anonymous class having any initialize method at all. The
whole situation has "closure" written all over it. So I tried to define
the hello method to accomplish the same thing using evals and blocks and
other tricky stuff, and became hopelessly confused. Could someone
straighten me out? Thx - m.
E7559e558ececa67c40f452483b9ac8c?d=identicon&s=25 Gary Wright (Guest)
on 2009-03-15 18:49
(Received via mailing list)
On Mar 15, 2009, at 1:17 PM, matt neuburg wrote:
> This works but seems heavy-handed. I feel I should be able to do it
> without the anonymous class having any initialize method at all. The
> whole situation has "closure" written all over it. So I tried to
> define
> the hello method to accomplish the same thing using evals and blocks
> and
> other tricky stuff, and became hopelessly confused. Could someone
> straighten me out? Thx - m.

Here is one possibility.  Note, I changed ClassMaker to a module
since it is really just a factory for creating classes and doesn't
really need to create intermediate instances of ClassMaker to
accomplish your goal.

module ClassMaker
  def self.greeter(greeting)
    Class.new do
      define_method :hello do
        puts greeting           # this is the nub of the matter
      end
    end
  end
end

ClassMaker.greeter("gday").new.hello
ClassMaker.greeter("aloha").new.hello


Gary Wright
Fbb4d027695dfdf76bf448b15d7e306a?d=identicon&s=25 matt neuburg (Guest)
on 2009-03-15 19:31
(Received via mailing list)
Gary Wright <gwtmp01@mac.com> wrote:

> Here is one possibility.  Note, I changed ClassMaker to a module
>     end
>   end
> end
> ClassMaker.greeter("gday").new.hello

Actually, I have to keep the architecture I've got - greeter is going to
be called without a parameter, that's just how it is. (The problem as I
posed it is just a schematic reduction of a vast existing architecture
that does something else entirely.)

However, your solution shows me exactly the trick I was missing: I need
to turn @howdy into a local so that a block can see it:

class ClassMaker
  def initialize(s); @howdy = s; end
  def greeter
    greeting = @howdy # the magic step!
    Class.new do
      define_method :hello do
        puts greeting
      end
    end.new
  end
end
ClassMaker.new("gday").greeter.hello

Thanks! It's nice to stop banging myself over the head with a hammer...
:)

m.
This topic is locked and can not be replied to.