From: matt@... (matt neuburg)
Date: 2009-03-16T03:27:44+09:00
Subject: Re: can I tighten this up with block or eval?

Gary Wright <gwtmp01@mac.com> wrote:

> 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

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.

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