From: Erik Veenstra <google@...>
Date: 2006-01-26T03:03:14+09:00
Subject: Re: LazyLoad

> Nuking data.txt is probably not the right response to a
> missing mapping.txt...

I got it. I introduced LazyException (see below) as a wrapper
for the original Exception. All methods are (once again)
delegated to the original exception with method_missing, except
for LazyException#exception.

(By the way, in your example, you use an exception to do some
business logic: "If the file doesn't exist, create it". In OO
theory, this isn't the right way to go...)

What more?

gegroet,
Erik V. - http://www.erikveen.dds.nl/

----------------------------------------------------------------

 class LazyException < Exception
   superclass.instance_methods(false).each do |method|
     undef_method(method)      unless method =~ /^__/
   end

   def initialize(exception)
     @original_exception       = exception
   end

   def exception
     self
   end

   def method_missing(method_name, *parms, &block)
     @original_exception.send(method_name, *parms, &block)
   end
 end

 class Lazy
   instance_methods(true).each do |method|
     undef_method(method)      unless method =~ /^__/
   end

   def initialize(mutex=Mutex.new, &block)
     @mutex            = mutex
     @block            = block
     @evaluated        = false
     @exception        = nil
     @real_object      = nil
   end

   def method_missing(method_name, *parms, &block)
     @mutex.synchronize do
       begin
         @real_object  = @block.call() unless @evaluated
       rescue Exception => e
         @exception    = LazyException.new(e)
       ensure
         @evaluated    = true
       end
     end

     raise @exception  if @exception

     @real_object.send(method_name, *parms, &block)
   end
 end

 module Kernel
   def lazy(*parms, &block)
     Lazy.new(*parms, &block)
   end
 end

----------------------------------------------------------------