From: Julian Snitow <vangczung@...>
Date: 2003-03-25T17:02:19+09:00
Subject: External iterators all nice and neat (was: Re: Iterate over two lists in parallel)

> 
> Sprry to reply to my own post, but another
> small issue is that the generator always
> returns nil after it exhausts the collection.
> 
> But what if a collection contains nil values?
> 
> I think I'd like an 'end?' method or something.
> At worst, we could define a meta-nil value, but
> I'm betting that's not necessary.
> 
> Hal
> 
>

I don't know about generators, but for traversing a collection, my 
iterator externalizer class (which I posted not 15 minutes ago ;-) can 
handle this with trivial changes.  Here it is, ready to eat, just add 
water, etc.  You will not find a nicer iterator class, or I will 
self.eat(@hat)  :-)

######## put this in 'iteration.rb' or something like that ###
module Iteration
   class ExternalIterator
     def initialize(collection, deathThroes=false)
       @collection = collection
       @cont = nil
       @deathThroes = deathThroes
     end
     def get
       @cont.call if @cont
       @collection.each { |elem|
         callcc { |@cont|
           return elem
         }
       }
       @cont = nil  # reset the continuation
       return nil unless @deathThroes
       throw :iter_done
     end
   end
   def iter(deathThroes=false)
     ExternalIterator.new(self, deathThroes)
   end
end
#####################################

And here's an example of it in use:

######### foo.rb ####################

require 'iteration.rb'

class FooCollectionClass  # anything with a FooClass#each method
   include Iteration
end

myFoo = FooCollectionClass.new

iterator = myFoo.iter(true)

catch(:iter_done)
   while(true)
     puts "The next element is: #{iterator.get}"
   end
end

####################################

There.  Now Ruby has external iterators.  ;-)

Julian