From: Julian Snitow <vangczung@...>
Date: 2003-03-25T15:42:02+09:00
Subject: Re: Iterate over two lists in parallel

Jim Weirich wrote:
> 
 > [snip!]
> 
> Here's my take on this ... you can find an HTML version (that's a bit
> more readable) at http://w3.one.net/~jweirich/talks/same_fringe/
>
 > [snippety!]

Ooh...  That is chock full of heady goodness! :-D

*throws out my admittedly much-inferior prototypical generator class*

Here's something else I've been poking around with.  It's a generic 
external iterator for any collection that supports the :each method.

#!/usr/bin/ruby -w

class ExternalIterator
   def initialize(collection)
     @collection = collection
     @cont = nil
   end
   def get
     @cont.call if @cont
     @collection.each { |elem|
       callcc { |@cont|
         return elem
       }
     }
     @cont = nil  # reset the continuation and return nil
   end
end

ex = ExternalIterator.new( ["foo", "bar", "baz", "qux", "florp", "zarg", 
"narg"] )


while(x = ex.get)
   puts x
end

puts "-----"
puts ex.get

##################################
# Here's the output:
foo
bar
baz
qux
florp
zarg
narg
-----
foo
###################################

Since it uses "each" to generate values, once you've gone through all 
the values (and the final 'nil'), you can use the same iterator object 
to iterate through the collection again.

This seems a lot less "magical" than your uber-cool Generator class, but 
with the ability to turn :each "inside out", can anyone think of some 
problems that still require generators?

Also, it just occured to me that it might be more Rubylike to have a 
mixin method like

module Iteration
   class ExternalIterator
      # class definition same as above
   end

   def iter
	ExternalIterator.new(self)
   end
end

Then you'd just do something like

class Array
   include Iteration
end

ex = ["foo", "bar", "baz", "qux", "florp", "bzaa", "worble"].iter

# then call ex.get whenever you want the next value
###################################