From: Paul Brannan <pbrannan@...>
Date: 2003-03-26T00:34:45+09:00
Subject: Re: External iterators all nice and neat (was: Re: Iterate over two lists in parallel)

On Tue, Mar 25, 2003 at 05:43:11PM +0900, Julian Snitow wrote:
> Apparently for some reason "#{iterator.get}" instead of using a static 
> expands weirdly in that loop. (Ruby 1.6.8)
> 
> /me nibbles the brim of his fedora  (not quite hat-eating-worthy, but 
> still... it's good to be humbled every now and then... )
> 
> I'm wondering what's inside the string-expansion that changes the 
> behavior like that...

I think the problem is that you are returning directly from your
continution.

The first time get() is called, you create a continuation and store it
in @cont.  When you return from the continuation, you always return to
the place where get() was first called.  It's a little clearer if you
unroll your loop:

  class Array
    include Iteration
  end

  foo = [1, 2, 3]
  iterator = foo.iter(false)

  p iterator.get() #=> 1
  p iterator.get() #=> 2
                       3
                       nil
                       1

iterator.get() gets called, returns 1, then 1 gets printed.
iterator.get() is called on the next line, then is returned to the
previous line, and 2 is printed.  This continues until iteration is
complete and nil is returned.

I think this is why most other parallel iterators are implemented using
two continuations instead of one.

Paul