From: Jim Weirich <jweirich@...>
Date: 2003-03-25T15:57:54+09:00
Subject: Re: Iterate over two lists in parallel

On Tue, 2003-03-25 at 00:25, Hal E. Fulton wrote:

> I've been playing with this for about an hour, 
> and I've only managed to convince myself that
> I don't actually understand it.
> 
> What I tried to do was encapsulate this 
> functionality in a module called Gen, so
> that I could, for instance, do this:
> 
>   class Array
>     include Gen
>   end
> 
>   x = [5, 6, 7, 8, 9]
> 
>   loop do
>     a = x.get
>     break if x.nil?
>     p a
>   end
> 
> Yes, I know the loop is trivial and an internal
> iterator would suffice here. The point is that
> I can't figure out this simple task. :(
> 
> Can you illuminate?

I don't know what you have in Gen, but I would expect it to work
something like this ...

   x = [5,6,7,8,9]
   it = x.iterator
   loop do
     a = it.get
     break if a.nil?
     p a
   end

A generator is really just a fancy external iterator.  So you need an
operation to create it (hence the iterator call).  You then use the
iterator to, ummm, iterator; not the array itself.

The iterator method will probably look something like this ....

  def iterator
    Generator.new { |g| each { |it| g.generate(it) } }
  end

That's the only thing you would need to mixin.  Generator can still
stand alone.

Does that help?

-- 
-- Jim Weirich     jweirich@one.net    http://w3.one.net/~jweirich
---------------------------------------------------------------------
"Beware of bugs in the above code; I have only proved it correct, 
not tried it." -- Donald Knuth (in a memo to Peter van Emde Boas)