From: "Hal E. Fulton" <hal9000@...>
Date: 2003-03-24T14:10:54+09:00
Subject: Re: Iteration styles (was Ruby lecture slides)

----- Original Message ----- 
From: "Jim Weirich" <jweirich@one.net>
To: "ruby-talk ML" <ruby-talk@ruby-lang.org>
Sent: Sunday, March 23, 2003 10:52 PM
Subject: Iteration styles (was Ruby lecture slides)

[snip fascinating discussion]

Thanks for saying all this so clearly and concisely,
Jim. This should be captured somewhere.

I have often felt the need for an external iterator
in Ruby. (If I'm understand your terminology correctly.)
Sometimes I want to "grab" the next item or next n items
without tangling my logic. In that case, a traditional
while loop with a counter works about as well as anything
else. To me, being stuck using the built-in iterators for
structures such as arrays is analogous to using *only* the
iterators for files. Often I want to iterate through a file,
just barrelling through it like a locomotive. But sometimes
I want to grab a char or a line, or several of them, and
proceed with my logic. The same with an array or tree or
whatever. 

This was part of my motivation for my "super-iterator" that
I posted several months ago.

> This is possible in Ruby, but it requires that we be able to return from
> a function in such a way we can resume the function at the return point
> (exactly as generators do).  We don't have generators in Ruby, but we do
> have a very powerful, mind-bending capability called continuations. 
> Someday I would like to write up how do use continutations to do this,
> but this note is already too long.  Suffice it to say that we can use
> continuations to implement generator capability in about 20 lines of
> code.  

Does this remind anyone but me of Fermat's Last Theorem? :)

> So, if we need external iterators AND we have a container class that
> only provides internal iterators AND we don't want to duplicate the loop
> iteration logic, THEN we can use generators (implemented in
> continuations) to get the effect we want.

That's fascinating. Could this be generalized to the point that
given an object and its external iterator, we could just mix in
a module to give us an external iterator?

If so, I think this would be a worthwhile thing indeed. In fact,
though it might be only 20 lines long or so, I would say it belongs
in the RAA (if not in the core).
 
> So which way is better?  I find it is fascinating that both Ruby and
> Python have almost equivalent power in its iteration, they have just
> taken different paths to get there.

I still want to learn Python. But it is nowhere near my list of
the top twenty things I want to do.

Cheers,
Hal