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On Mon, Apr 28, 2003 at 10:15:39PM -0700, Per Bothner wrote: > The iterator model is unclear to me. > First, the name "iterator" may not be the best choice. > When I use an "iterator" to iterate through a sequence, > in most languages I change the location of the iterator, > but the identity of the iterator object does not change. > In the proposed API, iterators themselvs are immutable, > and you move to a new position by getting a new iterator. > This is reasonable for a (mostly-)functional language > like Scheme, but perhaps "position" would be a better name? Well, they are still iterators, the difference is that instead of an iterator object which is mutated as in other languages, we behave functionally, returning a new state object from iterator-next. > > My conclusion is the model is that there are N+2 iterators for > a sequence of N elements. I guess this could be made to work, > but it is different from what I'd expect. (Input ports follow > the N+1 model.) I think there needs to be some motivation for > this model, and a little exploration of the consequences. There is a motivation for this model. It creates symmetry in the case that a collection supports reversing the iteration direction. A program written to walk backwards from within an iterator will have the same appearance as one which walks forwards: (if (not (iterator-at-end? i)) (begin (do-something) (loop (iterator-next i)))) and (if (not (iterator-at-start? i)) (begin (do-something) (loop (iterator-previous i)))) There are a few other reasons for this model, which I will try to remember when I leave work. Scott
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