On Mon, Jul 28, 2003 at 08:08:57AM -0700, Jim White wrote:
> I'm thinking that unordered collections are not quite fully specified.
>
> A key question is whether collection-fold-left will return the elements
> of the collection in the same (unspecified) order or not when used
> multiple times.

I've actually just added some language in the document about that.

>
> It seems to me that either unordered collections should either always
> enumerate in the same order as long as they are not modified, or there
> needs to be some way to distinguish unordered collections that make no
> representation about repeating the enumeration order.

Devils advocate may argue that garbage collection should be allowed to
compact a collection in a way that may affect its order.

>
> And if an unordered collection *will* preserve the enumeration order
> (which would be typical for many straightforward implementations) then
> collection-fold-right should be defined to enumerate in the reverse of
> that same unspecified (but invariant-while-not-modified) order.
>
That I disagree with completely.  Just because the order is unchanging
should not necessarily mean that a right fold is possible.  Take for
example a collection of the natural numbers.  Its unchanging from left
fold to left fold, but a right fold is completely impossible.  In less
dramatic cases, such as for long Scheme lists, a right fold may be
possible, but very inefficient.

	Scott