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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
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