xxxxxx@freenetproject.org wrote:
> On Mon, Jul 28, 2003 at 11:23:42AM -0700, Jim White wrote:
>
>>I have a second thought that addresses my concerns:
>>
>>collection-fold[-keys]-left and collection-fold[-keys]-right are not
>>specified with regard to the order of enumeration.   For unordered
>>collections, collection-fold-right the implementation may either raise
>>an error or provide an enumeration in reverse order or collection-fold-left.
>
> In other words, collection-fold-right is undefined on unordered
> collections.

It puts it in the same class as what the current document says about
(in)stability and enumeration with respect to mutation.

I think that collection-fold-right should be defined, but it is
permitted to raise an error if not supported.

>>collection-fold[-keys]-increasing and collection-fold[-keys]-decreasing
>>are specified to be an ordered enumeration and are only defined for
>>ordered collections.
>
> I like this proposal, but how does it address your enumeration stability
> concerns?

You raised the possibility that an enumeration might not even be stable
for a single enumeration.  I agree that that is a plausible scenario.
But it conflicts with this in the current document:

> Enumeration Stability
> ...
> Note that if a collection is purely functional, it will by
> definition be stable in enumeration, as the modified collection
> will be space-distinct from the enumerated collection.

The ones that especially have this problem are collection-fold-left and
collection-fold-right.  I wanted them to be defined as being reverses
(whenever reasonable).  But if we can't rely on an enumeration to
complete even without mutation (say because of side effects such a GC,
file i/o, or what-have-you), then clearly I can't expect that what I get
for two enumerations to be the same.

I do think that if an enumeration is begun, but cannot be continued for
any reason (say because of that GC which reordered things) that it must
raise an error (as you defined in the mutation/stability section).

Jim
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