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Re: Perlfunc for each(), keys(), values() has been changed

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From:
demerphq
Date:
March 29, 2013 17:26
Subject:
Re: Perlfunc for each(), keys(), values() has been changed
Message ID:
CANgJU+XLyEggwPA+TZPs3K2VkK5ZBO4kzKFOAMjHCPbOpKUWjg@mail.gmail.com
On 29 March 2013 18:06, Aristotle Pagaltzis <pagaltzis@gmx.de> wrote:
> * demerphq <demerphq@gmail.com> [2013-03-29 12:40]:
>> Each of the three functions entry in perlfunc now has the following
>> paragraph:
>>
>> +Hash entries are returned in an apparently random order. The actual
>> +random order is specific to a given hash; the exact same series of
>> +operations on two hashes may result in a different order for each
>> +hash. Any insertion into the hash may change the order, as will any
>> +deletion, with the exception that the most recent key returned by
>> +C<each> or C<keys> may be deleted without changing the order. So long
>> +as a given hash is unmodified you may rely on C<keys>, C<values> and
>> +C<each> to repeatedly return the same order as each other. See
>> +L<perlsec/"Algorithmic Complexity Attacks"> for details on why hash
>> +order is randomized. Aside from the guarantees provided here the
>> +exact details of Perl's hash algorithm and the hash traversal order
>> +are subject to change in any release of Perl.
>>
>> I'll postpone fixing it for awhile to see if there are any further
>> changes recommended by the list.
>
> IMO this mentions too many specifics of the current implementation’s
> behaviour – it would require revision the next time the hashing changes.

Such as? (Honest question.)

> But the specific behaviour now is really quite irrelevant. (Even more so
> is defending why it got to be that way; IMO that merits mention only as
> gory details for the curious.) The point of interest to a programmer
> consulting the docs is guidance on what to *do*: how to write correct
> code; not background on what Perl does and why. IMO that is the emphasis
> with which it should be written.

No arguments here...

> So how about this? Or something along these lines.
>
>     Entries in a hash have a repeatable but arbitrary order. Therefore
>     the order in which they are returned from C<keys>, C<values> and
>     C<each> will match. Adding or deleting any keys changes this order
>     arbitrarily and irreversibly, and you should expect the same set of
>     keys in different hashes (or even in the same hash after adding and
>     deleting keys) to yield a different order of entries.

I am inclined to say that should read:

"and you should NOT expect the same set of keys in different hashes (or even
in the same hash after adding or deleting keys) to yield the same
order of entries."

IOW, move the NOT from the end to the start....

>  A special
>     exception to this is made to allow you to safely delete keys from
>     a hash while iterating over it: the entry most recently returned
>     from C<each> or C<keys> can be deleted without changing the order of
>     the other entries. Any and all other assumptions about hash ordering
>     will make your code incorrect, even when it seems to work. (The
>     exact likelihood of this depends on the implementation of hashes,
>     which may differ between different versions of Perl.

I feel like we should make it really clear that this includes *any*
version of perl, not just major versions.

> See also
>     L<perlsec/"Algorithmic Complexity Attacks">.)

While I honestly don't think it really says anything different I do
feel yours says it more artfully and with the above points taken into
account I would be happy to see your wording replace mine.

Thanks a lot!

Also I was wondering if this paragraph shouldn't end up living in its
own pod file somewhere, and be linked to instead of repeated in
perlfunc.... Especially if we add more text to it like examples or
whatever.

cheers,
Yves


-- 
perl -Mre=debug -e "/just|another|perl|hacker/"

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