Re: [I18nrp] Mappings for IDNA2008 ?

[Andrew Sullivan <[e-mail]>]

I get your general point. My general point is that 2003 was not idempotent 
and therefore any backward compatible transition mechanism necessarily 
fails the same precondition, QED.

A
--
Andrew Sullivan
Please excuse my clumbsy thums.

On February 13, 2019 06:07:19 "Martin J. Dürst" <[e-mail]> wrote:

> Hello Andrew, others,
>
> On 2019/02/13 19:56, Andrew Sullivan wrote:
>> Surely the assumption fails because case folding and mapping of
>> characters I  IDNA2003 is not always idempotent -- the very thing 2008
>> set out to fix?
>
> If the general mapping is not idempotent, then indeed it has to be
> fixed. Given the purpose of the mapping, it seems obvious (at least in
> hindsight) that it should be idempotent.
>
> My general direction of arguing is that if things are reasonably
> designed (i.e. idempotent,...), then it makes a lot of sense to split
> the mapping into a general one and some locale-specific ones, and the
> locale-specific ones will go before the general one. Also, each
> locale-specific one should be rather small (the Turkish one, often
> brought as an example, essentially consists of mappings for 2 letters).
>
> I understand John's (Levine) point that locale-specific mapping data
> might not be available to some Python programmer writing some U-label ->
> A-label function or some such. I'm not very familiar with Python, but
> very familiar with Ruby, where it would be the same.
>
> But it wouldn't be too difficult to get the necessary data if one really
> wanted; it's not that this is something needing big bucks (as it seems
> to me Asmus is suggesting).
>
> Regards,   Martin.
>
>
>> A
>> --
>> Andrew Sullivan
>> Please excuse my clumbsy thums.
>>
>> On February 13, 2019 05:31:25 "Martin J. Dürst" <[e-mail]>
>> wrote:
>>
>>> Hello Patrik, others,
>>>
>>> On 2019/02/13 17:25, Patrik Fältström wrote:
>>>>
>>>>
>>>>> On 13 Feb 2019, at 09:19, Asmus Freytag (c) <[e-mail]>
>>>>> wrote:
>>>>>
>>>>>> On 2/12/2019 11:46 PM, Patrik Fältström wrote:
>>>>>>
>>>>>>> On 13 Feb 2019, at 08:18, Martin J. Dürst <[e-mail]>
>>>>>>> wrote:
>>>>>>>
>>>>>>> If f(x) is the generic mapping, and f'(x) is the mapping of the
>>>>>>> exceptions, then f(f'(x)), which applies f' *before* f, should do
>>>>>>> the job.
>>>>>> This works in some cases and not in other cases.
>>>>> Examples?
>>>>
>>>> This is math. :-)
>>>>
>>>> If f(x)=x’ and in the locale used f(x) should be x and not x’, you
>>>> have dependencies between f(x) and f’(x) if you want f(f’(x)) to be
>>>> x’ and f(f”(x)) to be x.
>>>
>>> Well, I have to admit that I didn't explicitly talk about that case in
>>> my 'proof'. For functions in general (e.g. a function such as f(x) =
>>> (x+7) mod 15), you are right.
>>>
>>> But f(x) (the general mapping function) is idempotent (i.e. f(x) =
>>> f(f(x)), which means that you can map as many times as you want, you
>>> always get the same result). This is also true for any locale-dependent
>>> mapping function. If it were not the case, human users would be very
>>> surprised: a maps to A, but then A maps to a: That wouldn't make any
>>> sense.
>>>
>>> Also, and even more important, for the general mapping function, if x is
>>> allowed in IDNs, then f(x) = x. This may not be true for a
>>> locale-dependent mapping function (i.e. it would be possible for a
>>> German mapping function to map (Hungarian) ȁ to ä, although that's not
>>> needed because German keyboards won't produce ȁ in the first place).
>>>
>>> So if f(x)=x', that would mean that x is not allowed in IDNs. That would
>>> mean that it is impossible that in a specific locale, the desired result
>>> of f(f'(x)) is x. It may be different from x', i.e. y, but we can easily
>>> get this by defining that f'(x) = y.
>>>
>>> So applying the (in most if not all cases very small) locale-specific
>>> mapping before the general mapping always will work.
>>>
>>> Regards,   Martin.
>
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