Re: [idn] Figuring out what can be displayed

["Adam M. Costello" <idn.amc+0@nicemice.net.RemoveThisWord>]

John C Klensin <klensin@jck.com> wrote:

> > The spec says that an application SHOULD NOT show the ACE form if it
> > can correctly display the Unicode form, and it SHOULD show the ACE
> > form if it cannot correctly display the Unicode form.
>
> So, an application must, ultimately, either decide that its operating
> environment can handle Unicode acceptably...
>
> Or the application must decide that it can't know

Those are not the symmetric choices.

> and therefore must display the ACE form always...

That does not follow from not knowing.

The cases are:

 1. The application can determine whether the environment can handle the
    non-ACE form.  In this case, exactly one of the two recommendations
    applies, and it's perfectly clear what to do.

 2. The application cannot know.  It is then faced with the following
    two symmetric choices:

    2a. Optimistically try to display the non-ACE form, which might
        or might not work.

    2b. Pessimistically display the ACE form, which is guaranteed to
        look like garbage to humans and guaranteed to be okay for
        software.

> If, in order to be fully-conformant, a specification requires an
> implementation to make a decision based on something it can't know,

The spec says "SHOULD".  RFC 2119 says "there may exist valid
reasons in particular circumstances to ignore" a SHOULD.  I think
the unavailability of sufficient information about the environment
to determine whether the recommendation applies is a particular
circumstance that qualifies as a valid reason to ignore the SHOULD.

> that is, IMO (in the language of RFC 2026) a "technical omission", and
> one of fairly serious proportions.

It's an omission if something has been omitted.  What has been omitted?

> If we cannot provide a better model, the text cannot, I think, say
> anything stronger than that the application MAY show either the ACE or
> the Unicode form, at its discretion, and leave this issue up to user
> pressure and implementation quality.

We can do better than that.  When the application can determine whether
the non-ACE form can be displayed, we don't need to leave it up to the
application's discretion, we can prescribe the proper choice.

AMC