[SG16] Draft proposal: Clarify guidance for use of a BOM as a UTF-8 encoding signature

[Shawn Steele]

I’m having trouble with the attempt to be this prescriptive.

These make sense:  “Use Unicode!”

  *   If possible, mandate use of UTF-8 without a BOM; diagnose the presence of a BOM in consumed text as an error, and produce text without a BOM.
  *   Alternatively, swallow the BOM if present.
After that the situation is clearly hopeless.  Applications should Use Unicode, eg: UTF-8, and clearly there are cases happening where that isn’t happening.  Trying to prescribe that negotiation should therefore happen, or that BOMs should be interpreted or whatever is fairly meaningless at that point.  Given that the higher-order guidance of “Use Unicode” has already been ignored, at this point it’s garbage-in, garbage-out.  Clearly the app/whatever is ignoring the “use unicode” guidance for some legacy reason.  If they could adapt, it should be to use UTF-8.   It *might* be helpful to say something about a BOM likely indicating UTF-8 text in otherwise unspecified data, but prescriptive stuff is pointless, it’s legacy stuff that behaves in a legacy fashion for a reason and saying they should have done it differently 20 years ago isn’t going to help 😊
-Shawn

From: Unicode <unicode-bounces at unicode.org> On Behalf Of Tom Honermann via Unicode
Sent: Monday, October 12, 2020 7:03 AM
To: Alisdair Meredith <alisdairm at me.com>
Cc: sg16 at lists.isocpp.org; Unicode List <unicode at unicode.org>
Subject: Re: [SG16] Draft proposal: Clarify guidance for use of a BOM as a UTF-8 encoding signature

Great, here is the change I'm making to address this:
Protocol designers:

  *   If possible, mandate use of UTF-8 without a BOM; diagnose the presence of a BOM in consumed text as an error, and produce text without a BOM.
  *   Otherwise, if possible, mandate use of UTF-8 with or without a BOM; accept and discard a BOM in consumed text, and produce text without a BOM.
  *   Otherwise, if possible, use UTF-8 as the default encoding with use of other encodings negotiated using information other than a BOM; accept and discard a BOM in consumed text, and produce text without a BOM.
  *   Otherwise, require the presence of a BOM to differentiate UTF-8 encoded text in both consumed and produced text unless the absence of a BOM would result in the text being interpreted as an ASCII-based encoding and the UTF-8 text contains no non-ASCII characters (the exception is intended to avoid the addition of a BOM to ASCII text thus rendering such text as non-ASCII). This approach should be reserved for scenarios in which UTF-8 cannot be adopted as a default due to backward compatibility concerns.
Tom.

On 10/12/20 8:40 AM, Alisdair Meredith wrote:
That addresses my main concern.  Essentially, best practice (for UTF-8) would be no BOM unless the document contains code points that require multiple code units to express.

AlisdairM


On Oct 11, 2020, at 23:22, Tom Honermann <tom at honermann.net<mailto:tom at honermann.net>> wrote:

On 10/10/20 7:58 PM, Alisdair Meredith via SG16 wrote:
One concern I have, that might lead into rationale for the current discouragement,
is that I would hate to see a best practice that pushes a BOM into ASCII files.
One of the nice properties of UTF-8 is that a valid ASCII file (still very common) is
also a valid UTF-8 file.  Changing best practice would encourage updating those
files to be no longer ASCII.
Thanks, Alisdair.  I think that concern is implicitly addressed by the suggested resolutions, but perhaps that can be made more clear.  One possibility would be to modify the "protocol designer" guidelines to address the case where a protocol's default encoding is ASCII based and to specify that a BOM is only required for UTF-8 text that contains non-ASCII characters.  Would that be helpful?
Tom.

AlisdairM


On Oct 10, 2020, at 14:54, Tom Honermann via SG16 <sg16 at lists.isocpp.org<mailto:sg16 at lists.isocpp.org>> wrote:

Attached is a draft proposal for the Unicode standard that intends to clarify the current recommendation regarding use of a BOM in UTF-8 text.  This is follow up to discussion on the Unicode mailing list<https://corp.unicode.org/pipermail/unicode/2020-June/008713.html> back in June.
Feedback is welcome.  I plan to submit<https://www.unicode.org/pending/docsubmit.html> this to the UTC in a week or so pending review feedback.
Tom.
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