Re: Empty lists in Structured Headers (#781)

[Julian Reschke <[e-mail]>]

On 02.05.2019 08:28, Mark Nottingham wrote:
> On 2 May 2019, at 4:24 pm, Julian Reschke <[e-mail]> wrote:
>>
>> In
>> <https://greenbytes.de/tech/webdav/draft-ietf-httpbis-header-structure-10.html#specify>,
>> we currently say:
>>
>> "Specify the header field’s allowed syntax for values, in terms of the
>> types described in Section 3, along with their associated semantics.
>> Syntax definitions are encouraged to use the ABNF rules beginning with
>> “sh-“ defined in this specification."
>>
>> Does this mean, that a definition like
>>
>>   MyField = [ sh-list ]
>>
>> is an acceptable use of the syntax? (see
>> <https://github.com/httpwg/http-extensions/issues/781#issue-426418064>).
>
> I think it's a *possible* use of the syntax; however, you'd need to accompany it with some prose that directed the parser what to do when SH parsing fails on an empty value. SH pretty strongly steers people away form doing that, so if by "acceptable" you mean "recommended", I think no.
> ...

The simplest possible processing model likely would be to parse as
sh-list, and then, upon failure, to inspect the value (after stripping
whitespace it should be empty or only contain commas).

> Happy to clarify the text you quote above to make that more clear (we probably need to take another pass at the author recommendations anyway).
 > ...

The outcome I'd like to see is: yes, it can be used that way (with care,
see above)

Best regards, Julian