Re: [apps-discuss] Review of: draft-yevstifeyev-abnf-separated-lists-02

[Julian Reschke <[e-mail]>]

On 13.12.2010 23:13, Thomson, Martin wrote:
> On 2010-12-13 at 20:56:08, Julian Reschke wrote:
>> "Wherever this construct is used, null elements are allowed, but do not
>> contribute to the count of elements present. That is, "(element), ,
>> (element) " is permitted, but counts as only two elements. Therefore,
>> where at least one element is required, at least one non-null element
>> MUST be present. Default values are 0 and infinity so that "#element"
>> allows any number, including zero; "1#element" requires at least one;
>> and "1#2element" allows one or two." --
>> <http://greenbytes.de/tech/webdav/rfc2616.html#rfc.section.2.1>
>
> For HTTP, the # rule can be handled with some tweaking...
>
>    a = #b
>
> ->
>
>    a = *LWS *^(*LWS "," *LWS)[b]
>
> It means that this isn't as directly usable by HTTP, though establishing a convention would make this a little easier.
>
>    a = *LWS *^COMMA[b]

Maybe I wasn't clear about the problem I see.

When in the HTTP grammar, we say

   1#2element

that matches the following header field values

   element

   element, element

   element, , element

So empty elements aren't counted.

I note that <http://tools.ietf.org/html/rfc822#section-2.7> has the same 
rule.

Is there a use case for this notation where empty list elements behave 
differently?

Best regards, Julian