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Date: Tue, 8 Jul 2014 12:21:08 +0100
From: Tony Finch <dot@dotat.at>
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Subject: Re: [abnf-discuss] defining "compatible" extensions
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Paul Kyzivat <pkyzivat@alum.mit.edu> wrote:
>
> The key thing is that an extension definition needs to satisfy *two* ABNF
> rules.

It is tempting perhaps to introduce a rule refinement operator that
parallels /=, e.g.

   attribute	 =	att-specific / att-generic
   att-generic   =	(att-field ":" att-value) / att-field

   att-specific	/=	new-att

   new-att	 =	att-generic
   new-att	&=	"new-att-name:" new-att-value

The fun thing about this is that context-free grammars are not closed
under intersection, so if you introduce a way of saying that a token
must be parsable by two syntax rules then you are significantly increasing
the power of your specification language :-) This has awkward consequences
for tools that might work with the grammar.

So I think the &= operator needs to have asymmetrical semantics: it should
mean that the grammar following &= must be a subset of the existing
grammar for the rule. That is, it is an error if a rule's refined grammar
can match an input that is not matched by its generic grammar. (Note I
have not properly checked if this restriction is sufficient to keep the
grammar context-free.)

Tony.
-- 
f.anthony.n.finch  <dot@dotat.at>  http://dotat.at/
FitzRoy: Northerly 4 or 5, increasing 6 or 7 in south. Moderate, occasionally
rough in south. Showers. Good.

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