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Re: [abnf-discuss] ABNF/RFC7405/ Update: EBNF syntactic exception

Carsten Bormann <cabo@tzi.org> Sun, 10 July 2022 01:06 UTC

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From: Carsten Bormann <cabo@tzi.org>
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Date: Sun, 10 Jul 2022 03:06:23 +0200
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To: "John R. Levine" <johnl@taugh.com>
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Subject: Re: [abnf-discuss] ABNF/RFC7405/ Update: EBNF syntactic exception
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On 10. Jul 2022, at 02:56, John Levine <johnl@taugh.com> wrote:
> 
>> This grammar is ambiguous.
> 
> That's not a problem. Lots of our ABNF is ambiguous. There are well-known ways to parse
> ambiguous CFGs, such as the GLR parser in GNU bison.

There is indeed no problem with recognizing ambiguous grammars, as long as they fall into a subclass of grammars that your parser generator of the day can process.
However, if different parses assign different semantics to the parse, it is not that useful that the sentence has been properly recognized as being part of the grammar.

In the proposed grammar, what does 

a \ b c \ d

mean?

(PEG has no problem here, because of prioritized choice.)

Grüße, Carsten

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