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RFC 5234, "Augmented BNF for Syntax Specifications: ABNF", January 2008

Area Assignment: app

Errata ID: 4361
Status: Rejected
Type: Editorial
Publication Format(s) : TEXT

Reported By: Brian S. Julin
Date Reported: 2015-05-10
Rejected by: Barry Leiba
Date Rejected: 2015-05-12

Section 2.1 says:

Unlike original BNF, angle brackets ("<", ">") are not required.
However, angle brackets may be used around a rule name whenever their
presence facilitates in discerning the use of a rule name.  This is
typically restricted to rule name references in free-form prose


Section 2.1 could be more explicit as to whether a parser, encountering a string that could be interpreted by humans as a rule name inside angles, should process it as a <rule-name> or not. The use of the words "typically restricted" allow for leeway in interpretation, and in light of section 4 note 1, implementors may be tempted to disregard the formal definition as being simply informative. Section 3.10 gives <prose-val> looser precedence than rule-name, adding weight to the idea that since there is a ambiguity to be resolved, angle brackets around rule names might be expected to appear in parser input.

It is also worth noting that the rule definition for <prose-val> prohibits the use of angle brackets around rule names within the contents of <prose-val> itself, and that the intended use of the <prose-val> rule is only described via comments.

Corrected text is not provided as the intent is unclear.
Within an ABNF parsing context, the handling of rulenames with or
without angle-brackets is a small distinction. Parsers can and do
easily handle rulenames without the brackets; obviously they also can
handle the presence of the angle-brackets, although they generally
don't. Omitting brackets from ABNF intended for parsing was generally
received as an improvement over BNF, 40 years ago.

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