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Editorial: Eliminate order-disambiguation from Annex B Pattern-grammar #2445

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Editorial: Eliminate order-disambiguation from Annex B Pattern-grammar #2445

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b1b26ed
Editorial: Rewrite some of the "Lookahead Restrictions" section
jmdyck Aug 12, 2024
575ebef
Editorial: Eliminate order-disambiguation from Annex B Pattern-grammar
jmdyck Jun 27, 2021
3a90b56
Editorial: Change the semantic basis for lookahead restrictions
jmdyck Aug 13, 2024
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17 changes: 14 additions & 3 deletions spec.html
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Expand Up @@ -819,8 +819,19 @@ <h1>[empty]</h1>

<emu-clause id="sec-lookahead-restrictions">
<h1>Lookahead Restrictions</h1>
<p>If the phrase “[lookahead = _seq_]” appears in the right-hand side of a production, it indicates that the production may only be used if the token sequence _seq_ is a prefix of the immediately following input token sequence. Similarly, “[lookahead ∈ _set_]”, where _set_ is a finite non-empty set of token sequences, indicates that the production may only be used if some element of _set_ is a prefix of the immediately following token sequence. For convenience, the set can also be written as a nonterminal, in which case it represents the set of all token sequences to which that nonterminal could expand. It is considered an editorial error if the nonterminal could expand to infinitely many distinct token sequences.</p>
<p>These conditions may be negated. “[lookahead ≠ _seq_]” indicates that the containing production may only be used if _seq_ is <em>not</em> a prefix of the immediately following input token sequence, and “[lookahead ∉ _set_]” indicates that the production may only be used if <em>no</em> element of _set_ is a prefix of the immediately following token sequence.</p>
<p>When a phrase of the form “[lookahead …]” appears in the right-hand side of a production, it indicates that the production may only be used if the “lookahead” (the items that immediately follow the corresponding point in the input) satisfies a specified constraint. If the production is in the syntactic grammar, the items of the lookahead are input elements (mainly tokens); otherwise, they are code points.</p>
<p>The forms of lookahead restriction, along with the constraint that each imposes on the lookahead, are as follows:</p>
<ul>
<li>“[lookahead = _seq_]”: _seq_ matches a prefix of the lookahead</li>
<li>“[lookahead ≠ _seq_]”: _seq_ does <em>not</em> match any prefix of the lookahead</li>
<li>“[lookahead ∈ _set_]”: some element of _set_ matches a prefix of the lookahead</li>
<li>“[lookahead ∉ _set_]”: <em>no</em> element of _set_ matches any prefix of the lookahead</li>
</ul>
<p>In the above:</p>
<ul>
<li>_seq_ is a sequence of terminal symbols from the production's grammar; and</li>
<li>_set_ is a finite non-empty set of terminal sequences. For convenience, _set_ can also be written as a nonterminal from the production's grammar, in which case it represents the set of all terminal sequences to which that nonterminal could expand. It is considered an editorial error if the nonterminal could expand to infinitely many distinct terminal sequences.</li>
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seq and set look too similar. I'd rename seq to sequence.

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The third commit merges them into _pattern_.

</ul>
<p>As an example, given the definitions:</p>
<emu-grammar type="definition" example>
DecimalDigit :: one of
Expand All @@ -837,7 +848,7 @@ <h1>Lookahead Restrictions</h1>
DecimalDigit [lookahead &notin; DecimalDigit]
</emu-grammar>
<p>matches either the letter `n` followed by one or more decimal digits the first of which is even, or a decimal digit not followed by another decimal digit.</p>
<p>Note that when these phrases are used in the syntactic grammar, it may not be possible to unambiguously identify the immediately following token sequence because determining later tokens requires knowing which lexical goal symbol to use at later positions. As such, when these are used in the syntactic grammar, it is considered an editorial error for a token sequence _seq_ to appear in a lookahead restriction (including as part of a set of sequences) if the choices of lexical goal symbols to use could change whether or not _seq_ would be a prefix of the resulting token sequence.</p>
<p>Note that when these phrases are used in the syntactic grammar, it may not be possible to unambiguously identify the tokens in the lookahead because determining later tokens requires knowing which lexical goal symbol to use at later positions. As such, when these are used in the syntactic grammar, it is considered an editorial error for a token sequence _seq_ to appear in a lookahead restriction (including as part of a set of sequences) if the choices of lexical goal symbols to use could change whether or not _seq_ would be a prefix of the resulting token sequence.</p>
</emu-clause>

<emu-clause id="sec-no-lineterminator-here">
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