It's a single-character lookahead, which is sufficient for an LR(1) language. All it does is validate that `/* * */` is a complete block comment, but not `/* *` or `/* * /`.
On Sun, Apr 9, 2017, 17:13 Darien Valentine <[email protected]> wrote: > I am curious about this lexical production, because if I understand > correctly, it seems to imply either backtracking or a lookahead that isn’t > made explicit. > > ``` > MultiLineComment :: > /* MultiLineCommentChars[opt] */ > > MultiLineCommentChars :: > MultiLineNotAsteriskChar MultiLineCommentChars[opt] > * PostAsteriskCommentChars[opt] > > PostAsteriskCommentChars :: > MultiLineNotForwardSlashOrAsteriskChar MultiLineCommentChars[opt] > * PostAsteriskCommentChars[opt] > > MultiLineNotAsteriskChar :: > SourceCharacterbut not * > > MultiLineNotForwardSlashOrAsteriskChar :: > SourceCharacterbut not one of / or * > ``` > > In the second choices within PostAsteriskCommentChars and > MultiLineCommentChars, the appearance of PostAsteriskCommentChars following > the literal asterisk is optional. Because of this, a naive match will be > made for PostAsteriskCommentChars against the `*` of a terminal `*/` of the > MultiLineComment. > > While this is not ultimately ambiguous because, having made that match, > the next attempt will fail and we can backtrack one step to find another > way out; or, more realistically, an implementation would look ahead at > whether the next character (after "*") is `/` before deciding that > PostAsteriskCommentChars/2 should _really_ be matched. However it seems > unusual that the grammar is written this way since elsewhere the grammar > seems to carefully avoid implied backtracking, and lookaheads are rare and > explicit. > > This section of the lexical grammar has remained unchanged since ES1, so > it may be that it just hasn’t been revisited as the grammar specification > style developed. Am I reading it correctly and, if so, is it actually > unusual, or does this sort of thing fall within the boundaries of how the > grammar is normally described? > _______________________________________________ > es-discuss mailing list > [email protected] > https://mail.mozilla.org/listinfo/es-discuss >
_______________________________________________ es-discuss mailing list [email protected] https://mail.mozilla.org/listinfo/es-discuss
