Frans Englich wrote:
Sometimes I write grammar constructs like this:
StringLiteral: STRING_LITERAL
and use StringLiteral in subsequent rules, instead of STRING_LITERAL directly.
The reason is readability, and to stay consistent with an EBNF specification.
I wonder, does this cause a negative performance impact? When I compile the
parser with extra debug output I see that it actually performs a
STRING_LITERAL --> StringLiteral reduction.
These rules are called "unit rules" in the literature; much effort was
devoted to wiping them out automatically from LR parsers since they
account for a non-negligible space and time overhead (an old paper by
Joliat mentionned a 47% time improvement when getting rid of them). If
you want to learn more about these techniques, the chapter on LR
optimization in Sippu and Soisalon-Soininen's _Parsing_Theory_ seems
rather exhaustive.
--
Sylvain
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