Global InformationPackrat parser information
| Class | Parsing grammars that are PEG |
|---|---|
| Data structure | String |
| Worst-case performance | or without special handling of iterative combinator |
| Best-case performance | |
| Average performance | |
| Worst-case space complexity |
The Packrat parser is a type of parser that shares similarities with the recursive descent parser in its construction. However, it differs because it takes parsing expression grammars (PEGs) as input rather than LL grammars.[1]
In 1970, Alexander Birman laid the groundwork for packrat parsing by introducing the "TMG recognition scheme" (TS), and "generalized TS" (gTS). TS was based upon Robert M. McClure's TMG compiler-compiler, and gTS was based upon Dewey Val Schorre's META compiler-compiler. Birman's work was later refined by Aho and Ullman; and renamed as Top-Down Parsing Language (TDPL), and Generalized TDPL (GTDPL), respectively. These algorithms were the first of their kind to employ deterministic top-down parsing with backtracking.[2][3]
Bryan Ford developed PEGs as an expansion of GTDPL and TS. Unlike CFGs, PEGs are unambiguous and can match well with machine-oriented languages. PEGs, similar to GTDPL and TS, can also express all LL(k) and LR(k). Bryan also introduced Packrat as a parser that uses memoization techniques on top of a simple PEG parser. This was done because PEGs have an unlimited lookahead capability resulting in a parser with exponential time performance in the worst case.[2][3]
Packrat keeps track of the intermediate results for all mutually recursive parsing functions. Each parsing function is only called once at a specific input position. In some instances of packrat implementation, if there is insufficient memory, certain parsing functions may need to be called multiple times at the same input position, causing the parser to take longer than linear time.[4]
- ^ Ford, Bryan (2006). "Packrat Parsing: Simple, Powerful, Lazy, Linear Time". arXiv:cs/0603077.
- ^ a b Ford, Bryan (2004-01-01). "Parsing expression grammars". Proceedings of the 31st ACM SIGPLAN-SIGACT symposium on Principles of programming languages. POPL '04. New York, NY, USA: Association for Computing Machinery. pp. 111–122. doi:10.1145/964001.964011. ISBN 978-1-58113-729-3. S2CID 7762102.
- ^ a b Flodin, Daniel. "A Comparison Between Packrat Parsing and Conventional Shift-Reduce Parsing on Real-World Grammars and Inputs" (PDF).
- ^ Mizushima, Kota; Maeda, Atusi; Yamaguchi, Yoshinori (2010-05-06). "Packrat parsers can handle practical grammars in mostly constant space". Proceedings of the 9th ACM SIGPLAN-SIGSOFT workshop on Program analysis for software tools and engineering. ACM. pp. 29–36. doi:10.1145/1806672.1806679. ISBN 978-1-4503-0082-7. S2CID 14498865.