History: PEG

Source of version: 13 (current)

Parsing Expression grammars. Parsing expression grammars allow for full lookahead and backtracking in time linear to the input size. This makes them more expressive than ((YACC)) or ((BNF)), which are limited in the how far they look ahead, and as a consequence how far they can backtrack. They also require more memory than either YACC or BNF to parse an equivalently sized input. PEG grammars also do not have a separate lexing stage. Lexing and parsing are performed at the same time, using the same language for both. See wikipedia for additional general information on [http://en.wikipedia.org/wiki/Parsing_expression_grammar|Parsing Expression Grammars]. ! 4th Baseline Machine Grammar Proposal ((.alyn.post.)) is working on a proposal for a 4th Machine Grammar Baseline, replacing the 3rd baseline's ((YACC)) grammar with a PEG grammar. This work is scheduled for inclusion into ((Suggestions for CLL, second edition|CLL version 2.0)) or ((CLL Peg Errata|CLL version 2.1)). !! Morphology Since PEG does not have a separate lexing stage, any PEG Machine Grammar will also need to express the Lojban Morphology in PEG. * ((BPFK Section: Formal Morphology)) !! Technical Points of the PEG language * The '.' characters means any character, in any character set. It is only used after fa'o, which unconditionally consumes the remaining characters. * '!.' is the way EOF is tested in PEG. * space is defined as the literal '.' (as opposed to the '.' operator of PEG), whitespace, and all punctuation other than ',' and '''. !! CLL The CLL will need to be updated to account for changes resulting from the translation of the grammar to PEG. Main Article: ((CLL PEG Errata)) !! Uncategorized Material ** ((Practicable syntax changes)) ** ((Exploiting the preparser)) *** ((rant about "exploiting the preparser")) ! Open Discussion Points * jbogenturfa'i further transforms the PEG grammar into an idealized representation. This parse tree is suitable for programmatic manipulation. Why is this idealized parse tree not the way the PEG is written? ! Lojban parsers that use PEG !!Of the current official grammar * ((camxes)), the original PEG parser from which all others are based. * ((jbogenturfa'i)) * ((jbominje)), John Leuner's PEG grammar. The [http://subvert-the-dominant-paradigm.net/~jbominji/code/lojban_grammar.peg|lojban_grammar.peg|grammar] is derived from camxes. * [http://mhagiwara.github.io/camxes.js/|mhagiwara's camxes.js] is Lojban Parser written in JavaScript based on ((camxes)). !!Of proposed grammars * [https://skami2.iocikun.jp/lojban/zasniGerna|iocixes] written in Haskell by la.iocikun. based on ((zasni gerna)) with ((MEX grammar proposal)) of la xorxes. ([https://skami2.iocikun.jp/lojban/zasniGernaPeg|zasni gerna peg]) * [http://ilmen.tk/lojban/camxes-exp.html|ilmentufa]: Created by la.ilmen. based on the [http://mhagiwara.github.io/camxes.js/|mhagiwara's camxes.js] with plenty of experimental propositions; because the core grammar is camxes.js, it does not include all the debuggings that were done in ((zasni gerna)) by la xorxes. ([http://ilmen.tk/lojban/camxes-exp.js.peg|ilmentufa's peg]) * [http://vrici.lojban.org/~gleki/mediawiki-1.19.2/extensions/ilmentufa/glosser.htm|Another frontend of ilmentufa]: Created by la.uilym. based on [http://ilmen.tk/lojban/camxes-exp.html|ilmentufa]. It is not necessarily based on the latest version of [http://ilmen.tk/lojban/camxes-exp.js.peg|ilmentufa's peg]. ! See Also * [http://www.teddyb.org/~rlpowell/hobbies/lojban/grammar/|Robin Powell's PEG Grammar Page]. This document builds on the work ((camgusmis)) and ((xorxes)) have done, documented on this page. * ((Grammar)), for a discussion of Lojban's grammar beyond PEG. * ((YACC)), the language in which Lojban's official grammar is defined. * ((BNF)), widely considered easier to read than the YACC grammar.