Rejected Alternatives

We did not seriously consider alternative ways to implement the new parser, but here's a brief discussion of LALR(1).

Thirty years ago the first author decided to go his own way with Python's parser rather than using LALR(1), which was the industry standard at the time (e.g. Bison and Yacc). The reasons were primarily emotional (gut feelings, intuition), based on past experience using Yacc in other projects, where grammar development took more effort than anticipated (in part due to shift-reduce conflicts). A specific criticism of Bison and Yacc that still holds is that their meta-grammar (the notation used to feed the grammar into the parser generator) does not support EBNF conveniences like [optional_clause] or (repeated_clause)*. Using a custom parser generator, a syntax tree matching the structure of the grammar could be generated automatically, and with EBNF that tree could match the "human-friendly" structure of the grammar.

Other variants of LR were not considered, nor was LL (e.g. ANTLR). PEG was selected because it was easy to understand given a basic understanding of recursive-descent parsing.

а если ты напишешь [[], []] - они таки разные будут

вот курсив зацени

я правда не доктор философии

ВЦИОМ: россияне назвали Толстого, Достоевского и Чехова великими поэтами

ничего не имею против иррационализма

( [ 0, 0, 0 ] ) x 3